RATES OF SPECIATION IN THE FOSSIL RECORD

Data from palaeontology and biodiversity suggest that the global biota should produce an average of three new species per year. However, the fossil record shows large variation around this mean. Rates of origin ation have declined through the Phanerozoic. This appears to have been largely a function of sorting among higher taxa (especially classes), which exhibit characteristic rates of speciation (and extinction) tha t differ among them by nearly an order of magnitude. Secular decline o f origination rates is hardly constant, however; many positive deviati ons reflect accelerated speciation during rebounds from mass extinctio ns. There has also been general decline in rates of speciation within major taxa through their histories, although rates have tended to rema in higher among members in tropical regions. Finally, pulses of specia tion appear sometimes to be associated with climate change, although m oderate oscillations of climate do not necessarily promote speciation despite forcing changes in species' geographical ranges.