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?



Introduction and 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.

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.

 

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.

Reminder - the genes on chromosomes code for, and control, how an organism develops.

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

In most animals, and many plants, the offspring get genes from both parents.

The combination of genes from a 'mother' and 'father' cause genetic variation because DNA sections get 'shuffled' around at random, albeit, to a small extent.

This means 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).

(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

 

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

(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,

Are environment effects inherited?

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

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

 

Combined effects

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 and how well you are cared for in your upbringing play a large part in what you become!

 

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.

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

Most mutations have no effect on an organism's phenotypes.

Some mutations can have a small effect, only slightly affecting the characteristics of an organism.

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.

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!)

 


Darwin's Theory of Evolution (Charles Darwin lived from 1809 to 1882)

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 that 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.

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.

Darwin recognised that organisms in a species showed quite wide variations in characteristics (we now recognise phenotype variations from different genotypes).

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.

 

Opposition to Darwin's theory of natural selection (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.

 

One very rare exception was the naturalist Alfred Russel Wallace (1823-1913), who developed a similar natural selection theory by studying plants and animals in the forests of South America.

Alfred Russel Wallace accepted by the scientific community as the co-founder or co-discoverer of the theory of evolution by natural selection.

 

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!

 


Other Theories of Evolution

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.


The mechanism of natural selection

  • 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).

    • Individuals with characteristics most suited to the environment are more likely to survive to breed successfully,

      • eg a successful competitive predator, a successful well camouflaged prey,

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

    • The genes that have enabled these individuals to survive more successfully in greater numbers, are then passed on to the next generation.

      • Unsuccessful genes-characteristics may well die out with the species!

    • You should develop an understanding and appreciation of the large timescales involved in evolution.

  • 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.

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

        • 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!

        • 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.

      • In fact, over millions of years and many mutations and natural selection, whole new species will emerge.


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

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 or millions of years ago.

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

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!

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.

Be able to explain 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 animals ranging from mammals, amphibians to reptiles.

Many pentadactyl limbs in mammals have a very similar bone structure, but not necessarily 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.

 


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?

Geographical separation due to a physical barrier is one of the most important ways in which speciation takes place.

Over a long period of time the phenotypes can change so much that a completely new species evolves.

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

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).

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.

 

The work of Alfred Russel Wallace (1813 - 1923)

The 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.

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.

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:

Many of the most spectacular butterflies had a peculiar odour and taste to deter predators.

Warning colours are used by some species of butterflies to deter predators from eating them - a 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.



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.


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.

 


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

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, and, unfortunately, from the bacteria's 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 formation of antibiotic-resistant bacteria.

The strains of bacteria that offer antibiotic resistance are more likely to multiply in the bacterial population, at the expense of less resistant bacteria, that will be killed/inhibited by the antibiotic - 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 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.

 

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


Selective breeding (artificial selection by humans)

Introduction

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 is quite simple.

You pick the plants and animals with the best features you want and interbreed them to get the best possible offspring.

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.

 

Examples of selective breeding and their advantages

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

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

Crops that are disease resistant.

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. cows have been bred from wild cattle.

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.

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

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

 

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.

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.


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.

 


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