SOME GENETIC CONSEQUENCES OF ICE AGES, AND THEIR ROLE IN DIVERGENCE AND SPECIATION

The genetic effects of pleistocene ice ages are approached by deductio n from paleoenvironmental information, by induction from the genetic s tructure of populations and species, and by their combination to infer likely consequences. (1) Recent palaeoclimatic information indicate r apid global reversals and changes in ranges of species which would inv olve elimination with spreading from the edge. Leading edge colonizati on during a rapid expansion would be leptokurtic and lead to homozygos ity and spatial assortment of genomes. In Europe and North America, ic e age contractions were into southern refugia, which would promote gen ome reorganization. (2) The present day genetic structure of species s hows frequent geographic subdivision, with parapatric genomes, hybrid zones and suture zones. A survey of recent DNA phylogeographic informa tion supports and extends earlier work. (3) The grasshopper Chorthippu s parallelus is used to illustrate such data and processes. Its range in Europe is divided on DNA sequences into five parapatric races, with southern genomes showing greater haplotype diversity - probably due t o southern mountain blocks acting as refugia and northern expansion re ducing diversity. (4) Comparison with other recent studies shows a con cordance of such phylogeographic data over pleistocene time scales. (5 ) The role that ice age range changes may have played in changing adap tations is explored, including the limits of range, rapid change in ne w invasions and refugial differentiation in a variety of organisms. (6 ) The effects of these events in causing divergence and speciation are explored using Chorthippus as a paradigm. Repeated contraction and ex pansion would accumulate genome differences and adaptations, protected from mixing by hybrid zones, and such a composite mode of speciation could apply to many organisms. (C) 1996 The Linnean Society of London