Question

How does geographic isolation contribute to evolution?

Answer 1

In 1963, Ernst Mayr contended that speciation takes place through geographic isolation (allopatry). Allopatric speciation (allo = other and patric = place) is genetic divergence permitted by geographic isolation.

Complete answer:
Allopatric speciation is dependent on random mutation, which accumulates steadily after a population has been subdivided in space. This subdivision can take place because of geographic barriers such as mountain ranges and water bodies.
Because the two subpopulations are then reproductively isolated, each of the populations accumulates in different mutations and the two populations diverge.

This divergence is strongest where the population size of one or both of the sub-populations is small because sampling error is high in smaller populations. These differences will eventually lead to reproductive incompatibilities that will be keeping the populations distinct.

One of the well known examples of allopatric speciation involves divergence of Drosophila populations in the Hawaiian archipelago.
Reproductive isolating mechanisms usually originate incidentally in the speciation process. That is they arise incidentally during the process of evolution in isolated populations rather than being selected for.

Note: When isolated populations come together again, incomplete isolating mechanisms may allow hybrids to form.
If the hybrids are normal, viable and can freely interbreed with individuals of each parent population, then no speciation has taken place.
However, if the hybrids are at disadvantage, natural selection may favour stronger isolating mechanisms. In this case, organisms that mate with individuals from the other population leave fewer offspring. The result is a more effective barrier to hybridization.
Regions in which previously isolated populations come into contact and produce hybrids are called hybrid zones.