Phyllodes and bipinnate leaves of Acacia exhibit contemporary continental-scale environmental correlation and evolutionary transition-rate heterogeneity

Abstract. In Acacia, 90% of species have drought-tolerant phyllodes as their adult foliage, the remaining species have bipinnate leaves. We conducted tests for relationships between foliage type and 35 bioclimatic variables at the continental scale and found significant correlations of both ‘moisture seasonality’ and ‘radiation in the coldest quarter’ with foliage type. Bipinnate species have lower species mean values of each variable, growing in stable soil moisture and generally darker environments (longer nights and lower incident radiation), on average. Evolutionary transformations between bipinnate and phyllodinous adult foliage exhibit asymmetry across the Acacia phylogeny, with transformations from bipinnate leaves to phyllodes occurring times faster than the reverse. At least three (and up to seven) transitions from phyllode to bipinnate adult foliage were inferred. Foliage type in the most recent common ancestor of extant Acacia is unresolved, some analyses favour a phyllodinous ancestor, others a bipinnate ancestor. Most ancestral nodes inferred as having bipinnate adult foliage had median age estimates of less than 5 million years (Ma), half having ages between 3 and 1.5 Ma. Acacia lineages with bipinnate adult foliage diversified during the Pliocene, perhaps in response to wetter climatic conditions experienced by the continental margin during this period.