22.4 SPECIATION AND SELECTION

The association of both the origin of species and natural selection with Charles Darwin may make it seem that one cannot occur without the other. However, speciation can occur in the presence or absence of natural selection, and natural selection does not always lead to speciation.

22.4.1 Speciation can occur with or without natural selection.

Natural selection may or may not play a role in speciation. The genetic divergence of two populations can be entirely due to genetic drift, for example, with no role for natural selection.

We have also seen two ways in which natural selection can be involved in speciation. First, sympatric speciation requires some form of disruptive natural selection, as when hybrid offspring are competitively inferior. Second, allopatric speciation (and adaptive radiation) may be facilitated by natural selection. For example, when a peripheral population is in a new environment, natural selection will act to promote its adaptation to the new conditions, accelerating in the process the rate of genetic divergence between it and its parent population.

22.4.2 Natural selection can enhance reproductive isolation.

There is a third way in which natural selection may play an important role in speciation. Recall the two hypothetical bird populations, one with large bills and the other with small bills, whose medium-billed hybrid offspring are at a disadvantage because of a lack of medium-sized seeds in their environment. If a large-billed individual cannot distinguish between large-and small-billed individuals as potential mates (that is, there is a lack of pre-zygotic isolation between them), it will frequently make the “wrong” mate choice, picking a short-billed individual and paying a considerable evolutionary cost of producing poorly adapted hybrid offspring.

Now imagine a new mutation in the large-billed population that permits individuals carrying it to distinguish between the two groups of birds and to mate only with other large-billed individuals. Such a mutation would spread under natural selection because it would prevent the wasted reproductive effort of producing disadvantaged hybrid offspring. This is an example of reinforcement of reproductive isolation, or reinforcement for short. Reinforcement is the process by which diverging populations undergo natural selection in favor of enhanced pre-zygotic isolation to prevent the production of less fit hybrid offspring. In this case, enhanced pre-zygotic isolation takes the form of mating discrimination, that is, an increased ability to recognize and mate with members of one’s own population.

The best evidence in support of reinforcement comes from a study of related fruit-fly species living either in allopatry (geographically separated) or sympatry (in the same place, without geographical barriers). Sympatric species evolve pre-zygotic isolating mechanisms more rapidly than allopatric species. Why would this be? When the two populations are geographically separated, the formation of less fit hybrids is impossible since the two populations do not interbreed, so a mutation that increases mating discrimination between the two groups would not be favored by natural selection. In sympatry, however, where the production of less fit hybrids is a day-to-day problem, such a mutation provides a fitness benefit, so natural selection will favor its spread in the population and reinforce reproductive isolation between the two groups.

Fig. 22.15 summarizes the evolutionary mechanisms that lead to speciation.

Visual Synthesis: Speciation

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