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June 08, 2003
THE SHIFTING BALANCE
I mentioned in a previous post that Sewall Wright’s ‘Shifting Balance’ theory of evolution had recently been criticised. It is only fair to add that it has also been defended. Key references are:
The original critique
No doubt this will rumble on.
To refresh memories about the Shifting Balance theory, Sewall Wright believed that evolution in large, randomly-mating populations would be slow and ineffective. The population would quickly evolve to an optimum equilibrium state (a peak in the ‘adaptive landscape’), but evolution would then stop, because natural selection could not cross the ‘valleys’ to other peaks. Since evolution clearly does take place, Wright proposed that the geographical substructure of populations was important. In small isolated sub-populations inbreeding and genetic drift could enable the sub-population to cross ‘valleys’ to new peaks of fitness despite adverse natural selection on the way. Successful new combinations of genes could then spread by migration or differential extinction of groups. The theory has been widely influential in American biology, notably on Ernst Mayr’s view of speciation, the Eldredge-Gould theory of punctuated equilibrium, and D.S. Wilson and Michael Wade’s theories of group selection.
On the other hand the Shifting Balance was never very popular among evolutionists in Britain. Haldane was sceptical, R. A. Fisher was hostile, and E. B. Ford reported empirical evidence against it. Fisher in particular denied that large populations would get stuck on peaks in the adaptive landscape, since in a large population there would always be numerous mutations arising to upset the equilibrium, and in any case environmental conditions (biotic and non-biotic) were constantly changing. The idea of a static ‘adaptive landscape’ was therefore misleading. Fisher also doubted whether sub-populations would remain isolated for long enough to diverge purely by genetic drift. The recent debate is largely about whether Fisher or Wright was closer to the truth.
I won’t (and can’t) answer that question, but I think the opposition between Fisher and Wright is sometimes misrepresented. Wright is represented as believing that species are divided into partly isolated sub-populations, while Fisher is represented as believing that an entire species is a single large randomly-mating population. If the disagreement is put in these terms, clearly Wright is right. But of course Fisher was well aware that species have a geographical structure, and that sub-populations are partially isolated from each other. Where he dissented from Wright was in doubting that the isolation was usually strict enough, or lasting enough, to be as important as Wright claimed. His objection is expressed in the second edition of his Genetical Theory of Natural Selection:
“The circumstance that smaller numbers, even less than 100, are sometimes found to reproduce themselves locally, does not, as has been supposed, add to the frequency of random extinction [of genes], or to the importance of the so-called ‘genetic drift’. For this, perfect isolation is required over a number of generations equally numerous with the population isolated. Even if perfect isolation could be postulated, which is always questionable, it is still improbable that the small isolated population would not ordinarily die out altogether before a period of evolutionary significance could elapse, or that it would not later be absorbed in other populations with a different genetic constitution” (Dover edition, p.10).
The underlying problem with the Shifting Balance is that populations will not diverge by genetic drift if there is, on average, more than one migrant between them per generation, regardless of the size of the populations. (The last point may be counter-intuitive, but is well-established. The effectiveness of genetic drift is inversely proportional to the size of the population, so that with larger populations a proportionately lower migration rate is needed to prevent them drifting apart. Of course, none of this applies to asexual species, mitochondria, or Y chromosomes.) Fisher considered this degree of isolation unlikely in the absence of major geographical barriers. Oddly, neither Fisher nor Wright seems to have mentioned in this context that most species have behavioural and/or morphological adaptations for dispersal at some stage in their life-cycle. Among humans, exogamous marriage rules, or capture of females from other tribes, would have the same effect.