Maintaining Genetic Isolation - In order for a new species to continue to exist, it must reproduce but continue to remain genetically distinct from other similar species. The speciation process typically involves the development of reproductive isolating mechanisms or genetic isolating mechanisms. These mechanisms prevent matings between species and therefore help maintain distinct species. A great many types of genetic isolating mechanisms are recognized.
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In central Mexico, two species of robin-sized birds called towhees live in different environmental settings. The collared towhee lives on the mountainsides in the pine forests; the spotted towhee is found at lower elevations in oak forests. Geography presents no barriers to these birds. They are perfectly capable of flying to each other’s habitats, but they do not. Because of their habitat preference or ecological isolation, mating between these two similar species does not occur. Similarly, areas with wet soil have different species of plants than nearby areas with drier soils.
Some plants flower only in the spring of the year, whereas other species that are closely related flower in midsummer or fall; therefore, the two species are not very likely to pollinate one another. Among insects there is a similar spacing of the reproductive periods of closely related species so that they do not overlap. Thus, seasonal isolation (differences in the time of the year at which reproduction takes place) is an effective genetic isolating mechanism.
Inborn behavior patterns that prevent breeding between species result in behavioral isolation. The mating calls of frogs and crickets are highly specific. The sound pattern produced by the males is species-specific and invites only females of the same species to engage in mating. The females have a built-in response to the particular species-specific call and only mate with those that produce the correct call.
The courtship behavior of birds involves both sound and visual signals that are species-specific. For example,
groups of male prairie chickens gather on meadows shortly before dawn in the early summer and begin their dances. The air sacs on either side of the neck are inflated so that the bright-colored skin is exposed. Their feet move up and down very rapidly and their wings are spread out and quiver slightly (figure 13.6). This combination of sight and sound attracts females. When they arrive, the males compete for th opportunity to mate with them. Other related species of birds conduct their own similar, but distinct, courtship displays. The differences among the dances are great enough so that a female can recognize the dance of a male of her own species.
Behavioral isolating mechanisms such as these occur among other types of animals as well. The strutting of a peacock, the fin display of Siamese fighting fish, and the flashing light patterns of “lightning bugs” of different species are all examples of behaviors that help individuals identify members of their own species and prevent different species from interbreeding.
The specific shapes of the structures involved in reproduction may prevent different species from interbreeding. Among insects, the structure of the penis and the reciprocal structures of the female fit like a lock and key and therefore breeding between different species is very difficult. This can be called mechanical or morphological isolation. Similarly the shapes of flowers may permit only certain animals to carry pollen from one flower to the next.
There are a vast number of biochemical activities that take place around the union of egg and sperm. Molecules on the outside of the egg or sperm may trigger events that prevent their union if they are not from the same species. This can be called biochemical isolation.
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