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GCSE biology notes: EVOLUTION - theories & evidence, variation, speciation

EVOLUTION - theories and evidence, variation and speciation

Causes of variation, Darwin's theory of 'Natural Selection', work of Wallace on insects, Lamarckian evolution, speciation - how do separate species arise?, extinctions, selective breeding, origin of life, the rise of anti-biotic bacteria

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

 What is, and what causes, variation in species?

 What did Darwin mean by 'Natural Selection'?

  How did Lamarck's theory of evolution differ from Darwin?

What is the evidence for Darwin's theory of evolution?  What do we mean by speciation and how does it happen?

What is the role of mutations in the theory of evolution?  What do we mean by selective breeding?


Sub-index for this page

Introduction to Variation - genetic and environmental factors, continuous and discontinuous

Introduction to Darwin's Theory of Evolution

The important contribution of A. R. Wallace

Other Theories of Evolution e.g. Lamarckism

The mechanism of natural selection - a modern genetic interpretation

Evidence of evolution from fossils and other sources

The evidence of human evolution

Speciation - old and new species

How did life begin?

Antibiotic-resistant bacteria - a contemporary and worrying story of evolution!

Selective breeding (artificial selection by humans)



Introduction to Variation

On this page I am assuming  you have studied some genetics - the science of inheritance, and are familiar with the concepts of genes, mutations etc.

Know and understand that there are not only differences between different species of plants and animals but also between individuals of the same species.

There are clearly major differences between plants and animals, but there can be even significant differences between members of the same animal/plant species or closely related species e.g.

in the human population there are differences in hair colour, skin colour, eye colour, facial features etc.

Differences between members of the same species is called variation.

Even within a family group you will see differences in hair colour, pattern of colours of hair (e.g. cats and dogs), facial shape, height etc.

All of these are examples of variation within a species.

These differences are due partly to the information in the cells they have inherited from their parents (the DNA) and partly to the different environments in which the individuals live and grow.

There are two types of variation, genetic variation and environmental variation.

 

Sub-index for evolution page

Genetic variation

All plants and animals have similar characteristic to their parents - but NOT an exact match.

This is a direct consequence of the genes inherited by an organism from its parents.

Reminders:

The genome is the complete genetic DNA code of an organism - arranged in chromosomes in the nucleus.

On the chromosomes are the shorter lengths of DNA called genes which code for protein production;

The proteins and control the characteristics of an organism and how it develops.

Genes can exist in different versions called alleles (variants), which can give different outcomes in terms of genotypes and phenotypes.

The diagram below summarises the 'chain of events'.

Genetic variation in a species is created by organisms having different alleles which lead to differences in phenotypes - the observed characteristics.

An organisms genes are inherited and passed on by parents to the next generation.

These genes are passed in by the gametes (haploid sex cells) from which offspring develop after fertilisation.

Genetic variation can be caused by (1) new alleles due to random changes in the DNA sequences known as occurrence of mutations.

In most animals, and many plants, the offspring get genes from both parents, so sexual reproduction is also a cause of genetic variation (see diagram below).

The combination of genes from a 'mother' and 'father' causes genetic variation (2) because DNA sections get 'shuffled' around at random - alleles combine in different arrangements, albeit, to a small extent.

See sexual reproduction - cell division by meiosis. from which the two diagrams above were copied.

AND, (3), new combinations of alleles may also interact with each other to produce new phenotypes.

So there are at least three causes of genetic variation.

As a consequence of these genetic variations, no two individuals in a species can be genetically identical (apart from identical twins), and this produces genetic variation and observed in differences in phenotype details (the results of gene expression).

Apart from 'identical twins', no two animals of the same species look exactly the same, there will always be differences in their visible characteristics.

(Note that recent detailed inspection of the genome of twins has revealed that they are not absolutely identical at the molecular level - but its hard for us to tell them apart because the DNA of them is so similar!)

Many characteristics are determined by genes alone:

For animals, examples include blood group, eye colour and inherited disorders like cystic fibrosis.

Many flower colours of plants are determined by their genetic make-up.

See Introduction to the inheritance of characteristics and genetic diagrams

 

Now natural selection comes into play!

From the above discussion on the origin of genetic variation, we can see that a population has pool of genetic variants (variations on genes-alleles).

Most genetic variants have little or no effect on the phenotype.

However, a genetic variant can have a significant effect on the phenotype.

Such a variant might give an organism some advantage within its habit, making it better suited to survive and breed in the environment of the population.

These advantageous phenotypes will be passed to future generations, increasing the prevalence of these 'advantageous alleles' in the population.

If e.g. competition for food or climate change becomes a survival factor, the organisms with the best adapted phenotypes are most likely to survive and reproduce - the process of 'natural selection' - survival of the fittest.

The process of selecting the most advantageous phenotypes, derived from the pool of genes, can be repeated so that a species can become more and more adapted to live in its environment.

This process can eventually to a new species evolving - see speciation notes.

 

 

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Environmental factors

In many ways an organism's 'full potential' is controlled by its genetic code in the chromosome genes.

However, whether or not an organism develops into a strong healthy specimen can be determined, to some extent, by environmental conditions e.g. the conditions in which the organism grows and develops.

The differences observed between individuals in a species is called environmental variation.

Examples of 'survival of the fittest'

(i) In third world countries, where food and medical services are scarce, children cannot grow and develop properly and suffer from malnutrition and are more prone to infectious diseases.

(ii) If fair skinned people are exposed to lots of sunlight, the body responds by producing more of the dark pigment called melanin, and your skin darkens in colour to protect you from the sun's harmful uv rays. People of e.g. African origin, already have an adaptation to produce a dark skin.

(iii) A plant that receives plenty of water, nutrients and sunlight can grow into a health 'lush' green plant. If the soil is too dry or grows in the shade, the lack of water, nutrients or light inhibits growth, resulting in e.g. a thin stem and yellowing leaves, and maybe won't even survive such harsher environmental conditions.

(iv) People who eat too much fatty food tend to be larger in size and have higher blood pressure, whereas people who watch their diet and take regular exercise tend to be leaner and fitter.

(v) 'In the wild' animals compete for resources including food, water and mates for reproduction.

(vi) How susceptible are animals to a predator? How susceptible to a disease?

The net result from ....

All of these acting as selection pressures,

and they all affect the chances of animal/plant surviving and reproducing.

Therefore, those plants/animals best suited/adapted to surviving are more likely to pass on their 'beneficial'/'advantageous' genes - in other words the alleles for the most useful characteristics will be passed on to the next generation and become more common in the population.

Those individuals, and even the whole population of a species (becoming extinct), which are less well adapted, are the least likely to be able to compete and survive and pass on the less effective alleles ....

... in other words the above describes what Darwin called the 'process of natural selection' and the 'survival of the fittest'

more on natural selection

 

Sub-index for evolution page

 

Are environment effects inherited?

Although 'local' conditions can affect an individual organism, the 'negative' characteristics described above are not usually inherited (the theory of Lamarckism).

The children of a couple of fair skinned parents who like sunbathing, are highly likely to be fair skinned too.

 

Combined effects of genetic and environmental factors

All scientific research suggests that an individual organisms detailed characteristics are a result of both genetic variation and environmental variation factors.

e.g. in the case of human characteristics - academic ability, body weight, height, physical strength  and athleticism, skin colour, teeth condition etc.

For many of your characteristics, your genes provide you initially with your maximum potential (as far as we know), BUT, e.g. good diet, physical activity, intellectual stimulation and how well you are cared for in your upbringing play a large part in what you become!

