If a species is defined as a group of potentially interbreeding populations, then it is necessary to determine what prevents certain groups of organisms from interbreeding. Is it a different anatomy, behavior, time available for mating, or some other barrier that separates populations into species? Ultimately, some form of reproductive isolation is involved. Reproductive isolation can be categorized according to whether the barrier blocks fertilization (a prezygotic barrier) or prevents complete development after a hybrid(an individual derived from two species) zygote has been formed (a postzygotic barrier). This figure summarizes how different isolation mechanisms can prevent interbreeding.
Prezygotic barriers to reproduction include habitat, timing, behavior, and gametic isolation; any barrier that functions before a zygote is formed. In the case of habitat isolation, two species may occupy an overlapping territory but use different habitats (this is also called ecological isolation). For example, seven lizard species from a single genus live in close proximity within forested areas of the Dominican Republic. However, one of these lizards (Anolis distichus) prefers sunny perches close to the ground; another (A. cybotes) perches in shady, mid-level tree branches. Occupying these separate microhabitats prevents them from interacting physically, therefore, hybrids are rarely produced. Habitat isolation is also used in breeding programs; cat fanciers who raise and sell Himalayans would not want their female cats in the same room with male Abyssinian cats.
Two species that are receptive to mating at different times of the year, or even day, exhibit temporal isolation. For example, the eastern spotted skunk and the western spotted skunk have overlapping ranges, but they do not produce hybrids because the western variety mates in the summer and the eastern variety mates in the winter. Similarly, there are many flower species that don't interbreed because they flower at different times of the day, season, or year, thus they cannot cross-pollinate.
Groups of organisms that do not interbreed because they exhibit different behaviors exhibit behavioral isolation. Eastern and western meadowlarks are difficult to tell apart based on size, shape, and color, however, their calls are quite distinct. Presumably this difference serves to allow the females to distinguish males from the two different species. Click on the icon beside each bird to hear its song. Can you hear a difference? Behavioral isolation may also prevent different firefly populations from mating because different species have their own pattern of light pulses. Other examples include mating dances and various courtship rituals.
In mechanical isolation, differences in size and shape of genitalia (or flower structures) prevent successful mating. This is commonly seen in insects, where the genitals function like a “lock and key”, so that the intromittent organ of a male will only fit into the opening of the reproductive tract of a female that is a member of the same species.
Gametic isolation is prezygotic isolation in the most literal sense. Here, gametes do not form a zygote; for any number of reasons (morphological, chemical, or environmental), the fusion of gametes does not occur.
Genetics Selection and Speciation Part 4 VoiceThread Transcript