Re: Haldane's dilemma
Mon, 28 Apr 1997 11:42:15 -0700

David Campbell wrote:
> Adrian replied:
> [most previous material sniped]
> >> >
> >> >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?
> If the difference between species is measured by comparing DNA, then
> neutral mutations could account for much molecular change, even though the
> morphology showed no change. What was the exact difference Haldane thought
> was too big? (i.e., genetic distance, gross morphology,...)?

As far as I know, error catastrophe occurs when the mutation rate gets
above one harmful mutation per progeny - .5 harmful mutations per gamete
per generation. Since Kimura (1983) estimated that amino-acid altering
mutations are ten times more likely to be harmful than neutral, then the
estimated rate of neutral mutation would be .05 per generation. That
figure seems to be too small to explain every much. In ten millions
years, using the example above, you get only about 25000 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?
> These events are generally rare (although many plants can easily form
> polyploid hybrids that act as genome duplication events, this is unusual in
> animals), but the time spans involved are also quite long.

Given the above example of ten million years, would it be long enough?
For starters, how rare are these events anyway?

> if the large differences that puzzled Haldane were morphological, a small
> mutation in DNA can have large phenotypic effects.

I think Haldane was talking about differences at the level of
nucleotides, but I have to check the original paper.