On Tue, 2 Mar 2004 04:48:40 +0000 (UTC), 
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.

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.

>> 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.

2. All kinds of traits can arise by chance. Take the ability of some
   people to roll their tongues as a simple example. Why do I have
   to explain why traits would develop for non-adaptive reasons? Is
   it because of your (irrational) belief that everything MUST be
   an adaptation?

[snip]

>> No offense. I'm a drifter in the sense that it's part of my 
>> self-proclaimed "mission" to educate people about the importance 
>> of random genetic drift
> 
> Genetic drift isn't very interesting, changes in phenotype are all
> that evolution can work with.

Two points ....

1. The mechanism of random genetic drift is the basic evolutionary 
   mechanism underlying most of molecular evolution. I find this very 
   interesting. You may not. Furthermore, I'm quite interested in 
   the organization of genomes and junk DNA. Part of the modern
   explanation of genome evolution requries random genetic drift. 
   You may not be interested in this either. Finally, I'm really
   interested in correct explanations of the main features of 
   modern species (and speciation in general). If you aren't interested
   in random genetic drift then you mustn't be interested in those
   things either. It's okay to focus all of your attention on 
   adaptations and to admit that nothing else about evolution interests 
   you. But please don't assume that others share your bias.

2. The most reasonable minimal definition of evolution is change in
   the frequency of heritable characteristics in a population over
   time. This definition includes fixation by random genetic drift of 
   almost neutral alleles. You seem to want to change that definition 
   to restrict it to changes in phenotype - where presumably you mean 
   only observable phenotypes. Furthermore, when you say that this is 
   "all that evolution can work" with you imply that evolution is 
   synonomous with natural selection. In other words, you would like to 
   change the definition of evolution so that the only "real" evolution 
   is adaptation. This is consistent with the fact that you aren't
   interested in any other kind of evolution but here you go one step
   farther. Now you are suggesting that we re-define evolution so 
   that my interests become illegitimate. If you succeed (highly 
   unlikely) then what word will you use for all those heritable changes 
   that aren't adaptations? Will this be pseudoevolution?  :-)

>> and evolution by accident.
> 
> Evolution by accident?  Can you explain?

Yes, I'm referring to evolution that takes place without natural selection
playing a significnat role. It includes things such as the random 
elimination of some species, and the survival of others, at the time of
mass extinctions. (The so-called "Field of Bullets" senario of David Raup.)
It includes lots of other things as well. When I use the phrase "evolution
by accident" I mean it to be a direct contrast to adaptionism. The phrase
is intended to provoke people into thinking outside their box. (It's
also a convenient way to emphasize the contrast between real evolution and 
intelligent design.)

>> 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.



Larry Moran
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