What are founder events?

Question

And how do they lead to allopatric speciation?

Answer 1

Founder events are situations where a small number of individuals are used to start a new population. For example, if you take two or four pandas from the wild and start breeding them in a zoo, that's a pretty small starting sample.

This leads to speciation because by limiting the starting reproductive population you increase the chances that rare mutations are fixed in the population. Mutations are the fuel that selection works on when driving evolution, so the new small population has increased chances to evolve in ways different from the original large population.

Answer 2

The founder effect is the effect that occurs when a small number of individuals in a species get geographically separated from the rest of the species. Because they are a small number of individuals, the distribution of genes they carry may not be typical of the rest of the species.

For example, if there is a population of 1000 members, where 100 individuals have blue eyes, that's 10% of the population that have the phenotype for blue eyes. But lets say that 100 individuals get separated and 30 of them have blue eyes. Now that subpopulation has gone from 10% to 30% blue eyes overnight. That's the founder effect.

Another way it can happen is by a population bottleneck ... some crisis (like a drought) wipes out much of the population, and changes the genotype distribution. (E.g. in our blue-eyed example ... the population drops from 1000 to 100 and 30 of those survivors have blue eyes. In this case this would be evidence that the blue eye gene is linked to some gene that is conferring some advantage.)

The way the founder effect can lead to allopatric speciation is that the newly isolated subpopulation already has a different genotype frequency than the rest of the species. Thus it takes less time for that subpopulation to become genetically incompatible with the original species than if it had the same genotype. When that subpopulation is so genetically incompatible with the rest of the species that they can no longer interbreed with them, then they are now a new species. That's allopatric speciation (speciation by geographic isolation).