 

More on genetic variation from mutations

As if the situation isn't complicated enough, there is yet another 'twist' in the science of variation.

Mutations are random changes in the sequence of bases on strands of DNA can affect the coding for proteins - as a result you can get altered versions of an allele in a gene which can be inherited.

For details see An introduction to genetic variation - causes and consequence of mutations

This change in the DNA order of the bases, changes the order of coding for the amino acids, so it can result in a change of protein it codes for.

Most mutations have no or little effect on the protein coded for and therefore little effect on the organism's phenotype - the characteristic. In fact many characteristics are controlled by several genes, so a small alteration in one of them, does not significantly change the gene expression - the phenotype - the observed characteristic.

e.g. you might see a change in eye colour, but the eye of the offspring is basically the same as those of the parents.

However, there are rare mutations that change a gene sufficiently (change of genotype) to produce a new phenotype in a species, and this may be important in an evolutionary development situation.

If the protein was an enzyme, it might not be synthesised in the correct shape - particularly the 'active site' - substrate molecule might not 'dock in' as effectively ('key and lock' mechanism). This will reduce, or even stop, the enzyme performing its function of catalysing a specific biochemical reaction.

The genetic disorder cystic fibrosis, is caused by the incorrect protein being produced.

See section on Inheritance of characteristics and genetic diagrams and inherited genetic disorders

For example, if environmental conditions change, the new phenotype characteristic might make the individual organism better adapted (suited) to the new situation.

If this is the case, the individual is more likely to survive and pass on this gene in reproduction to the next generation.

Therefore, this new phenotype can be spread throughout the population by natural selection (coming up next).

 

Variants - their affect on both coding and non-coding DNA

In the sections above, only the effects of mutations on coding DNA were discussed e.g. the correct structure of an enzyme protein might not be formed with the right shape, which will reduce, or even stop, the enzyme performing its function of catalysing a specific biochemical reaction.

BUT, DNA molecules incorporate lots of sequences of bases that don't appear to code for proteins.

These sections are referred to as non-coding DNA, and they are just as susceptible to mutation as the coding DNA previously described.

It now appears that mutations in the non-coding DNA can directly affect how genes are expressed.

This is often a case of whether genes are 'switched on', on 'not switched on'.

If the gene 'isn't switched on' the transcription of mRNA is inhibited and the protein coded for by that gene, might not be synthesised at all.

This can alter the expression of associated genes and change the resulting phenotype - characteristic.

Non-coding DNA mutations have been associated with certain types of cancer.

Research on non-coding DNA is relatively recent and there still much to discover and understand.

 

Variation and statistical representations

Variation can be continuous or discontinuous - illustrated by the two bar charts below.

Continuous variation - where the characteristic of an individual varies over a range of possibilities, with no distinct categories - no gaps between the bars, and each bar represents a narrow range e.g. 1.70 to 1.75 m in height.

We see this clearly in human statistics of height or weight.

The height of trees or their number of leaves also show continuous variation.

Characteristics that are controlled by several genes or are influenced by both genetic and environment factors, often show continuous variation.

e.g. the height of a human or a tree will be determined by both genetic inheritance AND how 'nutritious' the diet the organism receives.

Discontinuous variation - is where there are two or more distinct categories. The value/type of the characteristic must be in one of the categories, there is no range of intermediate values/types.

The four human blood groups of A, B, AB and O form a good example of a discontinuous variation of a type of characteristic - there is no continuous range of blood type..

For example, in the UK, The percentage of the population with the blood groups is A 42%, B 10%, AB 4% and O 44%. Four distinct groups, with gaps between them, only 4 possibilities.

Characteristics determined by one gene and not influenced by the lifestyle are likely to show a discontinuous variation.

Eye colour is another example of discontinuous variation e.g. irises are classified as being one of six colours: amber, blue, brown, grey, green, hazel, or red. It is not influenced by lifestyle conditions, but the colour is controlled by several genes.

However, things like a white/red blood cell count, may well show a continuous variation - especially as they depend on your lifestyle e.g. the state of your health and quality of your diet.

 


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Introduction to Darwin's Theory of Evolution - its initial rejection, its lasting influences on modern biology

(Charles Darwin lived from 1809 to 1882, first important publications on evolution in 1858 and 1859)

Jointly with the biologist Wallace, Darwin is credited with first describing the idea of evolution by natural selection - by way of the survival of the fittest plant/animal - the most adapted species to survive in their natural environment.

See later section for more on the work of Wallace

Be able to demonstrate an understanding of Darwin’s theory of evolution by natural selection including:

a) variation – most populations of organisms contain individuals which vary slightly from one to another, those with superior characteristics are more likely to survive,

b) over-production – most organisms produce more young than will survive to adulthood ensuring some will survive,

c) struggle for existence – because populations do not generally increase rapidly in size there must therefore be considerable competition for survival between the organisms,

d) survival - those with advantageous characteristics are more likely to survive this struggle,

e) advantageous characteristics inherited – better adapted organisms are more likely to reproduce successfully passing on the advantageous characteristics to their offspring

f) gradual change – over a period of time the proportion of individuals with the advantageous characteristics in the population will increase compared with the proportion of individuals with poorly adapted characteristics, and the poorly adapted characteristics may eventually be lost.

 

Darwin’s theory of evolution by natural selection states that all species of living things have evolved from simple life forms (we now know first developed more than three billion years ago).

He based his theory on a huge number of observations from fieldwork on a round-the-world trip, but also conducting many experiments in his own garden.

Some of his most important observations and studies of plants and animals were made on a five year trip around the world in a ship called HMS Beagle.

He also discussed his ideas with other scientists and took into considering the growing science of geology and the accumulation of more and more fossil specimens.

He noted the similarities and differences between fossils and realised there was some kind of progression in their sequence.

Darwin recognised that organisms, even of the same species, showed variations in characteristics (we now recognise phenotype variations from different genotypes).

He noted that the different characteristics within the same species, represented adaptations best suited to the local environment e.g. the different beaks of finches were adapted to best exploit particular food sources.

e.g. beaks changed ('adapted') as the birds developed different tastes for fruits, seeds, or insects picked from the ground or cacti. Long, pointed beaks made some of them more fit for picking seeds out of cactus fruits or picking out insects from a cavity. Shorter, stouter beaks served best for eating ('crunching') seeds found on the ground.

He also observed that these advantageous traits were passed on to their offspring.

It was obvious to him that organisms will compete for the limited resources in the ecosystem they belong to and be as successful as possible within part of a food chain.

Darwin concluded that organisms best adapted, that is, having the most suitable characteristics to live in the environment, would be the most successful competitors and more likely to survive and reproduce.

 This is summed up the phrase 'survival of the fittest'.

In other words, the successful competitive organisms pass on their genes of their successful characteristics to their offspring.

Organisms which are less well adapted to their environment are less likely to survive and reproduce.

This means they are less likely to pass their genes on to the next generation.

Such organisms are even more vulnerable if environmental conditions change, and, become less favourable, e.g. climate change, scarcity of food, so populations can fall, and a species might even become extinct.

Darwin reasoned that beneficial characteristics would become more common in the population of a given organism and the species evolves as it changes - this can lead eventually to new species - speciation.

 

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Opposition to Darwin's theory of natural selection (first published in 1859)

The theory of evolution by natural selection was only gradually accepted because:

The theory challenged the idea that God made all the animals and plants that live on Earth - challenging the way religious belief viewed how the Earth had developed.

It is/was common in religious belief systems that all we see around us was created by 'God', the divine creator.

Darwin's theory of evolution was the first important AND plausible theory for how life forms developed without the need for a 'Creator'.

