Evolutionary biologists have long recognized that developmental and genetic constraints can prevent natural selection from producing optimally adapted phenotypes ( Darwin 1859 Gould and Lewontin 1979 Cheverud 1984 Maynard Smith et al. Alternatively, as for any type of evolutionary change, absence of character displacement may also be due to constraints on the response to selection. The absence of character displacement in these examples may of course be due to absence of selection for divergence, although such absence is seldom documented.

However, interacting species do not always exhibit greater character divergence in sympatry than in allopatry ( Walker 1974 Noor 1997) we suspect that such cases are under-reported because of bias against negative results.

It thus seems reasonable to conclude that interactions between species often generate selection that favors character divergence. 2007), although studies involving plants are uncommon (but see Caruso 2000 Smith and Rausher 2008). In plants, both types of character displacement can arise when species share common pollinators, leading to evolutionary divergence in characters like flowering phenology, attractiveness to specific pollinators, rates of self-fertilization, and positioning of anthers and stamens ( Rathcke 1983 Waser 1983 Levin1985).Ī substantial literature documents patterns of selection for character displacement (i.e., Schluter 1994, 2003 Bolnick 2004 Pfennig et al. Two types of character displacement are recognized: ecological character displacement, which results from selection generated by competition for shared resources and reproductive character displacement, which results from selection to avoid disadvantageous hybridization. One major category of interaction-driven change is character displacement, which is expected to yield a characteristic pattern of variation: for a given character or set of characters, differences between two closely related species are more pronounced in sympatry than in allopatry ( Brown and Wilson 1956). hederacea is limited by the genetic architecture of floral traits.Įcologists and evolutionary biologists have long been interested in the role of species interactions (i.e., competition, mutualism, predation, parasitism, etc.) in driving evolutionary change. Our results suggest that the rate of evolution of reproductive character displacement in I. Most notably, the response to selection for reduced separation of the tallest and shortest anthers, which reduces the degree of detrimental heterospecific pollen flow, is substantially constrained. We also demonstrate, using new statistical approaches, that the predicted response to selection for four of these six traits is substantially constrained by their genetic correlation structure. We found that the six floral traits we measured are all positively genetically correlated. To assess the importance of genetic constraints for the evolution of character displacement, we examined the genetic architecture of a suite of floral traits previously shown to be under selection in the annual plant Ipomoea hederacea when this species co-occurs with Ipomoea purpurea. However, the rate and direction of character displacement will depend not only on the strength of selection for trait divergence, but also on the amount of genetic variation for selected traits and the nature of genetic correlations between them. Evolutionary theory predicts that interactions between species such as resource competition or reproductive interference will generate selection for character displacement where similar species co-occur.

Morning glory flower arch