The evolution of aposematism is accompanied by increased diversification.

Although the use of distinct colors to indicate unprofitability to predators (i.e. aposematism) is commonly thought of as an adaptation, our knowledge of its macroevolutionary effects is limited. Because aposematism is expected to decrease attacks by predators, we hypothesized that aposematic lineages should be larger on average than their non-aposematic sister groups because of the decreased probability of extinction and/or increased probability of speciation (i.e. increased diversification). The results of our sister-group analysis are consistent with the hypothesis that the evolution of aposematism is accompanied by increased diversification of lineages, with the aposematic focal group having more species in 11 of 14 pairs of sister groups. Despite the apparent advantages of reduced predation risk on diversification rates, the evolution of aposematism is relatively rare and reversions to a cryptic state are not uncommon. In addition to the difficulties in evolving a trait that initially decreases the survivorship of prey among naive predators, we discuss other factors that may limit the apparent prevalence and success of aposematism. It is hoped that the results of our analysis will encourage further analyses of the phylogenetic relationships among aposematic groups and their relatives, and of the evolutionary time scales over which the benefits of aposematism are the greatest to lineages with this condition.