Many scientists could not reconcile their religious views with Darwin's scientific approach to the origin of the diversity of species.

There was insufficient evidence at the time the theory was published to convince many scientists.

 

Lack of knowledge didn't help acceptance of Darwin's evolution theory

Darwin couldn't explain how beneficial and non-beneficial characteristics could occur and how beneficial characteristics were passed on.

All he could argue was that organisms with good survival characteristics would survive and thrive, and those without would die out.

He did actually recognise the effects of selective breeding for characteristics eg in breeding stronger faster racing pigeons, but had no idea why the pigeon fanciers methods worked!

There was also a lack of fundamental research on organisms, eg how plant/animal species may have changed over time, so there were very few scientist actually pursuing similar research.

Darwin couldn't explain how beneficial characteristics appeared and how they passed on to successfully reproducing offspring - he had no knowledge of genes and mutations.

Other scientists didn't think Darwin provided enough evidence for his theory - though few other scientists, particularly biologists, were not interested in researching how organisms had changed over time.

 

BUT, what we now know has made all the difference in biology!

The mechanism of inheritance and variation was not known until 50 years after the theory was published.

Although Darwin recognised, and argued, that species evolved he had no knowledge of genes, DNA and molecular genetics and so had no idea of how mutations occur ...

AND, therefore had no knowledge of how organisms passed on their beneficial adaptations to their offspring.

He did not know, as we now know, that observed phenotypes are controlled by genes (the alleles of genotypes) and that new phenotypes can arise by changes in the DNA (mutations) ...

AND we now know the mechanism of how beneficial genes are passed on from both parents to the offspring of future generations.

Modern research has fully vindicated Darwin's hypothesis on evolution by natural selection, and his theory is fully accepted by the scientific community, BUT, modern research has also shown that evolution and its mechanisms are much more complicated than Darwin could imagine - 'rock on' DNA!

The theory of evolution by natural selection is still important and still relevant to today's scientists.

The theories first described by Darwin and Wallace are still helping us to understand many aspects of plant and animal biology.

We now appreciate from the current total of scientific evidence that all life has descended from a common ancestor over 3 billion years ago and that changes in life forms have occurred through the process of evolution.

 

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Examples of how evolution theory is helping contemporary biology

(i) Understanding the problem of antibiotic resistance (discussed in detail in another section)

(ii) The systems of classifying all organisms are being updated thanks to evolution theory and advances in genetics - DNA genome analysis (notes on separate page).

(iii) Biodiversity: The conservation of many endangered species of plants and animals.

Understanding genetic diversity and its importance in populations adapting to changing conditions.

Around the world there are many conservation projects protecting rare and endangered species of plants and animals.

Many species and their habitat are endangered because of ever advancing movement into 'wild' areas to exploit the land for ourselves - we are changing the environment and many species are struggling to adapt.

For plants, one method is to build up stores of seeds - essentially a bank of plant genes.

A seedbank is a store of genetic material for the future - an important strategy for conservation.

If any plant becomes extinct in the wild, it can still be grown using the store seeds and the new plants introduced back into the wild.

ALSO, a the seed store provides a huge variety of alleles coding for different characteristics available for use in agriculture e.g. cross breeding species or genetically modifying plants.

The genetic variation in 'modern' crops can be quite limited and makes them susceptible to a particular animal pest or bacterial/fungal disease.

BUT, older, more traditional crops may have useful genes (allele variants) that may confer useful characteristics on newer crops.

 

Extra note on preserving the 'genetic lines' of plants

The Svalbard Global Seed Vault (Norwegian) is a secure seed bank on the Norwegian island of Spitsbergen, near Longyearbyen, in the remote Arctic Svalbard archipelago, it is approximately 1,300 kilometres from the North Pole. The idea of this 'cold' huge storage vault is to preserve a wide variety of plant seeds that are duplicate samples, or "spare" copies, of seeds held in gene banks worldwide. The seed vault is an attempt to ensure against the loss of seeds in other gene banks during large-scale regional or global crises. The seed vault now contains nearly a million seed samples.

 


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The important contribution of A. R. Wallace

One very rare exception to dismissal of Darwin's evolution theory by fellow scientists was the naturalist Alfred Russel Wallace (1823-1913), who independently developed a similar natural selection theory of evolution by studying plants and animals in the forests of South America.

Alfred Russel Wallace, working at the same time as Darwin, was eventually accepted by the scientific community as the co-founder or co-discoverer of the theory of evolution by natural selection.

Darwin and Wallace jointly published there papers on evolution and acknowledged each other's work and contributions to the theory of evolution.

However, they didn't always agree on the mechanism of evolution - how organisms change.

The work of Alfred Russel Wallace

The biological scientist A. R. Wallace independently from Darwin came up with his own (and correct) theory of evolution by natural selection. He tends not to get the same mention as Darwin.

His research led him to become a major contributor to the theory of speciation, mainly looking at the lives and species of various insects and he did collaborate with Darwin.

He was considered the 19th century's leading expert on the geographical distribution of animal species - an important factor in the theory of natural selection.

However, it was Darwin's publication in 1859 of 'On the Origin of Species' that caught the attention of scientists. This work on evolution was much more expansive than Wallace's scientific publications, and also presented lots of evidence to support his theories. The result, somewhat unfairly, is that Darwin is much better remembered than Wallace.

The practical and theoretical work of Wallace (and Darwin) has been developed as more research has done over the past 160+ years, particularly after 1858 when both scientists published major research papers on natural selection.

(Wallace's publication prompted Darwin to publish 'On the origin of species' in 1859)

Wallace travelled to many parts of the world making many observations that provided sound scientific evidence that species evolved by natural selection.

(Wallace collected more than 126,000 specimens of insects in the Malay Archipelago!)

Some examples of his research findings:

Wallace's thousands of observations provided lots of evidence to support the theory of evolution by natural selection.

Many species of butterflies had a (i) peculiar odour and taste or (ii) warning colours - all adaptations to deter potential predators from eating them - these beneficial characteristics had come about by natural selection - the fittest traits to help the species survive - beneficial characteristic passed on in the alleles of their offspring.

Another of his crucial set of observations and deductions fits in with what we understand about speciation. He noticed differences in subspecies of birds on the westernmost islands of the Malay Archipelago. He then noted their absence on the eastern islands, where other sub-species were present. He rightly concluded that this island isolation had led to the differentiation of the species.

What he and Darwin both realised was a mechanism of sub-species and even new species could arise if populations of the same species became geographically isolated - we call this speciation.

Groups of islands proved an ideal situation for observing 'speciation'.



On a somewhat grander scale, modern research has shown that species of animals in Africa and South America, now separated by the Atlantic Ocean, have common ancestors - the speciation deriving from
the geographical separation over millions of years as the American and African continental plates moved apart.

 


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Other Theories of Evolution e.g. Lamarckism

Other theories, including that of Jean-Baptiste Lamarck (1744-1829), are based mainly on the idea that changes that occur in an organism during its lifetime can be inherited.

Lamarckism theorises that an organism (plant/animal) can pass on characteristics that the organism has acquired in its lifetime and can pass on these acquired characteristics to its offspring.

Lamarckian inheritance is also known as the 'heritability of acquired characteristics'.

Lamarck envisaged that if some characteristic of an animal was particularly essential for its survival, then that characteristic could become enhanced through successive generations.

He imagined that if a beneficial characteristic is used a lot by an organism, then that organism could become more developed with this trait and the organism's offspring would inherit this acquired advantageous adaptation.

e.g. if a giraffe kept on stretching more and more for food, its neck would get longer and so offspring would be born with the potential to grow longer necks to reach for food other animals couldn't access.

