Cospeciation and Host Shift

Abstract

When two interacting lineages have been in intimate association during much or all of their diversification, as in the case of obligate pollination mutualisms or many host–parasite interactions, there is a probability that speciation in one group is paralleled by speciation in the other. This mode of diversification results in a pattern of shared evolutionary history between the two lineages, known as cospeciation. Cospeciation can be a nonadaptive process that occurs in the absence of selection. For example, repeated vicariance events followed by shared allopatric speciation can produce a pattern of parallel diversification (Roderick 1997). However, cospeciation can also be reinforced or directly result from an adaptive process. For example, in feather lice and their avian hosts, preening behavior of the host imposes selection on louse body size, which prevents lice from switching between hosts of different sizes (Clayton et al. 2003). In obligate pollination mutualisms, the pollinators are responsible for the fertilization among conspecific host flowers, and thus some adaptation in the plants to exclude nonlegitimate pollinators is likely present. In fact, there are several reciprocally selected traits that may reinforce plant–pollinator specialization, such as synchronized phenological patterns (Wiebes 1979; Patel and Hossaert-McKey 2000), species-specific olfactory signals (Hossaert-McKey et al. 1994; Song et al. 2001; Grison-Pigé et al. 2002, 2003; Okamoto et al. 2007; Svensson et al. 2008), and reciprocal adaptation between pollinator morphology and floral structure (Ramírez 1974; Herre 1989; van Noort and Compton 1996; Kato et al. 2003; Weiblen 2004). Thus, knowledge of the degree of cospeciation in obligate pollination mutualisms provides an essential step toward understanding the historical role of coevolution in shaping speciation and diversification in plants and pollinators.