PHYLOGENIES AND THE ANALYSIS OF EVOLUTIONARY SEQUENCES,WITH EXAMPLES FROM SEED PLANTS

Studies of character evolution have frequently relied on ahistorical correlations rather than on phylogenies. However, correlations do not estimate the number of times that a trait evolved, and they are insensitive to the direction or the temporal sequence of character transformation. In contrast, cladograms can provide this information. A cladistic test of the hypothesis that the evolution of dioecy is favored in animal-dispersed plants indicates that dioecy may have originated somewhat more often in such lineages. Nevertheless, differences in rates of speciation or extinction must largely account for the observed species-level correlation between dispersal and breeding system. In considering the evolution of individual traits, cladograms help identify the context in which a feature evolved and specify which organisms should be compared in evaluating the causes of character change. Determining whether a feature and a performance advantage were strictly historically correlated or followed one another in sequence helps to distinguish whether the trait is an adaptation or an exaptation for the function. For example, cladograms of seed plants suggest that double fertilization arose incidentally prior to the origin of angiosperms and that the resulting product was later co-opted and elaborated as a nutritive tissue for the developing embryo. The order of character assembly in a lineage also bears on the evolution of functional and developmental interdependencies. In particular, it may be possible to trace the evolution of a character's “burden” from an initial period, during which change is more likely, through later stages, wherein successful modification is less likely owing to the evolution of dependent characters. The evolution of vessels and of floral phyllotaxis in angiosperms may exemplify this pattern. Recognition that the likelihood of character transformation may change during the evolution of a group warns against character weighting in phylogenetic analysis.