This is quite contrary to Darwin's theory of evolution, which is based on the idea that variation occurs all the time and the plant/animal species whose characteristics are best suited for it to survive will survive!

In the vast majority of cases this type of inheritance cannot occur.

There is a small, but growing body of evidence to show, that environmental conditions may have an effect on the genes-DNA of subsequent generations ie the characteristics of 'gene expression' may be altered slightly. The mechanism is not fully understood yet, but, it does show how science can reject a theory and then, quite correctly, bring it back into recognition in a selected way.

Scientists like Charles Darwin (1809-1882) will always come up with different hypotheses to explain observed phenomena eg how animals have evolved over time, but ultimately an accepted theory must be supported by as much available evidence as possible.

To date, most evolution evidence fits in with Darwin's theory of evolution, a theory which is accepted by most scientists today.

Apart from naturalist's studies of the plant and animal characteristics and behaviour, the advent of molecular genetics is now providing an insight into the biological mechanism of how natural selection can happen.

 

Whenever scientists come up with alternative hypothesis to explain some phenomena, it should not be accepted without thorough research and observations (experimental evidence) to support the hypothesis and become accepted theory.

Lamarck's hypothesis was rejected because experimental observations did not support it.

The science of genetics has provided plenty of evidence to support Darwin's evolution theory.

Even the discovery of new fossils is providing fresh evidence to describe with increasing detail how organisms have developed over time.

There is also 'modern' examples of bacteria quite rapidly evolving to become resistant to antibiotics - a most unfortunate, and potentially fatal for us, case of natural selection.


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The mechanism of natural selection - a modern genetic interpretation - still advancing year by year!

  • Know and understand how evolution occurs via natural selection:

    • Individual organisms within a particular species may show a wide range of variation because of differences in their genes eg differences in height and weight (size).

    • This range of genetic variation in a population means there is quite a mix of alleles (variants of genes) from random mutations in the DNA.

    • Individuals with genetic variants giving characteristics most suited to the environment are more likely to survive to breed successfully, e.g.

      • a more successful competitive predator, a successful well camouflaged prey, faster runner,

      • white 'in winter' arctic birds are more likely survive from predator attack than those who are brown in winter,

      • larger ears or larger eyes for detecting prey lower in the food chain, or predator higher in the food chain,

      • and these are all examples of successful phenotypes,

      • which means they are more to likely to survive and breed, passing on successful genes to their offspring.

    • Since the genes that have enabled these individuals to survive more successfully in greater numbers, are then passed on to the next generation, not surprisingly, unsuccessful genes-characteristics may well die out with the species!

    • You should appreciate an understanding and appreciation of the large timescales involved in evolution e.g.

    • over many generation over many years, the best characteristics are naturally selected and the species becomes better and better adapted to its environment.

  • Know and understand that new forms of a gene result from mutation there may be relatively rapid change in a species if the environment changes.

    • It is possible by some means eg a chemical reaction induced by a foreign chemical, uv radiation, or just random chance of a small molecular change in the DNA of a gene for a mutation to occur.

    • Mutations are common, and most have no significant effect on the individual, and therefore significant effect on the species.

      • In fact, for a particular individual of a species, significant mutations can have harmful effects eg the development of cancer.

    • However, sometimes a mutation has a beneficial effect, and the change in the organisms characteristics may enable it survive, and therefore, survives and reproduces successfully.

      • This in turn means that successful genes-characteristics are passed on to the next generation.

      • Eventually, the cumulative effects of many mutations can lead to a much more successful and different, but similar species.

        • If a species of butterfly can exist in a light or dark winged form due to chance mutations, then in polluted 'darkened' industrial areas, the darker species will survive at the expense of the lighter form. In time this could lead to two closely related but separate and different coloured species as the 'dark coloured wing genes' survive in industrial areas and the 'lighter coloured' wing genes survive better in the countryside.

          • You can argue that is was a beneficial mutation for the darker coloured butterfly to help camouflage it in the industrial surroundings, but a non-beneficial mutation for the light coloured butterfly more easily seen by predators!

        • Now evolution doesn't have to take thousands or millions of years.

        • Organisms that can reproduce more quickly is an advantage in terms of the rate of evolution.

          • Rapid evolution means advantageous genes/traits are passed on to offspring more quickly and this reduces the time it takes for a population to adapt to a new environmental situation.

        • One unfortunate contemporary example is the MRSA bacterium (Methicillin-resistant Staphylococcus aureus).

          • Bacteria can mutate quite frequently and through evolution via natural selection, species are evolved that are resistant to antibiotics.

          • So we have to design new antibiotics to combat this new threat, but we're not always winning and tragic deaths have occurred in vulnerable young children or elderly people.

          • Bacteria can be ready to introduce in 20 minutes, its more like 20 years for humans.

      • BUT for most situation, over thousands to millions of years and many mutations and natural selection, whole new species exhibiting new phenotypes will always emerge, and perhaps others become extinct.


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doc b's Earth Science NotesEvidence of evolution from fossils and other sources

Introduction

Know and understand that evidence for early forms of life comes from fossils.

Know and understand that fossils are the 'traces' or ‘remains’ of organisms from many years ago, which are found in rocks which we find dating back thousands, millions, and even billions of years ago (it is thought life began 3.4 to 3.8 billion years ago, age of the Earth is ~4.5 billion years).

If fossils are well preserved they provide a wealth of information of the structure of the organism.

Generally speaking, in layers of sedimentary rock, the deeper the layer, the older the rock, hence the older the fossils.

It is therefore possible to arrange fossils in a chronological order and observe the gradual changes in the form of an organism.

These changes in fossil structure provides good evidence for evolution as it shows how species change and develop over time from thousands to millions of years.

The fossil record shows the emergence of new species and indicates extinctions when a species disappears from the fossil record.

 

Fossils may be formed in various ways:

(i) Fossils form from the hard parts of animals that do not decay easily eg bones, shells or teeth, but may stay buried as the original material for quite some time, though eventually most become replaced by surrounding minerals (*) so human bones might still be bones after a few thousand years, but dinosaur bones will be rock like after over 65 million years. The surrounding sediments might turn to a hard rock like limestone or sandstone, but the fossil structure can still be observed in its distinct detailed shape.

(*) Parts of the organism are very gradually replaced by other materials as they decay - eg mineralisation from surrounding sediments of sand or shale layers.

(ii) Fossils can form from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent e.g.

Insects in solidified amber resin (no air/microbes can get in), amber is itself a clear yellow stone formed from fossilised resin from a tree. The sticky resin traps the insect. The preservation detail can be quite remarkable.

Glaciers or permafrost ground (too cold for decay microorganisms to function), preserves the bodies of animals - woolly mammoth in the permanent frosty ground of Siberia in Russia, an iron age man found high in an alpine pass.

More of such 'bodies' are likely to emerge with global warming!

Very acid peat bogs where the pH is too low for decay microbes to function. The calcium based mineral bones dissolve rapidly in the acid water but flesh and human clothing/animal coats can be preserved in a sort of 'mummified' state - see picture below of a 'bogman'.

National Archaeology Museum, Dublin, Ireland - well worth a visit - a wonderful museum

Clonycavan Man, an iron age bogman dated, by radiocarbon dating to 392-201 BC (~25 years old)

(iii) Fossils occur as preserved traces of organisms by way of casts and impressions in sedimentary rock layers, eg footprints, burrows and rootlet traces. An organism may get buried in soft material like clay or shale. The enclosing mineral material hardens around it so that when the organism decays a cast of it is left in the sedimentary rock formed. You even find fossil footprints of dinosaurs or human beings left in soft sand or mud that became dry and buried under further layers of sand, eventually compressed into a sedimentary rock. As erosion takes place, footprints can appear!

