Jeffrey Turner wrote:
> Larry Moran wrote:
> 
>>Jeffrey Turner  wrote:
>>
>>>Larry Moran wrote:
>>>
>>>>William Morse  wrote:
>>>>
>>>>
>>>>
>>>>>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. 
>>>>
>>>>Hmmmm .... I would have thought that ear size in elephants and skin
>>>>color in humans were excellent example of drift.
>>>
>>>So where are all the small-eared elephants?
>>
>>Hmmmm ... I see where you're coming from. You observe that all
modern species 
>>of elphants have bigger ears than their ancestors and their modern cousins 
>>such as manatees, dugongs, and hyraxes. This leads you to the
conclusion that 
>>large ears are an adaptation. However, if the ancestors of modern elephants
>>just happened to have large ears, and large ears were not a disadvantage, 
>>then the presence of large ears could just as easily be due to drift. The 
>>fact that modern species possess a certain characteristic feature is not
>>prima facie evidence of adaptation since drift and the founder effect would
>>achieve the same result.
> 
> 
> True, but the advantage of large ears in a large mammal that can't
> really stay out of the tropical sun is obvious.  Why would you think
> that big ears would be a random occurrence?  Drift is probably a minor
> effect, while founders are interesting because of the advantage that
> made them a new species not because of their random minor traits.
> 
> 
>>But that's not what I had in mind. I was referring to the fact that Asian
>>elephants have much smaller ears than African elephants. If ear size is 
>>under strong selection then one has to account for this fact. On the other
>>hand, if ear size is non-adaptive then the difference between Asian and
>>African elephants could be due entirely to chance.
> 
> 
> Gee, why don't humans have large ears if they are just by chance?
> Ever think that climate differences account for the smaller ears in
> Indian elephants?  There's no reason to believe that non-adaptive
> mutations lead to speciation - and that's where all the marbles are.
> 
> 
>>>>Don't you see how
>>>>difficult it is to make general rules? Your decision about what the
>>>>"default" hypothesis should be depends to a great
extent on your
>>>>original biases. This is exactly the point that Lewontin and Gould
>>>>made in their original paper. If you tend to emphasize natural
>>>>selection in your thinking about evolution then you will look to 
>>>>adaptive explanations ahead of non-adaptive explanations.
>>>
>>>Considering genetic diversity, itself, gives a species an advantage,
>>>you're going to have to explain why traits would develop for
>>>non-adaptive reasons.
>>
>>
>>Two points ...
>>
>>1. Genetic diversity cannot be an adaptation since this requires a form
>>   of group selection that has been thoroughly discredited. If a species
>>   accidently possesses more diversity then it will be the lucky survivor
>>   when the environment changes. This is more like evolution by chance
>>   that real adaptation.
> 
> 
> Discredited?  We all know that in many species that have familial
> groups, say lions, the males leave or are expelled from the group to
> find mates elsewhere.  That type of behavior occurs across many
> species with the obvious effect of diversifying the genetics.  

The usual adaptive explanation for this behaviour is that it reduces 
kin-kin competition, so that it may have nothing to do with diversity.

Come to that, this behaviour may not have an effect on diversity at the 
species level, but only on how it is distributed.



>>>>I'm not having 
>>>>much luck on sci.bio.evolution. This newsgroup is heavily dominated 
>>>>by people who reject the very concept of drift or who have
deliberately
>>>>chosen not to understand it. I find this very strange in a newsgroup 
>>>>that's supposed to be devoted to discussing evolution at a serious 
>>>>level.
>>>
>>>Maybe because "drift" doesn't seem to be very well defined.
>>
>>Not true. It's as well-defined as natural selection. It may be a more
>>difficult concept to grasp and it may be unfamiliar to those who have
>>been brought up on a steady diet of adaptionist dogma, but that's a
>>different kettle of fish. You can't dismiss random genetic drift just
>>because you don't understand it.
> 
> 
> I can dismiss it because it's not significant in evolution.  You
> haven't shown any method by which it would cause speciation.
> 
I don't know if Larry knows (or cares) about the genetics of allopatric 
speciation, but that I interpret that evidence as showing that 
speciation occurs through two populations becoming physically isolated, 
and then becoming genetically isolated through drift.

Bob

-- 
Bob O'Hara

Dept. of Mathematics and Statistics
P.O. Box 4 (Yliopistonkatu 5)
FIN-00014 University of Helsinki
Finland
Telephone: +358-9-191 23743
Mobile: +358 50 599 0540
Fax:  +358-9-191 22 779
WWW:  http://www.RNI.Helsinki.FI/~boh/
Journal of Negative Results - EEB: http://www.jnr-eeb.org
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