CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

14. The Formation of Species and Evolutionary Change

14.3. The Maintenance of Reproductive Isolation Between Species

For a new species to continue to exist, it must reproduce and continue to remain genetically isolated from other, similar species. The speciation process involves the development of reproductive, or genetic, isolating mechanisms. These mechanisms prevent matings between members of two different species and, therefore, help maintain distinct species. There are several mechanisms for maintaining reproductive (genetic) isolation:

1. Habitat preference, or ecological, isolating mechanisms, occur when two species do not have the opportunity to interbreed because they typically live in different ecological settings. For example, in central Mexico, two species of robin-sized birds, called towhees, live in the same general region. However, the collared towhee lives on the mountainsides in the pine forest, whereas the spotted towhee is found at lower elevations in oak forests. Geography presents no barriers to these birds. They are capable of flying to each other’s habitats, but they do not. Therefore, they are reproductively isolated because of the habitats they prefer. Similarly, areas with wet soil have different species of plants than nearby areas with drier soils.

2. Seasonal isolating mechanisms (differences in the time of year at which reproduction takes place) are effective genetic isolating mechanisms. Some plants flower only in the spring, 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 are examples of similar spacing of the reproductive periods of closely related species, so that they do not overlap.

3. Behavioral isolating mechanisms occur when inborn behavior patterns prevent breeding between species. 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 mate only 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 both sides of the neck are inflated, so that the brightly colored skin is exposed. Their feet move up and down very rapidly and their wings are spread out and quiver slightly (figure 14.4).

FIGURE 14.4. Courtship Behavior (Behavioral Isolating Mechanism)

The dancing of a male prairie chicken attracts female prairie chickens, but not females of other species. This behavior tends to keep prairie chickens reproductively isolated from other species.

This combination of sight and sound is attractive to females. When the females arrive, the males compete for the 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 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 a beta (Siamese fighting) fish, and the flashing light patterns of “lightning bugs”/“fireflies” (they are actually beetles) of various species are all examples of behaviors that help individuals identify members of their own species and prevent different species from interbreeding (figure 14.5).

FIGURE 14.5. Animal Communication—Identifying Members of Their Own Species

Most animals use specific behaviors to communicate with others of the same species. (a) The trilling of a male American toad is specific to its species and is different from that of males of other species. (b) The visual displays of this orange octopus communicate to others of the same species.

4. Mechanical, or morphological, isolating mechanisms involve differences in the structure of organisms. 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; therefore, breeding between different species is very difficult. Similarly, the shapes of flowers may permit only certain animals to carry pollen from one flower to the next.

5. Biochemical isolating mechanisms occur when molecular incompatibility prevents successful mating. A vast number of biochemical activities 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. Among plants, biochemical interactions between the pollen and the receiving flower prevent the germination of the pollen grain and, therefore, prevent sexual reproduction between two closely related species.

6. Hybrid inviability, or infertility, mechanisms prevents the offspring of two different species from continuing to reproduce. This can occur in three ways: (a) the embryos of such a mating may not develop properly and die; (b) if offspring are produced, they die before they can reproduce; or (c) such hybrids may be sterile or have greatly reduced fertility (review figure 14.1).

14.3. CONCEPT REVIEW

8. Describe three kinds of reproductive isolating mechanisms that prevent interbreeding between different species.

9. Give an example of seasonal isolating mechanisms, habitat preference, and behavioral isolating mechanisms.