 

When arranged in chronological order, a series of fossils can show the gradual development changes in an organism - this is powerful evidence for evolution.

The evolution of the horse from the fossil record is illustrated on ...

https://en.wikipedia.org/wiki/Evolution_of_the_horse

Good diagram on the evolutionary record of the human skull (of importance to brain capacity and function) is on

http://www.bradshawfoundation.com/origins/short_story_of_human_evolution.php

 

BUT, be able to explain why there are gaps in the fossil record, including:

a) because fossils do not always form

b) because soft tissue decays

c) because many fossils are yet to be found

The fossil record is incomplete for several reasons e.g.

only a small proportion of organisms (creatures or plants) have by chance become fossilised, and same may be of an unsuitable structure to become fossilised,

many early forms of life were soft-bodied (very soft tissue), which means that they have left few traces behind,

and there are still lots other fossils have still to be discovered, in fact new species are being regularly discovered,

and many fossil traces have been destroyed by geological activity (volcanic, erosion etc.).

Know and understand that many early forms of life were soft-bodied, which means that they have left few traces behind.

What traces there were have been mainly destroyed by geological activity.

The fossil record is incomplete for other reasons e.g.

only a small proportion of organisms (creatures or plants) have by chance become fossilised,

and there are still lots other fossils have still to be discovered, in fact new species are being regularly discovered.

Know and understand that we can learn from fossils how much or how little different organisms have changed as life developed on Earth.

From fossils we can get some idea on what the animals and plants looked like e.g. general shape, skeletal structure and sometimes, though very rarely, detail of  internal organs.

Generally speaking, the deeper the layer of rock containing fossils, the older the fossils and this means we can follow the development and evolution of a species or the origin of new species by looking at similarities and differences, but its the gradual changes in the structure of plants and animals over millions of years that shows the evolutionary path of a species.

We know from the fossil record that many species don't exist today - they have become extinct.

Know and understand that extinction may be caused by:

changes to the environment over geological time - eg think of plate tectonic movement over millions of years from warm equatorial areas to cold arctic areas of the Earth's surface,

... species may adapt or change significantly over a long time ...

BUT, changes in environment-climate can be quite fast and species might not be able to adapt in time!

new predators - one species can consume another!, we humans have been responsible for many extinctions by 'over hunting'!

A classic example is the now extinct dodo bird became extinct on the small island of Mauritius. It was not only hunted (initially by Dutch sailors in the late 16th century), they also introduced dogs, pigs and rats, all of whom developed a taste for dodo eggs! no competition!!!

new diseases - eg an animal's immune system unable to cope with a new mutant bacteria or virus,

new, more successful, competitor for food invading a particular habitat,

a single catastrophic event, eg massive volcanic eruptions, collisions with asteroids (huge impact 65 million years ago may be responsible for the extinction of dinosaurs), onset of an ice age - through the cyclical nature of speciation - the evolution of a new species better able to cope with a rapid change in climate conditions.

 

Explaining how the anatomy of the pentadactyl limb provides scientists with evidence for evolution.

A pentadactyl limb is a hand or leg limb with five digits e.g. like your human hand or foot.

Pentadactyl digits are found in many species of animals ranging from mammals, amphibians to reptiles.

Many pentadactyl limbs in these animals (especially in mammals) have a very similar bone structure, but not necessarily evolved and used for the same function e.g.

a human or monkey's hand is used for grasping,

a dolphin's fin is adapted for swimming

a mole's feet are adapted for digging,

a bat's 'hands and feet' supports the wings for flying

This suggests, that all these species exhibiting a pentadactyl limb, with similarity in bone structure, all evolved from some common species and this common ancestor had itself evolved to have a pentadactyl limb.

It is highly unlikely that so many different species could have independently evolved to have the same specific anatomical characteristic like a pentadactyl limb.

 


Sub-index for evolution page


The evidence of human evolution

What does the fossil record and artifacts tell us about ourselves!

Since we are the species which has elucidated a theory of evolution, it seems appropriate to look at how we our selves (homo sapiens) have evolved!

As a child, I was fascinated by this picture of skeletons in a book my mother had called "Apes and Men" published by Oxford: The Clarendon Press (Oxford University Press) in 1927.

The skeletons alone suggest they all have a common ancestor! but note the changes in the relative length of arms and legs, and the structure of the feet and skull (including the brain containing cranium)

We (humans and like species, genus Homo) split from chimpanzees (genus Pan) from a common ancestor about 4-7 million years ago - its not that long ago in terms of geological time!

(The Earth is 4500 million years old, 4.5 x 109y)

The split is difficult to pin-point in time because only fragments of fossil bones are found and they, and the surrounding rock layers, are difficult to date precisely.

Rocks are dated from the ratios of elements in them including lead Pb, samarium Sm, neodymium Nd and argon Ar - the calculations are very complex and not without error, but improving with more research and cross-checking of data between different research groups. This branch of science is called radiometric dating.

Human beings and their ancestors are called hominids, but despite the scarcity of good quality fossils, from the fossil record, it is possible to show how we have evolved over the past ~5 million years.

The human fossil record and timeline - tabular style for a clear comparison

The early hominid fossils have characteristics somewhere between true apes and true modern humans.

The structure of feet, legs, arms and skull are of particular importance, and of course, a measurement of brain size.

You can also see evidence of evolution through the development of tools using stone and wood.

Gradually, stone tools became more sophisticated, along with brain development.

Generally speaking, the lower the layer of rock the fossils and tools are found in, the older the materials at that level.

The structural features of the tools tells you about their age - the more sophisticated, the more recent.

If a stone tool is found with a wooden handle, or non-mineralised skeleton (still bone), they will contain the element carbon and can be dated by carbon-14 dating (14C is a radioactive isotope of carbon with a half-life of ~5700 years).

For more on dating materials see Uses of decay data and half-life values - archaeological radiocarbon dating, dating ancient rocks  GCSE physics revision notes

Metals were first extracted and used around 11000 years ago, well after the first appearance of modern humans - copper was extracted from crushed rock and beaten into shape - but smelting came a bit later!

Fossil name and species ~Age

millions of years

Skull and brain size (cranial capacity - brain area in skull) Structure of feet Arms and legs Comments
'Ardi' Ardipithecus ramidus female from Ethiopia 4.4 Similar to chimpanzee ~35 kg, 350 cm3 Suggests she climbed trees - had a big toe to grasp branches. She had long arms and short legs - more like an ape than human, but leg structure suggests she walked upright (bipedal) and didn't use hands to walk like apes do. A mixture of ape and human features.
'Lucy' Australopithecus afarensis from Ethiopia 3.2 Brain slightly bigger than Ardi's ~37kg, 430 cm3 but still similar to chimps. Arched feet, more adapted to walking than climbing and no ape-like big toe. The arm and leg sizes are between that of apes and humans, and suggest she walked upright more efficiently than Ardi A mixture of ape and human features, but a bit more 'human' than Ardi.
'Turkana Boy' species of Homo erectus 1.6 Brain much larger than Lucy's,~1000 cm3,  but still less than modern humans Feet and leg structure suggest that he was even better adapted to walking upright than Lucy. He had short arms and long legs, much more human than an ape. Still a mixture of ape and human features, but significantly more 'human-like' than Lucy.
Modern humans Homo sapiens 0.1 - 0.2 significant increase in brain size ~57 kg, 1450 cm3      

 

Examples of stone too development

Homohabilis (2.5 to 1.5 million years ago)

Made simple crude stone tools by bashing rocks together to make sharp flakes that could be used to scrape meat off bone and extract marrow from cracked bones.

