Re: Haldane's dilemma

Adrian Teo (AdrianTeo@mailhost.net)
Fri, 25 Apr 1997 14:54:24 -0700

David Campbell wrote:
>
> 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.

I am a little confused. It is my understanding that neutral mutations
are just that, neutral. How then can they be said to effect sustained
change and still be called neutral?

> 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.

Aren't these events also exceedingly rare? Are they enough to solve the
problem?

-- 
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Adrian Teo
Institute of Child Development
University of Minnesota
E-mail: AdrianTeo@mailhost.net
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