I believe that macroevolution is generally taken to mean evolution above the species level (that leads to the production of higher taxonomic groups) and microevolution to that occurring at the level of species or populations. The question is, do the same processes account for both macro- and microevolution? The answer to this question probably depends on what one calls a significant difference. If macroevolutionary events depend upon the rather sudden introduction of radically new and divergent characters and a subsequent period of accelerated evolutionary change which leads to a major new taxonomc category, is this process different or is it just an extreme example of ordinary microevolutionary events? Macroevolutionary events must, surely, originate within populations and, therefore, they originate at the microevolutionary level. So we could ask what sort of populations would most likely encourage the production of "novel" types and the more or less rapid evolution of new morhphological, ecological, physiological and/or behavioral life styles which ultimately go on to become new higher taxonomic groups. Where would such "evolutionary revolutions" occur? Here, I would have to disagree with Gary. I think that the most likely scenario is that such evolutionary events would most often occur within small, isolated and usually peripheral populations. Such allopatric speciation provides a mechanism by which a population can rapidly diverge from and become reproductively incompatible with its parent group. The reasons for this are extensively discussed by Ernst Mayr in his various books. Before isolation, such populations may exist at the environmental extremes of a species' range and may thus also represent an unusual genetic subset within the parent species. The selective pressures experienced by the recently isolated population may also be quite different than those experienced by the parent group and, especially if the isolated population has become established in some sort of "virgin territory," permit the survival of mutations which could be quickly eliminated or "swamped" in the parent species. Favorable mutations will also spread more rapidly within a small population than a large one and so the rate of genetic change will be accelerated within the small isolated group. While it is possible that sympatric speciation may sometimes occur, it does not seem likely to me that it is the major source of speciation events simply because it requires the sudden production of individuals unable, or at least unlikely, to interbreed with their fellows. This would, of course, be a significant selection pressure against such mutants who would be forced to find similar mutants as mates within a population overwhelmingly dominated by nonmutant individuals. Polyploidy in plants may perhaps be a good example of a mode of sympatric speciation, but it is less likely to be a source of sympatric speciation in animal groups. Plants have the advantage, in many cases, of self fertilization or of vegetative reproduction giving rise to individuals which can then later interbreed. Isolated polyploid mutants may thus initially persist without finding a mate. Some animal groups have similar reproductive possibilities but most do not. kate