Homo erectus (includes Turkana Boy', 2.0 to 0.3 million years ago)

More sophisticated 'sculpturing' of rocks to make simple hand axes - these could be used for chopping meat/wood, scraping meat from bone and a hunting weapon.

Homo Neanderthals (species that went extinct around 30,000 to 40,000 years ago, emerged ~0.4 million years ago)

Able to make good quality flint tools e.g. sharper axes, pointed tools that could bore into wood and wooden spears for hunting.

Homo sapiens (200,000 years ago to the present)

Further improvements in working stone - precisely carved arrow heads. From 50,000 years ago (at least) a wider range of natural materials is being exploited e.g. spear heads fish hooks from carving antlers, buttons and needles

 

Further comments

The table only quotes a few of the many hominid fossils that have been found, but they are, I hope, sufficient, to illustrate the evolutionary path by which a species emerges that can investigate its own complex path of development!

From the table the general evolutionary trends from early hominids to modern humans are:

(i) A general increase in brain size - survival of the best 'thinkers'.

(ii) Arms are no longer required to help in walking - free to develop to do other skilful things!

(iii) Feet and legs evolve to give true bipedal upright walking.

(iv) The craftsmanship and sophistication of stone tool gradually improves.

In the 1980s a scientist called Richard Leakey led an expedition to Kenya in Africa to look for hominid fossils. He and his teams, discovered many important fossils of different species of the genus Australopithecus and genus Homo

Apparently for most of the past 2 million years, there was only a small increase in brain size, but from 100000 to 200000 years ago, evolutionary selection pressures produced a smaller body mass and larger brain.

 

Some examples of stone tools

Derby Museum - Stone tools, mainly flint, used by people 300,000 to 180,000 years ago.

 

Lincoln

Flint handaxe heads from 180,000 t0 450,000 years ago, mostly from Lincolnshire (The Collection Museum, Lincoln)

Some are a bit more sophisticated than the ones in Derby Museum.

 

Polished stone from the Neolithic Age found on Shetland, much more recent, 4500-6000 years old, much more recent, often produced with great skill, perhaps as ceremonial objects of status?

 

An elk antler fashioned into a digging tool

An inscribed pendant, 11,000 years old

An efficient barbed hunting too made from antler bone

A flint nodule from smaller flake scraper tools can be made

Examples of Mesolithic ('middle stone age) artefacts in the Yorkshire Museum, York, some from the famous Mesolithic site of Starr Carr near Scarborough. The Mesolithic period in Britain dates from around 11000 to 6000 years ago.

 


Sub-index for evolution page


Speciation - old and new species

Definitions

A species is defined as a similar group of organisms that can interbreed to reproduce to give fertile offspring.

Speciation is defined as the development of a new species.

Speciation happens when populations off the same species become sufficiently different genetically that the can no longer successfully interbreed to produce fertile offspring.

 

'Mechanism' - how does speciation happen?

A new species often arises as a result of geographical separation:

e.g. speciation can occur via isolation – two populations of a species become separated geographically.

This might be due to a physical barrier like a mountain chain.

Floods and earthquakes can isolate sections of a population.

In the two geographical regions, the climate might be different, the other plants and animals may be different - different ecosystems.

However, if each population can survive, by the process of natural selection, two distinct species can evolve.

(Or possibly one population remains the same, but the other has to adapt to a different environment).

If two populations of the same species become separated  ..

and encounter different environmental conditions, assuming neither population becomes extinct,

mutations will occur, and any advantageous genes are passed on through successive generations,

over a long period of time, the beneficial genotypes (alleles) and resulting phenotypes, can change so much that a completely new species evolves.

The fossil record shows changes that lead to the formation of new species and continues to this day.

 

Why can this happen?

Genetic variation – each population has a wide range of alleles that control their characteristics.

Since the environment is different in the separate geographical areas, different advantages characteristics will become more common - individuals better adapted will breed more successfully and pass their genes on to their offspring.

Natural selection – in each population, the alleles that control the characteristics which help the organism to survive are selected and passed on to the next generation - 'survival of the fittest'.

Now the populations become so different that successful interbreeding is no longer possible.

Therefore breeding continues separately within each distinct population - in time increasing further genetic distinction between the two populations, now different species.

 


Sub-index for evolution page


How did life begin?

To me, personally, this is the greatest of scientific mysteries yet to be solved.

Despite all the fossil and genetic evidence of evolution, there is no clear answer as to how it all began!

Current evidence suggests that life on Earth began about 3.7 billion years ago (3.7x 108 years).

There are various hypotheses to suggest how some simple form of self-replicating life evolved, but that is what they are - just hypotheses!

Did life evolve in some kind of pool of simple/complex organic molecules and mineral salts?

Did the molecules of life arrive on a comet?

Could life have evolved by hot volcanic springs under the sea?

The first problem is lack of evidence of the structure and function of early forms of life.

There are two principal reasons for this.

(i) The earliest fossil records are not clear, especially as early forms of life would have tiny soft bodies that would readily decay leaving little if any trace of the organism's existence.

(ii) Many fossils formed millions of years ago have been destroyed by the Earth's geological activity e.g. tectonic plate movements crushing and disrupting layers of sedimentary rock, volcanic activity, and the weathering and erosion of the earliest rocks - the latter means that much of the earliest layers of sedimentary rock containing many of the earliest fossils has disappeared.

 


Sub-index for evolution page


Antibiotic-resistant bacteria - a contemporary and worrying story of evolution!

The rapid growth and genetic development of bacteria provide strong evidence for evolution.

This is a relatively new area of research and is giving scientists for information as to how evolution works at the molecular level.

 

Introduction - what are antibiotics?

An antibiotic is a type of antimicrobial substance active against bacteria and is the most important type of antibacterial agent for fighting bacterial infections.

Antibiotic medications are widely used in the treatment and prevention of such infections.

They may either kill or inhibit the growth of bacteria.

Antibiotics like penicillin (discovered in 1928) have proved an amazing medical development for treating infectious (sometimes fatal) bacterial diseases.

The action of antibiotics in killing certain bacteria has saved millions of lives over 90 years.

However, they have their limitations and a particular antibiotic cannot provide a permanent long term solution to particular infections - the reasons why are discussed in detail further down.

 

How and why can bacteria become antibiotic-resistant?

Random mutations can occur in the DNA of any living organism, and bacteria are no exception.

These mutations can lead to a change in the bacteria's characteristics (phenotypes), and, unfortunately, from our point of view, less susceptible to a reaction with an antibiotic.

This is evolution in action!

Strains of bacteria can develop with genes-alleles that give the bacteria protection against a specific antibiotic.

The presence of these genetic variants in the genome of the organism leads to the formation of antibiotic-resistant bacteria.

The strains of bacteria that offer antibiotic resistance have an evolutionary advantage,

so are more likely to survive and multiply in the bacterial population (in the environment of the antibiotic),

at the expense of less antibiotic resistant bacteria, that will be killed/inhibited by the antibiotic,

so the gene for antibiotic resistance becomes more common in the population.

The ability of a bacterial strain to resist the effects of an administered antibiotic gives it an genetic advantage and these new strains are better able to survive by 'natural selection' in the host (your body!).

The situation is made worse because bacteria multiply rapidly, and this speed of reproduction allows the evolution of antibiotic-resistant strains of bacteria to take place more quickly.

