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.
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At the boundaries it's pretty obvious that reality is either "Wrightian" or "Fisherian."
Why "either" instead of "both"?
PS: Have you noticed that Tom Wolfe repeats some of the misconceptions you've previously pointed out, but this time in your mothership SEED? I found that rather annoying since I paid for the damn thing. If I wanted antiadaptationist pontification about human evolution, I could go read Sandwalk for free...
Why "either" instead of "both"?
just came out wrong. but yeah. and yeah...i saw that.
but this time in your mothership SEED?
and just to be clear, the relationship between SEED and any scienceblog is very much in the class of capitalist transactions between consenting adults. they're not responsible for me, and i'm not responsible for them, and if either gets caught doing a no-no the other is expected to disavow.
That is, over long periods of time, or across enormous expanses, and even surveying multiple taxa, I think Fisher's emphases might be warranted.
I'm not sure I understand that comment so let me put the facts in my own words and see if you agree.
When we compare the sequences of genes and proteins that have diverged for hundreds of millions of years we see that most of the differences are neutral or nearly neutral. Thus, most of the evolution that has occurred has been due to fixation of alleles by random genetic drift. Thus, over long periods of time random genetic drift is far more important in evolution than natural selection. This is why there's an approximate molecular clock, which you wouldn't see if most evolution at the molecular level was due to natural selection.
Well?
they're not responsible for me, and i'm not responsible for them, and if either gets caught doing a no-no the other is expected to disavow.
I know, I didn't imply any responsibility. Only that it seemed out of place...
larry, re: sequence level stuff they were both wrong. but i'm thinking more the adaptive landscape.
Larry: See Note 1 to my full post on gnxp classic. Rightly or wrongly, Wright and Fisher were not concerned with selectively neutral variants at the molecular level but only with genes that have some phenotypic effect.
razib says,
larry, re: sequence level stuff they were both wrong. but i'm thinking more the adaptive landscape.
Okay, I can accept that. Perhaps you should have clarified that in your posting by saying something like, "This is an old argument that has been made irrelevant by new data. It turns out that neither Wright or Fisher were correct."
"This is an old argument that has been made irrelevant by new data. It turns out that neither Wright or Fisher were correct."
no, that's not right either. sequence level variation is not the summum bonum.
The particular point affected by migration is the importance of genetic drift in differentiating populations at subspecfic level. I don't think the 'molecular clock' hypothesis resolves this.
As to whether most 'evolution' is neutral, it depends what you mean by 'evolution'. If you define it as phenotypic (whole organism) evolution, then probably most of it is not neutral. At a molecular level most of it probably is, though there are still arguments about how much.