Path: utzoo!attcan!uunet!cs.utexas.edu!usc!samsung!uakari.primate.wisc.edu!dali.cs.montana.edu!milton!wcalvin From: wcalvin@milton.u.washington.edu (William Calvin) Newsgroups: bionet.population-bio Subject: Re: geneflow:asymmetry,equilibrium Message-ID: <9703@milton.u.washington.edu> Date: 22 Oct 90 17:37:42 GMT References: <9010210525.AA06820@genbank.bio.net> Organization: University of Washington, Seattle Lines: 60 Rapid fluctuations in mammalian populations are likely to occur from the abrupt climate changes that have been recently documented from ice-core records of North Atlantic climate. Its minor fluctuations a thousand years ago were responsible for why Iceland was not named Greenland and vice versa (by the time that the coast of Greenland was settled by explorers from Iceland, things had warmed somewhat). Then came the Little Ice Age which wiped out the non-Inuit settlements on Greenland. Yet neither such changes of the last millennium, nor the occasional multiyear drought, is what is meant by *abrupt* climate change. The most recent abrupt episode was the Younger Dryas: in the midst of the rising CO2 and the general warming trend that melted the ice sheets of the last glaciation, there was a "cold spike" that lasted about 800 years, extremely sharp in both onset and release. It caused European forests to die within a decade or two; they were replaced with Arctic-adapted plants such as Dryas. It caused Scotland's glaciers to form once again. Because this happened when northern hemisphere summer sunshine was near its astronomical maximum, it served to alert climatologists that there was more to ice advance than just the familiar Milankovitch cycles. A good time for rapidly melting all that ice in the northern hemisphere is, just as Milankovitch predicted, when the earth's axial tilt is maximal and the earth's closest approach to the sun occurs in June -- but there appears to be something else going on that can occasionally override this general pacemaker of the ice cycles (probably a mode-switching change in ocean currents associated with a big "sink" near Iceland). Apropos speed, the Younger Dryas cooling started 11,500 years ago; it lasted until 10,700 years ago, when it ended even more suddenly than it began. Thanks to the year-by-year detail in the ice cores of Greenland studied by Dansgaard et al. (Nature, June '89, is a recent version), we know that rainfall returned over a 20 year period and, as Europe's land surface warmed up, the formerly severe winter storms diminished dramatically in that same two-decade-long period. Cooling episodes are just as rapid (though often with associated hot-and-cold "whiplash" chattering). Once triggered, mode- switching climatic "leaps" evidently operate on a far faster time scale than 20,000-to-100,000 year Milankovitch cycles, faster even than the century-long time scale of the predicted greenhouse warming. So just within a generation or so (for the larger mammals), there are enormous stresses on populations -- both crashes and booms. Add to this the 30-year drought cycle in the Sahara (that seems to affect the western side of the Atlantic as well) and you have a lot of natural selection operating on boom-and-bust, such as abilities to r-shift a little from K extremes (increases in twinning, etc.). This has been of particular interest to me because of how it might have affected hominid evolution (if you are interested, my book THE ASCENT OF MIND will be published next month in the US by Bantam) and how it might affect present-day human populations in Europe. The 500 million people in Europe who depend on that bonus of winter heat from the North Atlantic Current (perhaps 700 million: the Younger Dryas climate changes reached at least as far east as the Ukraine) have a considerable interest in preventing such unpleasant surprises as were experienced by the hunters and gatherers living in Europe 11,500 years ago. The thousand-fold population increase since then causes Europe to be particularly vulnerable to climatic shocks that arrive with little warning; the two-decade-long excursion of the proxy climate indicators should be interpreted to mean that significant changes could occur in several years. Essentially, a drought would start, get worse -- and then it would be too late for stockpiling. William H. Calvin University of Washington -- Biology NJ-15 Seattle WA 98195 wcalvin@u.washington.edu