This increases the relative population of antibiotic-resistant strains of bacteria.

This another case of 'natural selection', the antibiotic-resistant bacteria become more common in the population.

i.e. a case of 'survival of the fittest', because the development of antibiotic resistant genes/alleles in the organism, means that these evolved strains of bacteria are better able to survive in the environment of antibiotics!

 

These days research scientist can monitor the changes in the DNA of generations of bacteria and actually genetically plot their evolution into different strains.

 

What is the problem for us?

Its quite simple! If we become infected with a strain of bacteria that is resistant to the antibiotic treatment, the treatment is ineffective!

The person is not immune to the invasive bacteria AND the infection can spread more easily in the population.

Drug companies are able to develop new antibiotics BUT, unfortunately, they take a lot of time and money to develop AND 'superbugs' are evolving which are resistant to most common antibiotics - overuse is contributing to the problem.

MRSA is one of the most common superbugs that is difficult to treat and get rid of.

MRSA is often contracted by people in hospitals and can be fatal if it gets into the bloodstream.

Superbugs have to be treated with 'cocktail' of the strongest acting antibiotics.

 

Why is antibiotic resistance by bacteria on the increase?

Overuse and inappropriate use of antibiotics has led to a great increase in the 'evolution' of antibiotic-resistant strains of bacterial infections.

The more antibiotics are used, the greater the chance of antibiotic-resistant strains of bacteria evolving, the greater the problem becomes.

For many years antibiotics have been very successful in treating bacterial infections (NOT viruses).

The death rate from infectious bacterial diseases like pneumonia has fallen quite dramatically.

However, there are several reasons why antibiotic resistance is on the increase:

(i) The overuse of antibiotics

The more antibiotics are prescribed the bigger the problem of antibiotic resistance - you are giving more scope for different strains of rapidly reproducing bacteria to evolve.

The use of antibiotics doesn't cause resistant strains to develop, its just that by extensively using them, you unfortunately create a situation where there is an increased probability of an antibiotic-resistant strain developing, and these strains have a genetic advantage over the bacteria you are trying to treat.

(ii) The inappropriate prescribing of antibiotics

Antibiotics are often prescribed by doctors for viral conditions that antibiotics cannot treat - perhaps under pressure from patients feeling unwell and demanding a treatment? BUT, antibiotics are completely ineffective against viruses e.g. flue or common cold viruses.

(Note: Antiviral drugs are particularly costly to research and develop.)

(iii) Fully complete your prescription instructions

Despite point (i), for a genuine bacterial infection,  its really important you complete the full course of your prescribed antibiotics.

This ensures all the bacteria are destroyed, not only curing you of the infection, BUT preventing the bacteria form mutating into another antibiotic-resistant form.

(iv) Use of antibiotics in agriculture

Farmers treat animals with antibiotics to protect them against potential bacterial infections - obviously the prevention of illness will increase the yield of milk or meat from the herd.

Again, unfortunately, this mass treatment of farm animals will lead to the development of antibiotic-resistant strains of bacteria.

These could be passed on to humans in meat and milk based products.

The overuse of antibiotics in farming has caused sufficient concern for some countries to restrict the use of antibiotics with farm animals.

(v) Can we keep up with bacterial evolution?

Drug companies are always looking for a new market and, encouraged by both government and the medical profession, to develop new antibiotics that can combat these new deadly strains of antibiotic-resistant bacteria.

However, it takes time and a lot of money to develop new antibiotic products.

Because this process is so slow, it is difficult to produce new antibiotics in time to keep up with the rate of evolution of new strains of antibiotic-resistant bacteria - the stuff you are trying to treat!!!

I'm afraid this is a problem that is not going away, however clever pharmaceutical research chemists are!

 

See also Culturing microorganisms like bacteria - testing antibiotics and antiseptics  gcse biology revision notes

and Keeping healthy: defence against pathogens, infectious diseases, vaccination, drugs, monoclonal antibodies


Sub-index for evolution page


Selective breeding (artificial selection by humans)

Introduction - methodology

Darwin had noted and admired the successful breeding of livestock in agriculture and 'fancy' pigeon enthusiasts.

BUT note, this initially does not produce a new species.

In principle selective breeding of plants or animals by us humans, the basic process is quite simple.

You pick the plants and animals with the best features you want and interbreed them to get the best possible offspring - you are cross-breeding varieties to get the best outcome.

From your existing plant/animal stock you pick those with the best characteristics you desire.

Breed your selection with each other.

Repeat the process with the best offspring from your initial stock.

You continue this process over several generations so that the desired trait gets stronger and stronger.

The offspring should, in principle, display the desired characteristic to its full potential.

Unlike natural selection, 'in the wild', selective breeding is when we artificially pick the plants or animals to breed to keep the genes for the selected characteristic in the population.

This selective breeding develops the features that are e.g. most useful or attractive or resilient to the environment.

There is nothing new in selective breeding - for thousands of years, for their own use, people have been domesticating animals from the wild (e.g. dogs and cows) and producing useful edible grain like wheat, oats and barley from wild grasses.

 

Examples of selective breeding and their advantages

Crops that give the highest yield e.g. cereals or vegetables e.g.

Tall crop plants (e.g. wheat) give high grain yields but are easily damaged by rain and wind. Dwarf crop plants are better weather resistant, but give lower grain yields.

If you cross-breed the tall and the dwarf plant and then cross-breed the offspring, after several generations you get a new breed of crop variety that combines the good characteristics of the original varieties - a compromise of improved yields an weather resistance

Crops that are edible for our digestive system were bred from wild grasses thousands of years ago.

Crops that are disease resistant, without the controversy of genetically modified crops.

Farm animals give better yields e.g. milk from cows or beef from cattle.

You interbreed, through several generation, the best bulls or cows that give you the greatest volume of milk per cow or the greatest mass of meat per head of cattle.

This is not new in the 20th/21st centuries - this has been done for thousands of years e.g. domestic cows have been bred from wild cattle, domestic woolly sheep from wild sheep.

With these and other animals you may also want to breed for other good phenotypes e.g. good mothering skills, amiable temperament, successful fertility rates and good health in general.

Pretty flowers with bigger petals of particular/unusual colours produced in a plant nursery.

Developing a particular breed of dog - size, colour, quality of fur, facial looks, amiable temperament.

It is believed the first domesticated dogs were first bred from wolves by people in China 16,000 years ago.

Race horses and greyhounds are bred from thoroughbred stock known for their speed in racing.

Laboratory bred animals with reared with particular preferences (e.g. different foods, testing drugs?) and the results compared e.g. their behavioural activity or brain activity.

 

Disadvantages of selective breeding

Problems can arise because in selective breeding for specific characteristics, you are inevitably reducing the gene pool in the population.

This reduction in alleles results e.g. from the farmer/horticulturalist repeatedly breeding from the best animals/plants which are closely related genetically - this is known as inbreeding.

In general, for animals or plants, there is more chance of organisms inheriting harmful genetic defects from a more limited gene pool.

This is why it is inadvisable for close human relatives to interbreed - there is a much greater chance of inheriting a genetic disorder or disease.

Certain 'modern' dog breeds are quite susceptible to defects e.g. breathing problems in pugs, invertebral disc problems of the spine in corgis and dachshunds.

This raises ethical issues as to whether we should breed animals with negative characteristics.

The same ethical issue crops up if animals 'pure' bred for medical research.

Due to the narrower gene pool of a given organism population, serious problems can arise if a new disease crops up.

selective breeding ==> reduction in forms of genes (alleles) ==> less chance of alleles being present in the population that can help the organism resist the disease.

