Polyploidy and diversification: A phylogenetic investigation in Rosaceae

Polyploidy has been described both as an evolutionary dead end and as a major engine of diversification for angiosperms. Two recent studies have found that genera with higher proportions of polyploid species are more species rich. Here, we investigate patterns of diversification and polyploidy by performing traditional and phylogenetically corrected analyses within the Rosaceae. We find that polyploidy is associated with increased species richness and then differentiate between three alternative hypotheses for this pattern: (1) that polyploidy is associated with herbaceous growth habit, a trait that is in turn associated with increased species richness; (2) that polyploid clades are more evolutionarily successful (i.e., experience increased speciation and/or fewer extinction events) than diploid clades, perhaps because of the increase in genomic content or the increase in plant/flower size that often accompanies polyploidization; and (3) that the polyploidization events themselves, along with the reproductive isolation from the parental clade(s) that follows polyploidization, are responsible for the increased species richness observed in clades with a high proportion of polyploids. There is no evidence that polyploidy and herbaceous growth habit were correlated or that polyploid clades are more species rich than their diploid sister groups. We posit that the third hypothesis has the greatest potential for explaining the pattern of higher species richness of polyploid clades.