Question

Please describe a hypothetical scenario in which an initial ancestral population could split to form a new species through allopatric speciation. A fully correct answer will at least incorporate the following terms: gene flow, divergence, natural selection (including sexual selection), genetic drift, reproductive isolation, secondary contact and reinforcement. (3 points for correctly mentioning each of the terms, 8 terms including natural selection and sexual selection, and 6 points for the overall quality of the essay).

Answer 1

For speciation to occur, two populations must be formed from one original, and must evolve in such a way that it is impossible for individuals of both to reproduce. One type of speciation is allopatric, where organisms from one ancestral species evolve into two or more descendant species after a period of physical separation caused by a geographic barrier, such as a mountain range, river, or rockfall.

Once the groups are reproductively isolated, they can experience genetic divergence. That is, they can gradually accumulate differences in their genetic makeup over the course of several generations. Genetic divergence occurs through the action of natural selection, which can favor different characteristics in each environment, and other evolutionary forces such as gene drift.

As groups diverge, traits can be obtained that act as pre-zygotic or post-zygotic barriers against reproduction. For example, if one group evolves into a large body size while the other develops into a small one. Both organisms could become unable to mate with each other — a pre-zygotic barrier — if populations meet again.

If a newly separated population comes into contact again, reinforcement occurs in secondary contact between neighboring populations. This is represented when the biological populations commonly present a potszygous isolation, with hybrids of low biological efficacy. In this way, a pre-zygotic barrier appears favoring speciation. The natural selection model is responsible for the appearance of pre-zygotic isolation, which acts as a reinforcement in contact regions between populations. This exerts a pressure against the hybrids, with a low viability of reproduction.

If the reproductive barriers that arise are strong, and they effectively prevent gene flow, groups will continue to evolve in separate ways. That is, they will not exchange genes with each other even if geographic barriers are removed. At this point, groups can be considered as separate species.