Diversification rates in the Australasian endemic grass Austrostipa: 15 million years of constant evolution

Patterns seen in other Australian flora have led to hypotheses that early Miocene shifts in climate drove rapid radiation of major taxonomic groups such as Eucalyptus. Little is known about absolute dates and rates for Australian monocots, particularly grasses. I tested this early Miocene radiation hypothesis for Australian grasses using a calibrated phylogeny of the endemic stipoid genus Austrostipa and an analysis of diversification rates. The phylogeny was developed from a Bayesian likelihood analysis of the nuclear internal transcribed spacers region, and three calibration points were set based on fossil evidence. The results indicate that the genus arose in the early Miocene and underwent a species radiation, but the rate of diversification was not rapid compared to the current rate or to those of other taxa. Following an 8 million year period of fast molecular evolution but no taxonomic radiation, diversification rates have been constant for the past 15 million years. Comparable measures such as the gamma statistic can be used across taxa to make general conclusions about evolutionary rate constancy.