Nunney: The Cost of Natural Selection Revisited (Haldane Dilemma)

Never too lazy to do some research when ID creationists seem to be lost for arguments I ran across a paper by Nunney in which he shows how Haldane was wrong. All ReMine supposedly was able to do was to object to the fact that Nunney refused to share the software code with ReMine and that ReMine was unable to write the necessary code himself.

Haldane’s dillema hardly deserves the attention it is receiving from ID creationists but then again, there is not much else for them to focus on.

Nunney’s results differ markedly from those of Haldane and ReMine. Considering Nunney’s reticence to have his results critiqued and the divergence from other results, his results appear to lack credibility.

Enough whine, let’s look at the cheese… (see here for an earlier discussion on the non issue of Haldane’s dilemma by Ian Musgrave).

The paper in question: Nunney, Leonard, The cost of natural selection revisited, Ann. Zool. Fennici. 40:185-194, 2003

In a constantly changing environment, organisms must continuously adapt or face extinction. J. B. S. Haldane argued that the “cost of natural selection” (also called the cost of substitution) puts an upper limit on the rate of adaptation, and showed that the cost (C) was a decreasing function of the initial frequency of the beneficial alleles. Based on mutation-selection balance and 10% selective mortality, he suggested that the limit to adaptive evolution was about one allelic substitution per 300 generations. I have tested Haldane’s results using simulations of a population limited by density-dependent regulation and subject to a constantly changing environment that affects n (= 1–7) independent survival traits, each controlled by a single locus. I investigated the influence of carrying capacity (K), mutation rate (u), number of beneficial mutations per generation (approximated by M = 2Ku) and net reproductive rate (R). Of these, M has the predominant influence. The effect of large changes in R was relatively small. The cost of selection (C) was measured as the shortest number of generations between an allelic substitution at all loci under selection that was consistent with population persistence. The results differed from Haldane’s solution. Across a range of conditions, the cost of simultaneous selection at n loci was determined by the linear relationship C = C0(M) + nC1(M), where C0(M) is the intercept and C1(M) is the slope of the linear regression of C on n, for a given M. The intercept defined a positive fixed cost of substitution, that appears to reflect genetic deaths occurring during the stochastic phase when the beneficial alleles are rare. For M > 1/2, the cost of natural selection is substantially less than Haldane’s estimate; however, when M < 1/2, the cost (and particularly the fixed cost) increases in an accelerating fashion as M is lowered. This result has important implications for conserved populations, since for u ~ 5 x 10–6 the carrying capacity of the population must be 50000 for M = 1/2. To avoid low M, smaller populations should be linked together into a large metapopulation whenever possible. This large unit would be capable of adapting when the isolated parts could not. It also suggests that if M << 1, small gains in K through increases in habitat can have a very large positive influence on the future survival of the population in a changing environment.

In the conclusions Nunney mentions that

Haldane (1957) ended his paper by noting “I am quite aware that my conclusions will probably need drastic revision. But I am convinced that the quantitative arguments of the kind here put forward should play a part in all future discussions of evolution” (p. 523). The results presented here suggest that Haldane was correct that some revision of his conclusions is needed, but they also suggest that he was correct that the cost of natural selection is a real phenomenon that needs to be included in more discussions of adaptations in a changing environment. The concept is made even more relevant as we become increasingly aware of the potential for rapid environmental change, and as more and more natural populations become fragmented in many isolated units.

Perhaps it is time for ID creationists to become more familiar with the facts and depend less on hear-say? According to Scordova, Walter’s research is funded by the Discovery Institute, showing once again how ID fails to provide much of any relevant scientific research. As for Walter’s ‘response’ enjoy…