THE EVOLUTION OF FLIGHTLESSNESS - IS HISTORY IMPORTANT

Though most birds and insects are capable of flight ('volant') some sp ecies are flightless. In this paper I test the hypothesis that phyloge netic constraints have played a role in the evolution of flightlessnes s. If speciation occurred after the evolutionary transition to flightl essness, inferences concerning the importance of particular aspects of the environment on the probability of the evolution of flightlessness may be statistically spurious because of the inflation of the sample size. Among birds, ratites and penguins illustrate the phenomenon of c onsiderable speciation subsequent to the transition to the evolution o f flightlessness. In contrast, the rails represent a group in which ea ch flightless species probably represents a separate evolutionary tran sition. There are many more flightless insect species than bird specie s and several orders are monomorphically flightless, the sometimes eno rmous speciation within the order following and possibly being a conse quence of the evolution of flightlessness. While it can be shown in in sects that flightlessness has evolved independently many times, there are at least as many cases in which the question cannot be resolved. T herefore, in both birds and insects phylogenetic effects should not be ignored, for the number of evolutionary transitions may be much less than the number of species. The effect of incorporating phylogenetic ( or at least taxonomic) constraints into the analysis of habitat factor s associated with flightlessness is considered.