Quotation from Darwin: Haldane\'s Dilemma (Evolution)

by George Jelliss , Crewe, Thursday, July 03, 2008, 11:41 (5769 days ago) @ David Turell

This is not a subject on which I am any sort of expert, so I have not attempted to respond. However the subject of &quot;Haldane&apos;s Dilemma&quot; has come up on the Dawkins forum. The following is a copy of a response by one calling himself &quot;Calilasseia&quot; which gives up to date details: - === - Well first of all, Haldane&apos;s Dilemma has been roundly dealt with in the 49 years since Haldane first published his paper, not to mention the fact that Haldane himself stated that his conclusions would probably require drastic revision in the light of new data. He was, after all, operating with what was the state of the art of knowledge in 1957, and nowadays, we have acquired a lot more knowledge. - First of all, the assumptions upon which Haldane based his calculations include: - [1] That a gene, or collection of genes, is rare at the start of an evolutionary sequence, because they are initially detrimental; - [2] That a sudden environmental change now favours those genes; - [3] As a result, the large number of organisms possessing the previously favourable genes will all die. - First of all, the scenario was erected as an entirely hypothetical one. Second, assumption [3] is extremely suspect, given what we know from real world observation of actual living organisms. - However, more to the point, the idea that science has ignored the question or brushed it aside is errant nonsense. Take, for example, this interesting paper: - The Cost of Natural Selection Revisited by Leonard Nunney, Ann. Zool. Fennici, Vol 40, 185-194, 30 April 2003 - Let&apos;s take a look at the abstract: - Nunney, 2003 wrote:&#13;&#10;In a constantly changing environment, organisms must continuously adapt or face extinction. J.B.S. Haldane argued that the &quot;cost of natural selection&quot; (also called the cost of a substitution) puts an upper limit on the rate of adaptation, and showed that the cost (C) was a decreasing function of of the initial frequency of the beneficial alleles. Based upon a mutation-selection balance and 10% selective mortality, he suggested that the limits to adaptive evolution was about one allelic substitution per 300 generations. I have tested Haldane&apos;s results using simulations of 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 ws consistent with population persistence. The results differed from Haldane&apos;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>&#194;&#189;, the cost of natural selection is substantially less than Haldane&apos;s estimate; however, for M<&#194;&#189;, 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 approximately equal to 5 &#195;&#151; 10-6 the carrying capacity of the population must be 50,000 for M=&#194;&#189;. To avoid low M, smaller populations should be linked together into a larger metapopulation whenever possible. This large unit would be capable of adapting when the isolated parts could not. It also suggests that if M is much less than 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. - Interested readers can read the full paper by downloading it from here. - http://www.sekj.org/PDF/anz40-free/anz40-185.pdf - So in other words, once again, the peer reviewed scientific literature comes up trumps. Haldane was right to be suspicious about his own work and right to suggest that it was in need of revision. What a pity he didn&apos;t have access to powerful computers to test his ideas upon.