EVOLUTIONARY FLEXIBILITY AND FLOWERING PLANT FAMILIAL DIVERSITY: A COMMENT ON DODD, SILVERTOWN, AND CHASE

Abstract

Ricklefs and Renner (1994) analyzed the relationship between number of species in families of flowering plants and four classification variables: geographical distribution, growth form, pollination mode, and dispersal mode. They found that families with more than one state for any of these four variables had more species than did families exhibiting a single ‘‘character’’ state, and they proposed that evolutionary flexibility with respect to the classification variables promoted diversification. Dodd et al. (1999) reanalyzed these data in a phylogenetic context using sister-taxon comparisons and independent contrasts. Dodd et al. conducted a further test of the ‘‘flexibility hypothesis’’ involving analyses of proportions of sister taxon pairs with neither, either, or both being variable with respect to growth form, pollination mode, and dispersal mode. Based on this test, they rejected the idea that character variability results in high net speciation rate. Here we comment on two issues raised by Dodd et al.’s analysis. The first is the relative performance of phylogenetic and nonphylogenetic analyses of comparative data. We cannot detect substantial differences in the results of these two approaches. The second concerns difficulties in assessing evolutionary flexibility. Tests that involve analyses of the distribution of character variation among pairs of sister taxa are complicated by taxon size. We also examine plant radiation in the Hawaiian Islands to determine whether the probability that a colonizing lineage proliferates is related to character trait variability in the family to which it belongs, and we find no effect. P HYLOGENETIC AND NONPHYLOGENETIC COMPARISONS