David continues his series on the thinking of the great evolutionary geneticist Sewall Wright. Today's post, Notes on Sewall Wright: Migration. First, the general:
Continuing my series of notes on Sewall Wright's population genetics, I come to the subject of migration. This is important in understanding the differences between Wright and R. A. Fisher on the role of genetic drift in evolution. Fisher and Wright both agreed that genetic drift would be too weak a process to be of evolutionary significance in large populations (above, say, 10,000 in effective size)...Equally, they agreed that it would be important in small populations, provided these remained sufficiently isolated over sufficiently long periods of time. Their disagreement was over the probability that the necessary degree of isolation would occur. This depends largely on the rate of migration between populations.
Then the specific:
...But it remains possible that 'Wrightian' processes are important in some cases. A particularly interesting case is the modern human species itself. After the dispersal of modern humans out of Africa, it is likely that human populations for most of the last 100,000 years were small and scattered, with little migration between different continental groups. These are good conditions for Wrightian genetic drift. Whether the observed differences in gene frequencies between continental populations are due to drift or selection remains an active area of research....
At the boundaries it's pretty obvious that reality is either "Wrightian" or "Fisherian." That is, over long periods of time, or across enormous expanses, and even surveying multiple taxa, I think Fisher's emphases might be warranted. But as you shrink the time interval that you're evaluating, or are focused on small locale, or the evolutionary history of one taxon, you probably would observe more the dynamics which Wright would put the spotlight on. But obviously the crux is where you draw the line on the intervals in question. The only caution I would offer is that David does not mention epistasis, gene-gene interaction effects, and Wright's thinking in regard to population substructure was strongly influenced by this concept. If you want to follow the somewhat tortured and confusing trajectory of Wright's various thoughts about the connections between evolutionary process, epistasis and genetic drift, I recommend Sewall Wright and Evolutionary Biology by Will Provine. According to Provine even Wright's most prominent evangelists such as Ernst Mayr and Theodosius Dobzhansky muddled many aspects of his thinking on this issue. Update: I realized it is highly probable that David will take up Wright's views re: epistasis in detail in a subsequent post, explaining its notable omission here. Related:Notes on Sewall Wright: Population Size, Notes on Sewall Wright: the Measurement of Kinship, Notes on Sewall Wright: Path Analysis and On Reading Wright.