Haldane's dilemma

thumb|right|220px|J. B. S. Haldane in 1964

Haldane's dilemma is a limit on the speed of beneficial evolution, calculated by J. B. S. Haldane in 1957. Motoo Kimura's landmark paper on neutral theory in 1968 built on Haldane's work to suggest that most molecular evolution across species is neutral, framing this is as a resolution to the dilemma. However, adaptive evolution might explain a significant fraction of substitutions in protein coding sequence,. Other solutions have since been proposed.

Substitution cost

In the introduction to The Cost of Natural Selection Haldane writes that it is difficult for breeders to simultaneously select all the desired qualities, partly because the required genes may not be found together in the stock; but, he writes,

<blockquote>especially in slowly breeding animals such as cattle, one cannot cull even half the females, even though only one in a hundred of them combines the various qualities desired.

<blockquote>Haldane (1957 [= The Cost of Natural Selection]) drew attention to the fact that in the process of the evolutionary substitution of one allele for another, at any intensity of selection and no matter how slight the importance of the locus, a substantial number of individuals would usually be lost because they did not already possess the new allele. Kimura (1960, 1961) has referred to this loss as the substitutional (or evolutional) load, but because it necessarily involves either a completely new mutation or (more usually) previous change in the environment or the genome, I like to think of it as a dilemma for the population: for most organisms, rapid turnover in a few genes precludes rapid turnover in the others. A corollary of this is that, if an environmental change occurs that necessitates the rather rapid replacement of several genes if a population is to survive, the population becomes extinct. showed that a threshold model of selection, where individuals with a phenotype less than the threshold die and individuals with a phenotype above the threshold are all equally fit, allows for a greater substitution rate than Haldane's model (though no obvious upper limit was found, though tentative paths to calculate one were examined e.g. the death rate). John Maynard Smith and Peter O'Donald followed on the same track.

Additionally, the effects of density-dependent processes, epistasis, and soft selective sweeps on the maximum rate of substitution have been examined.

By looking at the polymorphisms within species and divergence between species an estimate can be obtained for the fraction of substitutions that occur due to selection. This parameter is generally called alpha (hence DFE-alpha), and appears to be large in some species, although almost all approaches suggest that the human-chimp divergence was primarily neutral. However, if divergence between Drosophila species was as adaptive as the alpha parameter suggests, then it would exceed Haldane's limit.

Haldane's dilemma

Substitution cost

See also

References

Further reading