THE GENETIC CONSEQUENCES OF THE ICE AGES

The realization of the scale and frequency of climatic oscillations in the last few million years is modifying views on evolution. Such major events caused extinction and repeated changes in the ranges of surving taxa, as witnessed in several paleo-records.

Spatial effects depend on latitude and topography, with extensive extinction and recolonization in higher latitudes and altitudinal shifts and complex refugia nearer the tropics. The population dynamics varied, with refugial bottlenecks, pioneer and phalanx colonization, and longer term metapopulation pulsation, depending on the organism and its geography.

These events can be expected to have effected the genetic constitution of the populations and species, which will carry signs of these past dynamics. Genetic variation may be assessed through allelic diversity, sequence divergence, phylogenetic trees and network shapes, and then placed in a geographic and historical context.

Such phylogeographic studies have burgeoned in the last few years, particularly in temperate regions. Present results will be reviewed from Arctic, Temperate and Tropical regions in the search for commonalities of cause in the resulting genetic patterns. A common pattern of southern richness and northern purity in north-temperate taxa is not apparent in arctic species, and is apparently more limited in tropical ones. Different patterns of species colonization across the same area may be deduced from the DNA evidence. Temperate refugial regions show relatively deep DNA divergence for many taxa, indicating their presence over several ice ages, and suggesting modes of speciation. A diverged genome found in several places, indicates a different colonization pattern in previous oscillations. Tropical montane regions contain deeply diverged lineages often in a relatively small geographic area, suggesting their survival there from the Pliocene. Man’s own glacial and postglacial history is similarly interpretable from such genetic studies.

The pattern of genetic diversity produced by the ice ages can inform us of the effects of these climatic changes on any species, and particularly when combined with extensive fossil and paleoclimatic data.