The What and Why of Research on Reinforcement

Reinforcement, like sympatric speciation (see Box 1), has charisma. Evolutionary biologists are still deeply uncertain about how often these processes take place, and hence how important they are in explaining the biological diversity we see today. Empirical and theoretical support for both ideas has waxed and waned over recent decades. Yet both ideas have consistently garnered an unusual amount of attention. Box 1. Glossary Sympatry—Area(s) of overlap in the ranges of populations, enabling potential interbreeding. Allopatry—Area(s) of population ranges that do not overlap with one another, preventing interbreeding. Sympatric speciation—Speciation that occurs within a range of sympatry. Reinforcement—The evolution of mechanisms that prevent interbreeding between newly interacting incipient species, as a result of selection against hybrids (narrow definition) or interspecific matings (broad definition) (See Figure 1). Figure 1 Schematic Diagram of Reinforcement Conspecific sperm precedence—Disproportional fertilization of a female by sperm of a conspecific male, when that female has mated with both conspecific and heterospecific males. Much of the appeal of both reinforcement and sympatric speciation lies in the way they unite micro- and macroevolution. Reinforcement, a concept popularized by Dobzhansky (1937), is a process by which speciation, a macroevolutionary process, can be driven directly by natural selection, one of the primary microevolutionary forces. Sympatric speciation can make the same claim. Because of this close linkage between the concepts, the study of one can tell us a great deal about the other (see Kirkpatrick and Ravigne 2002). Such studies can also reveal a lot about the general role of microevolution in species divergence. Reinforcement provides a pathway toward the completion of the speciation process. Imagine that two divergent populations (potentially even classified as separate species) come into contact after a period of allopatry (Figure 1). If the populations have been apart for a long time, evolved differences between them will cause a certain degree of incompatibility when the populations come together. Often, this incompatibility comes in the form of low hybrid fitness (postzygotic isolation) or mismatched mating characteristics (premating isolation). The degree of the development of these isolating mechanisms is roughly proportional to the genetic distance between the populations, reflecting the fact that incompatibilities accumulate over time (Coyne and Orr 1989). If the isolating mechanisms between these populations are only partially complete, extensive hybridization may occur. This can result in fusion back into a single population, or in the swamping of one population's gene pool by the genes of the other (extinction). But there is another possibility, one that can cause the speciation between the two populations to proceed. Remember that if the populations have been separated for long enough, it is likely that hybrids between them will have relatively low fitness. Individuals who mate with members of the opposing population will therefore produce offspring of poor quality, and hence have lower fitness than individuals that mate within their own population. This favors the evolution (or further divergence) of characteristics that cause mating within, rather than between, populations (Figure 1C). Speciation between the populations is driven further towards completion through this increase in premating isolation. This process, the evolution of premating isolation after secondary contact due to selection against hybrids, is reinforcement sensu Dobzhansky (1937). Recent authors have broadened the definition of reinforcement to include as a driving force any form of selection against mating between populations (e.g., Servedio and Noor 2003). This could include, for example, lower fertility, or higher mortality of females that mate with members of other populations. In all definitions, however, the microevolutionary process of selection is essential for reinforcement. In fact, in reinforcement, speciation itself can be thought of as an adaptive response to selection. It is little wonder that this causal linking of micro- and macroevolution has appeal for many evolutionary biologists.