Why Evolution Is True

by Greg Mayer (Updates below.)

A new study just published in Science by Maureen O’Leary and colleagues examines the phylogeny of 40 fossil and 46 extant mammals based on a very large data set of morphological and molecular characters (the latter only from the living taxa). The study has gotten a fair amount of attention in the press, where it seems to have been misinterpreted; more on that later. First, let’s see what they were trying to do and what they found.

There are three major groups of mammals alive today: the egg-laying monotremes (the platypus and the echidnas), the marsupials (opossums, kangaroos, bandicoots, etc.: a few hundred species) and the placentals (cats, dogs, cattle, deer, and all the rest, including primates: several thousand species in about 18 orders in all). The placentals are the overwhelming majority of extant mammals, and dominate the mammalian fauna of all parts…

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