Detecting Differential Speciation Rate in Ipomoea by a Continuous-Time Markov Model

Preliminary study on Ipomoea phylogeny shows that species selection may influence flower-color evolution in this genus. Evolutionary transitions between pigmented flower and white flower appear to be highly asymmetrical. White-flowered lineages are less speciose than their pigmented sister clade and evolutionary transitions to white flowers occurred almost exclusively at the tip of phylogeny. A hypothesis to explain this pattern is that species selection acts against white-flowered lineages and prevents them from increasing in relative abundance whiten the genus Ipomoea. A continuous-time Markov model is developed to test differential speciation rates within genus Ipomoea. Simulation result shows that the observed distribution of white lineages is not compatible with equal transition rates between pigmented flower and white flower. Only highly asymmetrical transition rates that favor transitions to white flower are compatible with the observed distribution. This result implies that there is a tendency to increase the proportion of white-flowered species in the genus. The only process that could prevent this outcome is species selection or some similar higher-level evolutionary process that acts against white-flowered lineages. The absence of any deep white-flowered clades suggests that it might be white-flowered species have greatly elevated extinction rates compared to pigmented species.