lamoran{at}bioinfo.med.utoronto.ca (Larry Moran) wrote in
news:c2i7gu$14qo$1{at}darwin.ediacara.org: 

> On Mon, 8 Mar 2004 04:12:39 +0000 (UTC), 
> William Morse  wrote:
>> lamoran{at}bioinfo.med.utoronto.ca (Larry Moran) wrote in
>> news:c25pfm$4j0$1{at}darwin.ediacara.org: 

(snip)

> Nobody is arguing that positive natural selection doesn't exist at the
> molecular level. However, I hope we can agree that most change in the
> sequences of genes is not due to adaptation. 


Yes. Wilson gives a nice explanation of why the observed degree of 
polymorphism cannot be maintained by adaptation in Chapter 4 of 
Sociobiology, which I am just getting around to reading. Of course I could 
have read  Kimura himself, but then they might have revoked my membership 
in the Adaptationists Club :-)


>The question is whether 
> this observation should apply to nucleotide substitutions that give 
> rise to visible phenotypes or whether it is confined to substitutions
> that don't have any obviously visible phenotype.

Or whether drift (via both neutral change and sampling error) generates 
substitutions and movement away from the trap of local adaptive peaks that 
then get  sculpted by selection to give rise to the observed morphology. I 
threatened before to move away from the basic drift vs. selection debate - 
which I think we have flogged to death at this point - and get into the 
subject of speciation.    

But before I do:

(snip my discussion of both forces working)

> Why does there have to be a "default assumption" when we know for a
> fact that both random genetic drift and natural selection play
> important roles in evolution?

Because I don't want to have to fight over every trait. As I pointed out in 
my initial division between what types of morphological traits are likely 
due to selection and what types are likely due to drift (and I agree the 
latter exist), I think there are clues as to which traits are _likely_ to 
be due to which. This allows for simplification in discussions and I think 
we can benefit from that - it is one of the essences of creating models.


Now onward and (hopefully) upward. 


A famous criticism of "On the Origin of Species" is that it doesn't say 
much about the origin of species. The classic (over half a century old) 
neoDarwinian explanation is allopatric speciation. The mountain rises, the 
populations are cut off, they adapt separately. Another classic assumption 
is that gene flow between populations is significant, so that effective 
population sizes are large. There are some problems with this:  birds 
(which can fly over the mountain) should all be one species; a recently 
appeared spatially small geologic feature, like Lake Victoria, should have 
only a few species, while  a widely dispersed long lived species such as 
horseshoe crabs should show great variation. This may just prove Orgel's 
Second Law - "evolution is cleverer than you are" - or it may mean that it 
is time for a Post Modern Synthesis (sorry Josh - sometimes I can't help 
myself).

If I actually knew what would comprise this PMS (ouch!), I would be an 
evolutionary biologist instead of a civil engineer, but I can note some 
notes. First, we need to stop using the  term  "drift" as a
catchall phrase 
for stochastic processes, and we need to stop using the term "adaptation" 
as a catchall phrase for selective processes. 

There are events that are selective and stochastic, but are clearly not 
adaptive. The survival of species during a calamity is one such. Species 
with large numbers of individuals and large geographic ranges predictably 
survive calamities better than species with the opposite. The calamity is 
random, but the odds of survival is based on heritable characteristics of 
the species. 

There are events that are stochastic and non-selective, but that are 
clearly not drift even if they might be called neutral. The founder effect 
is an obvious example, and even Mayr acknowledges its potential importance 
in evolution.

There are events that are not stochastic and not adaptive (in the sense of 
being a reaction to a niche), but are still selective. An example is the 
self-organization of neural tissue during development of the brain. 

There are events that are non-stochastic, selective, but not adaptive, such 
as sexual selection for the peacock's tail.

There are events that are stochastic and adaptive, such as recombination.

There is true neutral drift, that is stochastic and usually thought of as 
non-selective and non-adaptive. 


There is symbiosis, that really isn't explained very well by any current 
evolutionary theory that I have seen - game theory  says it should be 
advantageous if it can be made stable, but doesn't say how it can get 
started in a stable fashion.

There is sampling error (there has _got_ to be a better term for this) that 
is stochastic but also interacts in complex ways with selective and 
adaptive processes.


Focusing on sampling error for the nonce, a key question (which Bob O'Hara 
raised) is effective population size.  For modern humans and mayflies, the 
effective population size is very large. For many other species, it is 
probably only on the order of hundreds of individuals, at least in the 
short term. This gives the opportunity for fairly significant sampling 
error, even for adaptive traits , at least in the short term.  Mini 
calamities (the 100 year storm beloved of civil engineers) can further 
enhance this effect. Even if populations later intermix, the genetic 
variation created make them subject to the disruptive selection that Tim 
Tyler mentioned. The net result may be sympatric speciation, as well as 
evolution away from adaptive peaks. By golly, I've just successfully 
rediscovered Shifting Balance Theory ;-) 


Yours,

Bill Morse
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