Mammalian extinctions seem to be driven more by a failure to keep up with evolutionary pace than by random swings in diversity.

Tiago Quental at the University of Sao Paulo in Brazil and Charles Marshall at the University of California, Berkeley, analysed 19 mammalian clades — groups of species descended from a common ancestor — that had well-preserved fossil records and had either become extinct or declined in diversity within the past 66 million years.

Diversity loss was due to new lineages arising at lower rates and extinctions occurring at higher rates. The authors say clades' decline can be explained by the Red Queen hypothesis: that species must continue to evolve to keep pace with a deteriorating environment.

Science http://dx.doi.org/10.1126/science.1239431 (2013)