STASIS IN NEOPHRONTOPS AMERICANUS (EGYPTIAN VULTURES) FROM LA BREA TAR PITS DURING THE LAST GLACIAL-INTERGLACIAL CYCLE

According to conventional evolutionary theory, small changes in organisms from year to year, such as in the beaks of the Galápagos finches, are the mechanism by which birds evolve in response to environmental changes. Yet, all published studies of the fossil birds at Rancho La Brea have found no evidence of evolutionary responses to the glacial-interglacial cycle climate changes over the last 35,000 years. None of the birds exhibit any change in body size or limb proportions, even during the last glacial maximum about 18,000-20,000 years ago, when snowy coniferous forests replaced the southern California chaparral. To further assess this conclusion, we measured the leg bones of Neophrontops americanus, an extinct relative of the Egyptian vulture. We found complete stasis from 37,000 years ago until the end of the Pleistocene in size and limb robustness. This is consistent with the idea that small-scale changes, as seen in Galápagos finches, are random fluctuations around a mean and do not lead to speciation. Instead, the data show that speciation and lasting morphological change rarely occur over short time scales and are only visible in long-term records of hundreds of thousands to millions of years.