Phylogeny, specialization, and brood parasite-host coevolution: some possible pitfalls of parsimony

Coevolutionary hypotheses (COEV) predict that parasitic birds become more specialized in host selection over time as more host species evolve defenses. A contrasting model, PHYLO, suggests that brood parasites exhibit a phylogenetic trajectory toward increasing generalization because there is a positive correlation between present-day numbers of host species and the branching order of parasitic cowbird species in a DNA-based phylogeny. However, this apparent phylogenetic pattern does not conflict with COEV, as some have concluded. Assuming allopatric speciation, which is supported by an area cladogram, COEV predicts a correlation between branching order and host number because the potential hosts of the earliest cowbirds to branch off have had the greatest amount of time to evolve defenses. Although PHYLO is more parsimonious than COEV, the difference is trivial, with the latter requiring only one more evolutionary change in the entire cowbird clade to produce the pattern that exists today. Support for COEV over PHYLO comes from brood parasitic cuckoos, which are much more specialized than parasitic cowbirds and represent an older clade, as shown by new DNA data. Cuckoos also have lower interspecific variance in host numbers than do cowbirds, which conflicts with PHYLO. Unlike COEV, which assumes that the number of hosts a parasite uses is related at least as much to present ecological conditions as to phylogenetic history, PHYLO assumes that current host numbers reflect historical character states. However, host number is labile, with as much variation within as between species. Nor are published host numbers reliable measures of parasite host selectivity, as they are due in part to researcher effort and range size. Although the comparative approach can provide insights into evolutionary history, some coevolved features may be too dynamic to retain a phylogenetic signature, and, in the case of parasitic birds, neither PHYLO nor COEV can be invalidated, although the latter is more consistent with available evidence. Strict adherence to parsimony may often be inappropriate when assessing coevolved characters.