Haldane\'s Dilemma: a new review (Evolution)

by David Turell , Thursday, July 30, 2015, 18:35 (3402 days ago) @ George Jelliss

George brought this up a long time ago, but a news review has been presented covering all the new theories to try to solve the problem, which roughly is: the time is too short, as accepted intervals in evolutionary history, for beneficial mutations to create speciation especially when looking at the human split from monkeys:-http://inference-review.com/article/haldanes-dilemma-This is a long mathematically intense article.-Haldane, one of the founders (along with Ronald Fisher and Sewall Wright) of mathematical population genetics, was the first to quantify such a limit on the speed of adaptive evolution.7 He concluded that the cost of selection “defines one of the factors, perhaps the main one, determining the speed of evolution.” Cost was the main reason Motoo Kimura proposed the neutral theory of molecular evolution. Many others cite its importance. -The implications for mammalian evolution were considered so severe that the issue became known as Haldane's dilemma.-Despite Haldane's work, a massive body of literature has accumulated asserting the primary role of natural selection in evolutionary change, often implying rates of adaptive evolution that exceed plausible limits. I maintain that cost, though often neglected in contemporary studies, remains as important as Haldane's mathematical theory of selection. By setting a limit on the number of selective evolutionary changes, Haldane provided a simple way to test the plausibility of many evolutionary scenarios.-***-It is easy to see why Haldane's conclusions posed a dilemma for biologists interested in mammalian evolution. Human and chimpanzee species diverged from a common ancestor approximately 4.5 to 13 million years ago. Humans currently have an average generation time of 30 years, chimpanzees 20 years. At most, 500,000 generations have elapsed. Given Haldane's limit, this makes for 3333.3 adaptive differences.-Can roughly 3000 changes explain all of the complex adaptive differences between humans and chimpanzees? -This is Haldane's dilemma.-It is a dilemma that has been exacerbated by genome sequencing. Humans and chimpanzees both have genome sizes of roughly 30 billion nucleotides. Yet these species differ by some 30 million fixed nucleotide differences.37 If these differences were fixed individually by positive selection, then the substitution rate would have been 1.5 substitutions per year in each line of descent, or 30 per generation—a biological impossibility.-ReMine's work: "Can this formulation of cost be used to calculate an achievable number of substitutions over the course of human evolution? If we assume a total fertility rate of five per female, R = 2.5 per individual.54 If the historic population size N = 10,000, there are then roughly t ? 10 generations per substitution, implying a maximum of 50,000 substitutions.55-The dilemma remains.-***-With respect to neutral substitutions, it follows that there is no real dilemma. They can easily account for the approximately 30 million nucleotide differences observed between humans and chimpanzees. But neutral mutations have no effect on fitness, and so by definition do not contribute to complex adaptations. If we consider the case of beneficial mutations, then positive natural selection appears limited to driving the substitution of fewer than 10,000 nucleotides.-We are faced with a different formulation of the same dilemma. Can so few differences account for the complex adaptive changes and innovations that shaped humans over the 10 million years since their divergence from the chimpanzees? -Either Haldane's limit on adaptive changes can be modified while still assuming positive natural selection, or natural selection must be supplemented by a different adaptive mechanism.-***-Genetic drift: "In The Origins of Genome Architecture, Michael Lynch has argued that random genetic drift is, by itself, quite sufficient to explain many of the features of complex genomes. Neutral mutations that fix over the course of evolution might provide novel genetic contexts that can be converted into beneficial alleles by subsequent mutations. As few as one in 1077 protein domain-sized polypeptides may be able to form functional folds. This suggests that the number of neutral mutations needed to result in a functional gene product is vast. Lynch and Adam Abegg have observed that a mutation involving three nucleotide sites and two neutral steps would take ~760 million years to be fixed.-But processes of this sort do not explain functional innovations in structural genes, let alone genes that might be unique to the human species.