Cospeciation and Phylogenies
Farenholz's Rule (originally proposed in 1913) posits that parasites and their hosts speciate in synchrony. This process, the joint speciation of two or more lineages that are ecologically associated (coevolving), has since been termed cospeciation (or parallel cladogenesis). While most research to date has examined parasite-host interactions, other coevolutionary relationships may also exhibit cospeciation; however, we will focus on parasites and hosts for this discussion.
If the process of cospeciation were the only one operating, then phylogenetic trees of parasites and their hosts should be topologically identical (i.e., exact mirror images of each other; Figure 2). However, virtually all such phy-logenies are not perfectly concordant. This implies that other processes must also be at work, such as host switching, speciating independently of their host, members going extinct, failure to colonize all descendants of a speciating
Parasite tree host lineage, or failure to speciate when its host does. Further, even when concordant, it is possible that one of the groups (often the parasite) has colonized the other (the host) - host shifts might correspond to the host phylogeny because closely related hosts are more liable to colonization by closely related parasites. Comparisons of species' phylo-genies can produce insight (with limitations) into the coevolutionary history of interacting organisms. Thus, as phylogenetic data, and more sophisticated tree-building methods, have become widely available, phylogenies have become very useful in the study of coevolution. A number of empirical studies now, at least partially, support the general notion of cospeciation occurring with some regularity in some parasite-host interactions.
Coevolution, Divergence, and Adaptive Radiation
Coevolutionary interactions can lead to phenotypic divergence among populations, speciation, and adaptive radiation. In addition to the process of cospeciation described earlier, diversifying coevolution across landscapes, which has now been demonstrated in several empirical systems, can (and apparently does) contribute to the formation of new species, as well as phenotypic differentiation within species. One hypothesis of coevolutionary diversification has its roots in the original article that first coined the term coevolution (Ehrlich and Raven article mentioned above), and proposes that reciprocal diversification among members of a coevolu-tionary association (often parasite-host) results from reciprocal adaptation, geographic differentiation, speciation, and periods of noninteraction in the diversifying lineages. This process is called escape-and-radiate coevolution (Figure 3). In this process, one member evolves a defense (or some other innovation greatly reducing impact of interaction with other member), which enables a radiation due to the expansion of ecological opportunity. During the
Figure 2 Hypothetical phylogenies for host and parasite clades illustrating evidence for cospeciation.
Figure 3 Hypothetical illustration of escape-and-radiate coevolution. In this example, a host lineage has undergone an adaptive radiation after evolving an effective defense against the parasite clade. During this time, interaction with the parasites does not occur. Then, the parasite lineage evolved a counter-defense and also radiated. Because the parasite clade did not cospeciate or colonize each new host in a systematic manner, the parasite-host associations are not anticipated to exhibit one-for-one matching as in Figure 2.
radiation, interaction among the members is minimal or nonexistent. Then, the other member evolves a counter-defense to overcome the innovation and radiates as well, producing reciprocal radiation events among the members. It is believed that diffuse coevolution among plants and herbivores may often follow such a coevolu-tionary diversification process.
Coevolutionary interactions may also often produce character displacement (exaggerated phenotypic divergence in sympatry) within local hot spots. This coevolutionary displacement is typically embedded within the broader geographic mosaic of coevolution among species, but may also result in fixation or specia-tion. A number of potential outcomes may result from coevolutionary displacement: character displacement among competitors in coevolutionary hot spots, displacement via apparent competition in hot spots, replicated community structure in hot spots, and trait overdispersion in competitive networks. Such displacement has been demonstrated in numerous systems in nature.
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