Re: Haldane's dilemma

David Campbell (
Wed, 23 Apr 1997 11:16:53 -0500

Adrian responded to my speculation on Haldane's dilemna:
>> A population bottleneck or isolation of a small subpopulation could
>> allow new rare traits to become prevalent.
>This is the argument that Mayr used to countere Haldane. The problem, as
>far as I understand is that in small populations, beneficial mutations
>are exceedingly rare.
>> If those with the new were a
>> significant part of the small population, genetic drift could make the new
>> trait dominate among the small population's descendants.
>Through genetic drift, the odds are that rare beneficial mutations are
>likely to be eliminated. And if you were to take inbreeding into account
>the problem gets worse. You get rapid change, but the overall change is
>not beneficial.

>> Extreme selective
>> pressure, in the unlikely event of a significantly beneficial mutation,
>> could also act more rapidly.
>Unlikely is the word.
>> I suspect that calculation of the amount of genetic change
>> necessary for generating a given species may be dificult to asses, though
>> perhaps half of the difference between the most disparate modern species
>> would be a reasonable approximation (assuming each evolved an equal amount
>> away from the last common ancestor would minimize the maximum change to
>> explain). I don't know whether this problem would increase or decrease the
>> amount of time apparently needed relative to his calculation.
>I don't know, but here is the example given by Remine:
>Say a given prehuman ancestor species has a generation time of 20 years.
>Given a span of 10 million years (which is amply generous), that's
>enough time for 500 000 generations.
>Imagine a population of 100 000 of these, and a scenario that wildly
>speeds up evolution. Say, every generation, one male and one female
>receive a mutation so beneficial that all the other 999 998 of them die
>off in opne generation., and the population is replenished (back up to
>100 000) in one single generation. At that crashing rate of one
>beneficial mutation per generation for 500 000 generation, you get 500
>000 new nucleotides - approximately .014 % of the genome. Is that enough
>to get a human out of some chimp-like ancestor?
>Haldane calculated that it takes on average 300 generations to pay for
>the cost of one substitution. That leaves us 1667 beneficial mutations.
>What is wrong with this argument?
Although noticably beneficial mutations are likely to be rare, nearly
neutral mutations are much more frequent and could account for much of the
change. In particular, a minor change affecting courtship behavior could
be enough to isolate two subpopulations. Also, one mutation may easily
involve more than one nucleotide. Gene duplications (e.g., unequal
crossing over) and retroviruses can produce large changes in a single step.

David Campbell

"Old Seashells"
Department of Geology
CB 3315 Mitchell Hall
University of North Carolina at Chapel Hill
Chapel Hill NC 27599-3315

"He had discovered an unknown bivalve, forming a new genus"-E. A. Poe, The
Gold Bug