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Learning Goals
- Identify prezygotic and postzygotic reproductive barriers
- Discuss the role of habitat in the process of speciation
- Describe the process of adaptive radiation
Introduction
For speciation to occur, two new populations must form from one original population and they must evolve in such a way that it becomes impossible for individuals from the two new populations to interbreed. Biologists think of speciation events as the splitting of one ancestral species into two descendant species. However, there is no reason why more than two species might not form at one time except that it is less likely and multiple events are most likely single splits occurring close in time.
Reproductive Isolation
Given enough time, the genetic and phenotypic divergence between populations will affect characters that influence reproduction: if individuals of the two populations were brought together, mating would be less likely, but if mating occurred, offspring would be nonviable or infertile. Many types of diverging characters may affect the reproductive isolation, the ability to interbreed, of the two populations. Scientists organize the means of reproductive isolation into two groups: prezygotic barriers and postzygotic barriers. Recall that a zygote is a fertilized egg: the first cell of a sexually reproducing organism’s development. Therefore, a prezygotic barrier is a mechanism that blocks reproduction from taking place, thus no zygote is formed. A postzygotic barrier occurs after zygote formation. This includes organisms that don’t survive the embryonic stage and those that are born sterile.
Prezygotic Barriers
Some types of prezygotic barriers prevent reproduction entirely. Many organisms only reproduce at certain times of the year, often just annually. Differences in breeding schedules, which we call temporal isolation, can act as a form of reproductive isolation. For example, two frog species inhabit the same area, but one reproduces from January to March; whereas, the other reproduces from March to May (Fig. 1).
In some cases, sub-populations of a species move or are moved to a new habitat and that no longer overlaps with the same species’ original population. Reproduction with the parent species ceases, and a new group exists that is now reproductively and genetically independent. We call this situation habitat isolation. For example, a cricket population that was divided after a flood could no longer interact with each other (Fig. 2). Over time, natural selection forces, mutation, and genetic drift will likely result in the two groups diverging. The habitats need not be far apart for reproduction isolation to occur.
Behavioral isolation occurs when the presence or absence of a specific behavior prevents reproduction. For example, male fireflies use specific light patterns to attract females. Various firefly species display their lights differently. If a male of one species tried to attract the female of another, she would not recognize th