Natural Selection
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’
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
Adaptation by Natural Selection
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 is how evolution occurs
This means populations of organisms become better suited to their environment
A good example of this is the development of antibiotic resistance by bacteria