Patterns of Evolution in late Pleistocene Mammals and Birds from La Brea Tar Pits, California

Conventional neo-Darwinian theory views organisms as responsive to their environments on neontological timescales, and able to readily change size or shape due to selection pressures (as exemplified by the famous case of Galápagos finches). But since 1863, it has been well established that Pleistocene animals and plants show limited morphologic change in response to the glacial-interglacial cycles. Rancho La Brea tar pits in Los Angeles, California, preserves a large and diverse fauna from many well-dated deposits, spanning 37,000 years. Pollen evidence shows that climate changed from oak-chaparral about 37 ka to snowy piñon-juniper-ponderosa pine forests during the peak glacial about 18 ka, then returned to the modern chaparral since the glacial-interglacial transition. We have studied all the sufficiently abundant mammals (dire wolves, saber-toothed cats, giant lions, Harlan’s ground sloths, camels, bison, and horses) and all the common birds (28 species, ranging from eagles, hawks, vultures, condors, owls, to yellow-billed magpies, ravens, and Western meadowlarks). We found complete stasis in size and robustness as measured by the major limb bones in all 28 species. There was no significant response even at 20 ka to 18 ka, during the peak glacial period, when climate and vegetation were very different at La Brea. The larger species might be so wide-ranging and versatile that they would not respond to environmental changes, but this is inadequate to explain stasis in even the smallest birds, such as meadowlarks and burrowing owls. While the Galápagos finch model suggests rapid morphological change in response to environmental change, the fossil record shows that such small-scale changes are transient and do not accumulate to result in speciation.