Because all the animal/plant stock are closely related, there is not much genetic variation in the population.

So if one individual in the population is susceptible to a disease, all the others may be equally at risk and can result in the death of most, if not all, of a population of animals or plants.


Sub-index for evolution page


Typical learning objectives for this page

  • Be able to demonstrate an understanding of the causes of variation, including:
    • a) genetic variation – different characteristics as a result of ...
      • (i) mutation - mutations that are inherited may change the characteristics of the species
      • (ii) reproduction - the 'controlled randomness' of the possible gene combinations of the offspring inherited from their parents ensures that no offspring can be identical to either parent.
    • b) environmental variation – different characteristics caused by an organism’s environment (acquired characteristics) eg
      • sun tan caused by extra melanin pigment on exposure to lots of sunlight,
      • withering unhealthy plants drying to grow in dry soil, or too shaded light conditions
  • You should know and understand that particular genes or accidental changes in the genes of plants or animals may give them characteristics which enable them to survive better.

    • Know that over time this may result in entirely new species.

  • Know and understand that there are different theories of evolution.

  • Know that Darwin’s theory of evolution by natural selection is the most widely accepted..

  • You are expected to use your skills, knowledge and understanding to:

    • Be able to interpret evidence relating to evolutionary theory.

      • You may be given data to work from.

    • Be able to suggest reasons why Darwin’s theory of natural selection was only gradually accepted.

    • Be able to identify the differences between Darwin’s theory of evolution and conflicting theories, such as that of Lamarck.

    • Be able to suggest reasons for the different theories.

      • Understand that scientists may produce different hypotheses to explain similar observations and it is only when these hypotheses are investigated that data will support or refute hypotheses.

  • Demonstrate an understanding of how speciation occurs as a result of geographic isolation.
    • A species is group of similar organisms that can interbreed to give fertile offspring.

    • Speciation is the development of a new species and can happen when populations of the same original species becomes so different (genetically) that they can no longer interbreed to give fertile offspring.

    • Speciation can occur via isolation – two populations of a species become separated, eg geographically,

    • In the two geographical regions, the climate might be different, the other plants and animals may be different.

    • However, if each population can survive, by the process of natural selection, two distinct species can evolve (or perhaps one population remains the same, but the other has to adapt to a different environment).

  • In the context of evolution theory and genetics, be able to explain the role of the scientific community in validating new evidence, including the use of:
    • a) scientific journals - enable new findings on evolution theory and genetics to be communicated to other scientists working in the same areas of science, so ideas and knowledge are widely spread AND other scientists can check whether the research is valid eg do other scientists get the same results? do other scientists draw the same conclusions? do other scientists agree with, and find the theory valid?
    • b) the peer review process - a sort of refereeing system, research papers on evolution theory and genetics are read and checked by people competent to understand the contents of research papers (their peers) - this ensures standards are high in terms of 'good scientific practice'.
    • c) scientific conferences enable scientists to meet and present and discuss their findings on evolution theory and genetics, compare their work, listen to new ideas, get ideas to take back to their own research project. Its also a forum for other scientists to hear about research which isn't necessarily exactly their own specialist field, but broadens their own knowledge of related fields of science e.g. evolution theory and genetics.
  • Know and understand that the information that results in plants and animals having similar characteristics to their parents is carried by genes, which are passed on in the sex cells (gametes) from which the offspring develop.

    • You should understand that genes operate at a molecular level to develop characteristics that can be seen - the phrase 'gene expression' is sometimes used to describe the 'genetic outcome'.

    • An organism's characteristics are the result of the genes inherited from its parents.

    • It is the genetic code in the genes that controls the development of the organism.

  • Know that the nucleus of a cell contains chromosomes and it is the chromosomes that carry the genes that control the characteristics of the body.

    • It is the specific sex cells or gametes, which pass the chromosomes of genes on from one generation to another.

  • Know and understand that different genes control the development of different characteristics of an organism.

  • Know and understand that differences in the characteristics of different individuals of the same kind may be due to differences in:

    • (i) the genes they have inherited (genetic causes), genetic variation

      • The combination of 'male' and 'female' genes automatically produces variation.

      • Genes determine characteristics like blood group, eye colour and unfortunately inherited disorders like cystic fibrosis and haemophilia.

    • (ii) the conditions in which they have developed (environmental causes), environmental variation

      • The environment that an organism grows in can have significant effects on its development and produce variation in the species (quality of diet, environmental pollution, extent of physical activity, access to sunlight - vitamin D) eg

        • people who eat too much fatty food tend to be larger in size and have higher blood pressure,

        • people who watch their diet and take regular exercise tend to be leaner and fitter,

        • plants growing in poor soil devoid of a good supply of nutrients, tend to be smaller and less healthy, eg poor compost or lack of muck gives poorer quality of vegetables for eating,

        • people breathing in polluted air or smoke are much more likely to suffer from asthma or lung disease,

        • plants which are too shaded tend to be less green, pale coloured and thin in structure,

        • plants treated with fertiliser will grow faster and bigger (might not taste as good though!),

    • (iii) or a combination of both (i) and (ii).

      • How an organism finally ends up is often a combination of genetic and environmental factors.

      • Characteristics like academic ability, athletics performance, height, health of teeth, skin colour and condition, weight are the result of genes + environment ('nature + nurture').

  • Be able to explain how new evidence from DNA research and the emergence of resistant organisms supports Darwin’s theory.
    • DNA research suggests that all life has common origins, we all have a line of ancestors going back hundreds of thousands or millions of years.
    • DNA analysis shows a close relationship between species that have relatively recently diverged from a common ancestor (a high percentage of our DNA is the same as the DNA of apes!).
    • Evolution has been driven by small changes in DNA over many generations and this gradually changes the nature of the species and due to speciation, can lead to new species.
    • Today we can see evolution in action and the survival of the 'fittest genes' eg
      • The deadly bacteria MSRA is a strain of microorganism that has survived and prospered by having genetic characteristics making it resistant to most antibiotics.
      • Bacteria (and viruses) can mutate quite quickly and those most resistant (and carried by us!) will tend to multiply at the expense of bacteria killed by antibiotics (less carried by us!).
      • Certain strains of rats have become resistant to the poison Warfarin.
  • Know and understand that changes in the environment of plants and animals may cause them to die out.

  • Know and understand that the fossil record shows that new organisms arise, flourish, and after a time become extinct.

  • Know and understand that the record also shows changes that lead to the formation of new species.

  • You should be able to use your skills, knowledge and understanding to suggest reasons why scientists cannot be certain about how life began on Earth.

  • The uncertainty arises from the lack of enough valid and reliable evidence.

  • Know and understand that new species arise as a result of:

    • A species is group of similar organisms that can interbreed to give fertile offspring.

    • Speciation is the development of a new species and can happen when populations of the same original species becomes so different (genetically) that they can no longer interbreed to give fertile offspring.

    • Speciation can occur via isolation – two populations of a species become separated, eg geographically,

      • In the two geographical regions, the climate might be different, the other plants and animals may be different.

        • However, if each population can survive, by the process of natural selection, two distinct species can evolve (or perhaps one population remains the same, but the other has to adapt to a different environment).

    • Genetic variation – each population has a wide range of alleles that control their characteristics,

    • Natural selection – in each population, the alleles that control the characteristics which help the organism to survive are selected and passed on to the next generation - 'survival of the fittest'.

    • Speciation – the populations become so different that successful interbreeding is no longer possible.

      • Therefore breeding continues separately within each distinct population - in time producing further genetic distinction.

 


Sub-index for evolution page


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