Evolution (AQA GCSE Biology: Combined Science)

• In any environment, the individuals that have the best adaptive features are the ones most likely to survive and reproduce
• This results in natural selection:
• Individuals in a species show a range of variation caused by differences in genes
• When organisms reproduce, they produce more offspring than the environment is able to support
• This leads to competition for food and other resources which results in a ‘struggle for survival’
• Individuals with characteristics most suited to the environment have a higher chance of survival and more chances to reproduce
• Therefore the alleles resulting in these characteristics are passed to their offspring at a higher rate than those with characteristics less suited to survival
• This means that in the next generation, there will be a greater number of individuals with the better adapted variations in characteristics
• This theory of natural selection was put forward by Charles Darwin and became known as ‘survival of the fittest’

Examples of natural selection

Natural selection illustrated by snail shell colour

• Within the population of snails there is variation in shell colour
• Normal varieties of shell colours in this snail species is black or grey (as evidenced by the first picture)
• Chance mutations lead to a small number of snails / one snail having a white shell
• This ‘small number’ is shown in the second diagram where there are less white shelled snails than black or grey shelled snails
• The white shelled snail(s) survive longer
• This is the ‘survival of the fittest’, a term used to explain why some organisms succeed in the competitive struggle for survival against other members of their population
• The reason the white shelled snail(s) survive longer is because they are better camouflaged
• This means that they are less likely to be seen by predators and eaten
• As they survive longer they get more opportunities to reproduce
• And so the allele for white shells is passed onto offspring more frequently than the alleles for black or grey shells
• Over generations, this is repeated until the majority of snails in the population have white shells

Another good example of natural selection is the evolution of the peppered moths

Evolution 

• If the environment does not change, selection does not change
• This will favour individuals with the same characteristics as their parents
• If the environment changes, or a chance mutation produces a new allele, selection might now favour individuals with different characteristics or with the new allele
• So the individuals that survive and reproduce will have a different set of alleles that they pass on to their offspring
• Over time, this will bring about a change in the characteristics of the species – it will produce evolution
• Evolution is defined as the change in adaptive features of a population over time as a result of natural selection
• Natural selection results in a process of adaptation, which means that, over generations, those features that are better adapted to the environment become more common
• This means populations of organisms become better suited to their environment
• If two populations of one species become so different in phenotype that they can no longer interbreed to produce fertile offspring, they have formed two new species