THE PHYLOGENETIC COMPONENT OF COOPERATIVE BREEDING IN PERCHING BIRDS

The appreciation by earlier workers of the importance of studying avian cooperative breeding (CB) in an explicitly phylogenetic context has waned in most recent studies of the subject. Newer statistical and conceptual methods correct for correlations among species inherent in their phylogenetic relationships and are used to study the evolution and adaptive status of CB in the context of phylogenetic trees. Statistical, simulation, and phylogenetic analyses of the taxonomic distribution of CB among passerine genera confirm the suspicion that CB is nonrandomly distributed among genera and extend the conclusion of E. Russell that CB may be ancient in some lineages, many of which include well-studied species. Phylogenetic reconstruction of ancestral states of ecological factors hypothesized to have promoted CB revealed a variety of temporal relationships between the inferred invasion of selective environments and the origin of CB that were not immediately apparent from nonphylogenetic analyses and that clarified the mechanistic relationship between these events. In some lineages the persistence of CB after substantial change in the selective environments presumed responsible for its origin suggests that ''phylogenetic inertia'' may partly explain the observed taxonomic distribution of CB. Phylogenetic effects cannot explain the observed plasticity and context-specific variation in many aspects of CB and helping; the joint effects of phylogeny and ecology for explaining such variation are illustrated. The data suggest that many lineages experience evolutionary forces promoting long-term stasis in life histories conducive to CB in addition to the better-characterized environmental responses modifying its short-term expression.