STASIS IN LATE PLEISTOCENE BIRDS AND MAMMALS FROM LA BREA TAR PITS OVER THE LAST GLACIAL-INTERGLACIAL CYCLE

One of the great puzzles of evolutionary biology is how organisms remain static in the face of dramatic climatic changes, contradicting the “Galapagos finch” model of organisms that are constantly changing in response to their environment. Such stasis was documented in Pleistocene mammoths as early as 1863 by Darwin's friend, Hugh Falconer, and is widely recognized among most Pleistocene large mammals. We examined all the common birds and mammals from the Rancho La Brea tar pits in the Page Museum in Los Angeles. The studied taxa included the extinct horse, bison, camel, larger ground sloth, the sabertooth cat, and the Ice Age lion, as well as the extant golden eagle and the ancestral California condor. We measured large samples (usually more than 100 of each element) of several dimensions of the most common bones (typically leg or foot bones) from all the pits with good radiocarbon dates. Even though pollen, plants, snails, and isotopic studies provide evidence of dramatic climatic and vegetational change from the previous interglacial (40 ka-20 ka) to the peak glacial (20 ka-15 ka) to the glacial-interglacial transition (15 ka-10 ka) to the Holocene, none of these taxa show any statistically significant differences in size or shape of their bones from one level to the next. Such remarkable stasis among all the common mammals and birds over the late Pleistocene-Holocene, despite dramatic climatic changes, casts doubt on the responsiveness of birds and mammals to environmental variables, and suggests that intrinsic rather than extrinsic factors are more important in evolution.