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From: straney@msudoc.ee.mich-state.edu (Ronald W. DeBry)
Newsgroups: sci.bio,talk.origins
Subject: Re: sexual selection and investment
Message-ID: <368@nancy.UUCP>
Date: 1 Apr 88 02:11:46 GMT
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I *think* that I correctly sorted out all the >>>>>>s - I hope so.


Michale McNeil wrote:

>>>I might also add that since our closest living relatives are the
>>>gorillas and chimpanzees, which are much more markedly dimorphic,
>>>humans most probably lost much dimorphism during our evolution.  

Robert Firth replied:

>>Well, since both pongos and chimpanzees are more highly differentiated
>>than humans (and read that as "more evolved" if you wish), it is not
>>impossible that the opposite is the case - that they have gained
>>some dimorphism rather than that we have lost some.
>

I don't know what you mean by several of your terms here.  Did you intend
"pongo" to mean gorilla?  _Pongo_ is the genus name for orangutans.
Currently, as a family name (Pongidae), it refers to orangs, gorillas
and chimps (its only our keen sense of anthropocentrism that keeps 
a separate family for humans, there ought to be no such thing as the
Hominidae).  Anyway, next, what do you mean by "more highly
differentiated" than humans?  More different from what?  A common
ancestor?  More different from each other than either is to humans?  If
I had to pick one as "more different from a common ancestor" I would
pick humans, on the basis of their radically enlongated development
compared to any other primate.  Even so, that has no necessary bearing
on any other character.  It could, of course, be true that the
developmental differences have pleiotropic effects on sexual dimorphism,
but we don't have anything like the data needed to show that.

Of course, just because your resoning wasn't perfect, that doesn't
guarantee that your conclusion must be wrong.  In fact, it certainly is
possible that chimps and gorillas have gained dimorphism.  (by the way -
where did the measures of dimorphism come from in the first place?  I
mean, is it really true that humans are less dimorphic than the rest of
the Pongidae?)

Michael McNeil replied (to Robert):

>I agree it's *not impossible* that chimpanzees and gorillas gained
>dimorphism rather than humans having lost it, but it's less probable.
>The reason is due to the fact that humans, chimpanzees, and gorillas
>apparently are roughly equally divergent from a common ancestor.  In
>such a three-way branching situation, if two species share a trait --
>such as the knuckle-walking behavior of chimpanzees and gorillas -- it
>is quite likely that our common ancestor possessed that trait as well.  
>The alternative -- that convergent evolution independently invented a
>trait within different species -- is of course possible, but generally
>considered somewhat less likely, depending on the trait's complexity.


No iron-clad guarantee that the answer you get will be right, but the
best approach in this situation is to look at the next "outgroup", in
this case the orang.  If they are strongly dimorphic, then it
strengthens the case that humans became less so.  

Another by the way:  The causes (meaning selective pressures) that lead
to any given level of sexual dimorphism are one of the big, hotly
contested areas in evolutionary population dynamics.  Theories are about
a dime a dozen, and equations purporting to support of theory or another
go for about 50 cents :-). 


Ron DeBry  Dept. of Zoology  Michigan State University