Tim Tyler  wrote in
news:c1tbc1$85n$1{at}darwin.ediacara.org: 

> William Morse  wrote or quoted:
> 
>> If the trait shows little
>> variation throughout a large population or is strongly correlated
>> with an obvious environmental variable, and has an obvious relation
>> to an aspect of species behavior, one can make the default assumption
>> that it is an adaptation, especially if it has existed for a long
>> period.(Again based on recollection, Wirt did a much better job than
>> I just did of defining what traits could be considered adaptive as a
>> default). Examples would include large ears in elephants, skin color
>> in humans, and almost any morphological feature of horseshoe crabs. 
>> 
>> If the trait shows wide variation throughout a population regardless
>> of environmental variables, or is confined to isolated subpopulations
>> with no obvious relation to fitness, one can make the default
>> assumption that it is due to drift. Examples include coat color in
>> domestic cats and the Rh- blood type in humans. 
> 
> Genes for traits can reach fixation by accident in small populations.

Surely. This is why I left the caveat that the default assumption is only 
an assumption - and why I did not include traits with little variation in 
small populations. 

 
> They can remain fixed due to selective lock-in if other adaptations
> come to depend on their presence.

In which case they will still show little variation in large populations 
- but those populations will also give more opportunity for competing 
suites of adaptations to develop and overcome the lock-in.
 
> Old, non-variable traits (of any sort) need not /necessarily/
> represent adaptations.

I disagree strongly. If they are not being fixed by stabilizing 
selection, they will drift. 

> Incidentally, are you /really/ suggesting a human blood type is
> neutral? Are not blood types often critical in disease resistance? 
> That is surely a case of parasite-driven selection favouring
> diversity. 

The Rh- example comes from Cavalli-Sforza. But yes he is really 
suggesting that Rh- is due to drift.It apparently comes from a relatively 
small (historically) population in central Europe - and given the extreme 
effects on reproduction (you generally only get one child when mated with 
an Rh+) it is unlikely to be parasite-driven.  In fact he notes that the 
absence of B blood types in Amerindians could also be due to drift - 
although that one is open to question since it could also be parasite-
driven by syphilis.


Yours,

Bill Morse
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