GCSE biology coding non-coding DNA mechanism of natural selection modern genetic interpretation genetic variation, mutations, variants, effect on evolution Doc Brown's biology revision study notes

GCSE level exam revision notes on Evolution: 6.

Genetic variation, mutations, variants, effect on coding and non-coding DNA and the mechanism of natural selection - a modern genetic interpretation of evolution

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Sub-index of biology notes on all aspects of EVOLUTION

(6) More on genetic variation, mutations, variants, effect on coding and non-coding DNA and the mechanism of natural selection - a modern genetic interpretation

(6a) Genetic variation from mutations - variants and their effect on both coding and non-coding DNA

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).

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.

(6b) 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.

Genetic Variation and Its Importance in Evolution

Genetic variation refers to differences in DNA sequences among individuals of a species. It is essential for evolution because it provides the raw material for natural selection to act upon.

Sources of Genetic Variation

Mutation – Random changes in DNA sequence.
Genetic recombination – Crossing over during meiosis increases genetic diversity.
Gene flow – Movement of genes between populations through migration.

Mutations and Variants

A mutation is a change in the DNA sequence. Some mutations are beneficial, some are neutral, and others can be harmful.

Mutations can create variants, which are different forms of a gene within a population.

Types of Mutations

Point mutations – A single nucleotide change (e.g., sickle-cell disease).
Insertion/deletion – Addition or removal of bases that can lead to frame shift mutations.
Chromosomal mutations – Large-scale changes, such as deletions or duplications of chromosome segments.

Mutations can occur spontaneously or due to external factors like radiation or chemicals.

Effect on Coding versus Non-Coding DNA

DNA contains coding and non-coding regions, both influencing genetic function.

Coding DNA (Genes)

Contains exons that directly code for proteins.
Mutations in coding DNA may change the amino acid sequence, altering protein function.
Example: Cystic fibrosis results from a mutation affecting the CFTR protein.

Non-Coding DNA (Regulatory Sequences)

Does not code for proteins but plays a role in gene expression.
Includes enhancers, promoters, and introns.
Mutations in non-coding DNA can disrupt regulation, affecting how genes are turned on or off (e.g., cancer can result from mutations in regulatory genes).

Mechanism of Natural Selection – A Modern Genetic Interpretation

Natural selection is the process where organisms with advantageous traits survive and reproduce more successfully.

How Genetic Variation Drives Natural Selection

Mutation introduces variation – Some traits give survival advantages.
Selection pressures (e.g., predators, climate change) favour beneficial traits.
Individuals with favorable genes reproduce, passing on advantageous alleles.
Over time, the population changes genetically—this leads to evolution.

Examples of Natural Selection

Antibiotic resistance – Bacteria evolve resistance due to mutations.
Peppered moths – Shift in moth coloration due to industrial pollution.

Why This Is Important in Understanding Evolution

Explains species adaptation and survival.
Helps in medicine (e.g., understanding genetic disorders and treatments).
Supports conservation efforts by studying genetic diversity in endangered species.

Understanding genetic variation and natural selection provides a modern view of evolution, integrating genetics, molecular biology, and environmental factors into Darwin’s original theory.

Summary of learning objectives and key words or phrases

The influence of coding non-coding DNA on the mechanism of natural selection modern.

Be able to describe the genetic interpretation of variation, mutations, and variants, and their effects on evolution of species of plants or animals.

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.

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