Re: Haldane's dilemma

David Campbell (
Mon, 28 Apr 1997 09:43:49 -0500

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,...)?

>> 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

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

David Campbell