Now that we have addressed several mechanisms that keep members of different species from interbreeding, we will focus on some theories about how new species arise (speciation). Essentially, gene flow between closely related populations must be interrupted. This can happen in several ways; we will discuss two of them. Allopatric speciation occurs when populations become physically isolated due to some sort of geographical barrier. Sympatric speciation occurs when populations become genetically isolated, even though they have not been physically separated from each other.
Allopatric speciation involves some sort of geographical isolation that physically blocks migration of individuals (and therefore gene flow) between populations. Geographical isolation may arise as a result of changes in the path of a river, the uplift of a mountain range, the formation of a canyon, or other landmass changes. A good example of allopatric speciation involves two species of antelope squirrels whose populations are separated by the Grand Canyon. Presumably they evolved from once-interbreeding populations that were isolated on either rim as the canyon was formed by the Colorado River.
Sympatric speciation is speciation in the absence of geographic separation. As you can probably predict, this occurs more rarely than allopatric speciation. However, sympatric speciation is fairly common in plants, although it also has been documented in a few animals. It is not unusual for errors in cell division (meiosis) to produce gametes with an extra set of chromosomes if the homologous pairs are not separated in Meiosis I. If a plant is able to self-fertilize, the union of two of these unreduced gametes results is a condition called polyploidy. For example, if a diploid plant produces diploid instead of haploid gametes, then undergoes self-fertilization, the offspring would be tetraploid, with four sets of chromosomes. These tetraploid individuals cannot interbreed successfully with diploid plants of the parent population, thus resulting in speciation. In this manner, speciation can occur quite rapidly (in one generation) within a sympatric population of plants.
Adaptive radiation refers to the relatively rapid evolution of many new species from a single common ancestor into diverse habitats. Adaptive radiations are most often observed on island groups where new opportunities exist for immigrant species. The immigrant arrives on the island, and over time, as it moves into new habitats, the different populations become adapted to the different environments within those habitats. Genetics diversification occurs, and eventually the populations becomes reproductively isolated from each other. Examples include the speciation of Darwin's finches in the Galapagos Islands, the diversification of honeycreepers (another group of birds) and fruit flies in the Hawaiian islands, and the radiation of Anolis lizards on different islands in the Caribbean.
Genetics Selection and Speciation Part 6 VoiceThread Transcript