Question

Consider the following scenario. A large population of lizards occupies an extensive range that is relatively uniform ecologically (i.e., in terms of climate and co-occurring species). At a certain point in time, the ancestral population becomes divided down the middle into two similar-sized portions by a barrier that completely prevents the movement of lizards between the descendent populations. Around the same time, a major flood occurs, and a small number of individuals are swept away from the mainland on floating debris and arrive alive on a small offshore island that has no lizards of this species. The habitat on the island is similar to their homeland, but not identical (the climate is slightly different, many mainland species of plants and animals are absent, and a few island species of plants and animals are different from any present on the mainland. Thus, there are now three separate populations. Compare and contrast the expected evolutionary consequences for these populations in the future.

Answer 1

Populations that were divided by a geographic barrier (for example as a result of plate tectonics) will undergo allopatric speciation. Due to natural selection, which is a powerful evolutionary force in large populations, evolutionary adaptation probably causes the genetic changes that result in reproductive isolation in vicariant speciation.

The population that arrived on the island, with time a genetic divergence and/or morphological will occur that will lead to a new species. The constitution of the small population implies the possibility of sudden fluctuations in gene frequencies (random genetic drift), which create new arrangements based on the available gene pool and the selective pressures (natural selection) that drive or direct the process. This situation is reinforced by the isolation to which the populations are subjected, as a result of colonization processes in different environments, as a strategy primarily for dispersal of the population.