AQA A-Level Biology: Speciation and Genetic Drift

Speciation is how one species splits into two. This guide explains what has to happen for a new species to form, the two ways it can occur depending on whether the populations are physically separated, and genetic drift, a second mechanism of evolution that works by chance rather than selection and matters most in small populations. Running through it all is the idea that two gene pools have to be kept apart long enough to become incompatible.

What speciation is

A new species arises when two populations of the same species become reproductively isolated, so they can no longer interbreed to produce fertile offspring.

The process has three parts:

• Two populations become reproductively separated, so their gene pools stop mixing.
• Over time, differences accumulate in their gene pools, through mutation and different selection pressures.
• Eventually the genetic differences are so great that members of the two populations cannot interbreed to produce fertile offspring. They are now separate species.

The two forms of speciation differ only in what causes the reproductive separation in the first place.

Allopatric speciation

Allopatric speciation is caused by geographical isolation. A physical barrier splits a population in two.

• A population is split by a geographical barrier, such as a new river, mountain range or stretch of sea.
• This causes reproductive isolation: the two groups can no longer interbreed, so their gene pools are separated and gene flow stops.
• Random mutations create new genetic variation independently in each population.
• The two areas have different environments and selection pressures, so different alleles are advantageous in each.
• Over many generations, the allele frequencies of the two gene pools change in different directions.
• Eventually the populations are so different that they cannot interbreed to produce fertile offspring, even if brought back together.

Each stage of the figure is one step of the sequence above.

• One population. A single species on one island, its alleles shared across the whole population because every individual can breed with every other.
• Geographical isolation. A barrier splits the island, so the two groups are separated and gene flow between them stops.
• Two new species. Mutation and selection have changed the allele frequencies of the two gene pools in different directions, which is why the dots on one side have changed colour and those on the other have not.

The colour of the dots is the part of the figure doing the work. Nothing about the barrier itself makes two species; it only stops gene flow. What actually separates them is what happens afterwards, in the third stage: different mutations arise in each group, different selection pressures act on them, and the allele frequencies of the two gene pools drift apart.

That is also the order an answer has to go in, and marks are lost by stopping too early. Say that the population is geographically isolated, that this prevents interbreeding so the gene pools are separate, that mutation and selection then change the allele frequencies differently in each, and only then that the two groups can no longer interbreed to produce fertile offspring even if the barrier is removed.

Sympatric speciation

Sympatric speciation happens without geographical isolation, within a single area. Here the reproductive isolation comes from changes that stop members of the same population breeding with each other, such as:

• gamete incompatibility, so the gametes cannot fuse;
• different breeding seasons or flowering times, so the groups are never fertile at the same time;
• different courtship behaviour, so members of the two groups do not recognise one another as mates;
• changes in body shape or size that physically prevent mating.

Once a barrier like this splits the gene pool, the rest of the process is the same as before: different selection pressures favour different alleles in each group, allele frequencies diverge over many generations, and eventually the two groups cannot interbreed to produce fertile offspring.

Genetic drift

Selection is not the only thing that changes allele frequencies. Genetic drift is a mechanism of evolution in which allele frequencies change over generations purely by chance, not by natural selection.

By chance alone, some alleles are passed on to the next generation more often than others, regardless of whether they give any advantage. In a large population these chance effects average out and make little difference. In a small population, though, the gene pool is small enough that chance has a large influence, so drift has its strongest effect in small, isolated populations with little gene flow. Two situations make this especially likely:

• The bottleneck effect, when a population is sharply reduced in size, for example by a disaster, so the survivors carry only a fraction of the original variety of alleles.
• The founder effect, when a small group breaks away to start a new colony, carrying only a small, possibly unrepresentative, sample of the parent population's alleles.

In both cases genetic drift reduces genetic diversity: some alleles end up at much higher frequencies, and others are lost from the population altogether.

How this fits together

Speciation and genetic drift complete the picture of how populations change: selection and chance alter allele frequencies, and reproductive isolation lets two gene pools diverge until they become separate species. Populations do not exist in isolation, though; they interact with each other and their surroundings, which is the subject of the populations in ecosystems guide.