Re: Adaptive landscapes and drift Was Re: Ears under pressure.

Thomas Clarke (
6 Nov 1995 03:54:54 GMT

In article <> (Bill Burnett) writes:

>I wrote...

>>>Quite so. In fact the whole concept of an adaptive landscape works much
>>>better when you include genetic drift, and Wright's 'Shifting Balance'
>>>hypothesis is heavily dependent upon it.

>and Thomas Clarke writes:

>>But genetic drift can only produce significant change when the
>>landscape is flat in the relevant dimensions subject to drift.

>Not strictly true. In large stable populations yes, but drift should be
>inoperative in an effectively infinite population anyway. In Wright's
>conception of the adaptive landscape is disturbed by population fluctuations.
>A population crash or bottleneck following e.g. a catastrophic or colonisation
>event can result in very rapid drift and the formation of new allelic
>combinations which might be disadvantageous compared to the original status
>quo but which may allow the recovering population to scale a new (different)
>adaptive peak... which may or may not be 'higher' than the starting one.
>For a better explanation I can only point you back to Hartl and Clark (1988)
>again. The point is that drift provides a very convincing mechanism for
>change in fluctuating populations in combination with selection... The whole
>vitriolic selectionist/neutralist argument was/is in my opinion (and from my
>perspective) a complete waste of time.

Oh, I really must look up your references. I think I heard an
implicit "all things being equal" that wasn't there in you statement -
my mistake.

If other conditions obtain, bottlenecks, isolated populations etc.

No need too. I am an AAT agnostic now. And since other things are ot
equal, I accede to the significant of genetic drift in the presence
of population fluctuations.
>>You are going to have to work very hard to convince, me or I suspect
>>most people, that bipedalism arose entirely through genetic drift.

>No of course it didn't, that would be silly.

Well not without population fluctuations.
But a small population of miocene apes isolated as in the
"West Side Story" might just tip over toward bipedalism whereas
another population evolves toward knuckle walking under exactly
the same selection pressures. So maybe its not so silly.

Tom Clarke