lamoran{at}bioinfo.med.utoronto.ca (Larry Moran) wrote in
news:c1ikj8$2pkb$1{at}darwin.ediacara.org: 

> On Tue, 24 Feb 2004 20:57:28 +0000 (UTC), Anon. 
>  wrote:
>> dkomo wrote:
>>> Tim Tyler wrote:
> 
> [snip]
> 
>>>>Drift may have something to say about small populations on islands
>>>>around speciation events - but it's probably a bit player in most
>>>>other places - as far as the *features* of organisms go (as opposed
>>>>to the makeup of their genes).
 
>>> Really?  You mean the fact that people have brown, green, or blue
>>> eyes, are short or tall, fat or skinny, beautiful or ugly, have red,
>>> black or blonde hair, are prone to get different diseases in old
>>> age, have different reactions to medications, have different
>>> physical abilities like being able to run for long distances, sing
>>> very well, be really strong, and so on and so on -- these are all
>>> the result of adaptations and natural selection?
 
>> Why not?  Their ancestors have experienced different environments, so
>> hte selective pressures will be different.  Add into the mix
>> variation in the environment (which can help maintain polymorphism),
>> and disruptive selection, we have plenty of adaptive explanations for
>> the diversity we see.
 
>> Of course, drift may also be an explanation, but I believe that we
>> can only separate out the contributions of the different causes
>> empirically. 
 
> Yes. Until we can actually test the hypotheses it's wise not to
> *assume* that all morphological features are adaptations, don't you
> think? 
> 
> Tim seems to be making the default assumption that most morphological
> features are due to natural selection. Dawkins certainly makes that 
> assumption. Is this valid? 

Haven't we had this debate before on sbe? I believe it was termed at
that time the "null hypothesis" rather than the "default
assumption". My
personal opinion, FWIW, is that we can make a good first guess about a
trait without the need for a default. 

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. 

Note that I am cheating somewhat, as I am assuming there is at least
some data available to make a "default" assumption. Also, the above
_are_ only default assumptions - either may be proved false by better
data, so ultimately I agree with Bob.  Also note that there will be a
lot of traits - morphological features and otherwise - that will not
fall into either category.  In this case the
adaptationists (including myself) will dream up unsubstantiated Just So 
stories to which the drifters (sorry -
the term has both a nice slightly pejorative ring to my ears as well as
reminding me of the 60's rock band) will recoil in horror :-)  

Yours,

Bill Morse
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