Evolution's Wedge: Competition and the Origins of Diversity

Chapter 3. When Character Displacement Occurs

57 in the previous two chapters, we examined how character displacement arises as an adaptive response to resource competition or deleterious reproductive interactions between species. a key unresolved issue, however, is why some populations and species are more likely to undergo character displacement as opposed to the alternative outcomes of competitive exclusion or reproductive exclusion (schluter 2000; rice and Pfennig 2007). in this chapter, we address this issue by considering when character displacement occurs. We begin by discussing six general factors that facilitate character displacement. Like other forms of local adaptation, character displacement is more likely to occur when various proximate and evolutionary factors are in place; here, we examine these factors in detail. Furthermore, we describe how variation in the occurrence of these factors can generate variation in the incidence and extent of character displacement. indeed, variation in these facilitative factors can explain why different interacting species, populations , and even conspecific individuals within the same population may differ in the expression of traits that minimize competitive interactions with other species. We then consider how one form of character displacement (be it ecological or reproductive ) potentially facilitates the alternative form. Because species that are similar enough to compete for resources will also likely impede each other’s ability to reproduce successfully (and vice versa), populations that experience one form of character displacement would generally be expected to experience the other form. Once one form of 3 WhEn ChaRaCtER disPlaCEMEnt oCCuRs 58 • C h a P t E R t h R E E character displacement has occurred, it can then facilitate the other form by generating variation on which selection can act. indeed, either form of character displacement may be decisive in determining whether or not the alternative form also transpires. Understanding when character displacement is more likely to proceed is important, because differences in its occurrence could explain ecological and evolutionary patterns of diversity. in particular, although character displacement is taxonomically widespread, it appears to be more prevalent in some taxa, communities, and populations than in others (schluter 2000). those taxa or communities that are more prone to undergo character displacement should also be more diverse, for at least two reasons. First, species that undergo character displacement are less likely to become extinct through competitive or reproductive exclusion. second, as we describe in chapter 8, character displacement may promote speciation. therefore identifying the facilitators of character displacement should ultimately provide key insights into the origins of diversity. fACiliTATOrS Of CHArACTEr DiSPlACEMENT six non–mutually exclusive factors appear to be important in facilitating character displacement : (1) standing variation; (2) strong selection favoring the avoidance of interactions with heterospecifics; (3) ecological opportunity; (4) initial trait differences between species; (5) gene flow; and (6) a lack of antagonistic genetic correlations. Below, we briefly discuss each factor’s impacts on character displacement. standing vaRiation a key facilitator of character displacement is standing variation (roughgarden 1976; slatkin 1979, 1980; taper and Case 1985, 1992; doebeli 1996). Here, we use the term “standing variation” to mean pre-existing phenotypic variants within species that ultimately minimize competitive interactions with other species when they come into contact . We focus on standing phenotypic variation, because it is phenotypic variation on which selection acts. We assume, however, that this phenotypic variation is underlain by genetic variation, which drives an evolutionary response to this selection (in the next chapter, we discuss the proximate causes of phenotypic variation). although character displacement can eventually occur without standing variation, when such variation is present, character displacement should transpire more rapidly and be more likely to come about before the alternative outcome of competitive or reproductive exclusion takes place (rice and Pfennig 2007; Pfennig and Pfennig 2009). to understand why standing variation facilitates character displacement, consider that character displacement can evolve through two non–mutually exclusive routes, which differ in whether or not standing variation is present (Figure 3.1; rice and Pfennig 2007). in the first route—which we refer to as “in situ evolution of novel phenotypes ”—novel traits that happen to arise in sympatry (for example, through a new W h E n C h a R a C t E R d i s P l a C E M E n t o C C u R s • 59 mutation) after competitors encounter each other are selectively favored and increase in frequency if they minimize competitive interactions (rice and Pfennig 2007). thus, this route occurs when competitively...