How does natural selection work to make a population look different?

We can break down the process of natural selection into a few steps: genetic variation within a population, a selective pressure which favours a particular trait, increased survival and therefore reproductive potential for individuals who have those traits, and finally the increase in proportion of favourable alleles in the next generation.

This is most easily understood through an example: let’s use giraffes. Giraffes’ long necks have evolved through natural selection. If we imagine a population of the ancestors of giraffes, they would be antelope-like animals in the African savanna. In this population, there would be variation in the lengths of the necks of the animals as a result of different individuals having different alleles which control neck length. Those individuals with longer necks might be able to reach better leaves on bushes or trees and would have less competition for food, so they would be able to feed more and would be more likely to survive to reproductive age and reproduce successfully. This means that longer-necked animals would have more offspring, so the alleles which they have would be passed on to the next generation more frequently. Over time, this would change the frequencies of different alleles in the population: more offspring would be born with ‘long neck alleles’, so gradually the average neck length of the animals would increase until, after millions of years, the phenotype of the long neck of the giraffe is achieved. In this example, access to food was a selection pressure: animals with long necks got more food and so survived and reproduced more, leading to an increase in individuals with long necks in the next generation. This framework can be applied to hundreds of different examples, from peppered moths to plant leaf shapes.