Annuality and C4 photosynthesis co-occur but evolved independently in warm, dry environments

Warm, dry environments create living conditions that challenge plant growth, reproduction and survival. Plants in these environments have evolved adaptive strategies to enhance water-use efficiency and ensure reproductive success, two of which are annuality and C4 photosynthesis. However, life history variation is rarely included in large-scale studies of plant diversity, and the extent to which these traits coevolve and are jointly selected for is not known. To address this, we used Pagel's models of independent and correlated evolution for over 4000 species of grasses (Poaceae), while accounting for evolutionary rate heterogeneity and potential type I statistical errors. We found that there are more C4 than C3 annuals and that C4 origins predate evolution of annuality, but no support for correlated evolution between the two traits. Our results indicate that any habitat or trait similarities (e.g. small seeds, fast growth) between annuals and C4 species reflect independent adaptations to similar environmental conditions or are contingent on the two traits themselves, rather than the result of evolutionary or functional links between them. Our results further highlight the importance of appropriate null model specification for testing evolutionary hypotheses across large, old clades.