Natural selectionOxford AQA IGCSE Biology: Revision notes

Evolution is the process by which species change over long periods of time. The main scientific explanation for evolution is the theory of evolution by natural selection.

Other theories have also been proposed. Lamarck's theory was based mainly on the idea that changes which occur in an organism during its own lifetime (acquired characteristics) can be passed on to its offspring — for example, the idea that an animal that stretches its neck during its life would have offspring with longer necks. We now know that, in the vast majority of cases, this type of inheritance cannot occur, because changes to an organism's body during its lifetime do not change the genetic information in its gametes.

Evolution by natural selection instead depends on genetic variation that already exists within a population, and on which individuals survive and reproduce. This is a crucial distinction: Lamarck's theory says the environment causes the useful change directly in an individual, and that change is then inherited; natural selection says the useful variation already exists in some individuals for other reasons (for example, mutation), and the environment simply determines which of the existing variants survive and breed.

Natural selection occurs through the following sequence of events:

1. Individual organisms within a particular species show a wide range of variation, because of differences in their genes.
2. Some of these variations make certain individuals better suited (better adapted) to their environment than others.
3. Individuals with characteristics most suited to the environment are more likely to survive to reproduce successfully.
4. Because these individuals reproduce, the genes that gave them their advantageous characteristics are passed on to the next generation.
5. Over many generations, the proportion of individuals with the advantageous characteristics increases within the population.

This process explains how populations gradually become better suited to their environment over time, without any individual organism intentionally changing. Natural selection acts on existing genetic variation — it does not create new variation itself; that comes from mutation and from the mixing of genes during sexual reproduction.

It is important to remember that natural selection acts on the population, not on a single organism. An individual does not evolve during its own lifetime; instead, the proportions of different characteristics within the whole population shift, generation after generation, as better-adapted individuals leave more offspring than less well-adapted ones.

Evolution by natural selection is generally a very slow process. Noticeable changes in a species usually take place over many generations, which for most organisms means thousands to millions of years, rather than happening within a single lifetime or a few generations.

This long time scale is one reason evolution is difficult to observe directly. Evidence for evolution instead comes from sources such as the fossil record, which shows how species have changed gradually over very long periods of geological time. Some organisms with very short generation times (such as bacteria) can show detectable evolutionary change over much shorter periods, such as months or years, because so many generations occur in a short time.

The length of time needed for evolutionary change to become noticeable depends on how quickly a species reproduces: species with short generation times pass through many generations, and therefore many rounds of selection, within a given period, so change can accumulate faster than in species with long generation times.

A new species can arise through a process involving four linked stages:

1. Isolation — two populations of the same species become separated from one another, for example by a geographical barrier such as a river, mountain range or stretch of ocean, so that they can no longer interbreed.
2. Genetic variation — each of the two separated populations already has a wide range of alleles that control their characteristics, and this variation continues within each population.
3. Natural selection — because the two populations live in different environments, or face different conditions, different alleles are favoured by natural selection in each population; the alleles that help organisms survive in their particular environment become more common in that population.
4. Speciation — over time, the two populations become so different (genetically and in their characteristics) that, even if they were brought back together, successful interbreeding leading to fertile offspring is no longer possible. At this point, the two populations are considered separate species.

The key test of whether two populations have become separate species is whether they can produce fertile offspring together — if they cannot, speciation has occurred.

Notice that isolation on its own does not create a new species — it simply stops the two populations exchanging genes. It is the combination of continuing genetic variation and natural selection acting differently in each separated environment, sustained over a long time scale, that gradually drives the two populations apart until they can no longer interbreed successfully.