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From: kliman@mbcl.rutgers.edu
Newsgroups: sci.bio
Subject: Re: Coelocanth and evolution:x
Message-ID: <472.28511bf9@mbcl.rutgers.edu>
Date: 8 Jun 91 22:03:37 GMT
References: <17580003@hpfcdj.HP.COM> <18@tdatirv.UUCP> <471.284d6041@mbcl.rutgers.edu> <29@tdatirv.UUCP>
Lines: 50

In article <29@tdatirv.UUCP>, sarima@tdatirv.UUCP (Stanley Friesen) writes:
> In article <471.284d6041@mbcl.rutgers.edu> kliman@mbcl.rutgers.edu writes:
>>In article <18@tdatirv.UUCP>, sarima@tdatirv.UUCP (Stanley Friesen) writes:
>>> *No* species are unchanged for this long.  NGE was wrong.
>> 
>>This is clearly a question of semantics.  As far as I know, there is no rule 
>>that phenotypic change must occur within some specified period of time (e.g., 
>>400 MY).
> 
> Perhaps so.  But there is no known living *species* that is more than a few
> million years old (certainly no more than 10 MY).  And *most* living species
> are around one million years old (or even younger).
> 
> I would require truly impressive evidence before I would accept a claim
> that a *species* was anything like 400 MY old.  Even genera almost never
> survive *that* long.  

What is the definition of species here?  Certainly bifurcations have occurred 
in the phylogenies of all extant species.  However, does this cause the
extinction of the ancestral species?  I don't think we can determine if any
extant species can produce fertile offspring in a cross with some distant
ancestor (if we use that particular definition of species), so to say that no
species is more than 10 MY old is just an assumption.  I'm not saying that
the coelocanth is morphologically identical to its ancestor 400 MY back, or
that it is even the same biological species.  However, I stick to my 
prediction that *some* species (not necessarily a chordate) has not changed
substantially for 400 MY.  That is not to say that diversification through
offshoot lineages (e.g., by "founder effect") has not occurred. 
 
>>Still, it seems to me that there must be plenty of cases out there of organisms
>>resembling, at a morphological level, their far distant ancestors; natural 
>>selection can favor ancient phenotypes if they remain superior, thereby 
>>overwhelming the force of mutation.  Why not?
 
> Quite true.  But none are so closely identical (even morphologically) as to
> be considered to be the same species.  Why this is so is, perhaps,
> a problem worth studying, but it is nonetheless true (by observation).
> 
> My guess would be that no environment has remained sufficiently unchanged
> for more than a few million years for identical phenotypes to continue
> to be advantageous.  The least changed forms I know of are probably
> crocodiles, but even they are detectably different.

What I'm saying is purely hypothetical.  I'm inclined to agree that all extant
chordates probably differ from their ancestors from 400 MY.  All I originally
wanted to convey was disagreement with a statement that I considered too
rigid.  We simply don't know what the phylogenies of all living species look 
like, and all "rules" are bound to be broken occasionally.

- Rich Kliman