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2nd piece on clades



Jeffrey Martz wrote:
     
>     Wait a minute; both these viewpoints are based on evidence
>regarding the type of skin present in Dimetrodon's ancestor; the
>viewpoint you call "cladistic" is simply based on a more recent
>evaluation of the evidence.

The first viewpoint, which evolutionary taxonomy may not endorse but does 
encourage, is based on an evolutionary sequence from fish to amphibian to 
reptile to mammal.  This viewpoint ignores the value of modern amphibians and 
lungfish as outgroups (especially as different outgroups).  It also assumes 
that because some of the features reptiles share are primitive for amniotes, 
any character they share is most likely primitive.

It's perfectly true that one can carry out phylogenetic analysis without using 
cladistic taxonomy.  However, if you are unable to use a taxon in 
reconstructing evolutionary processes, that is a severe blow to its utility.  
Apart from being a (conceptual) box to put certain organisms in, what use is 
the class "Reptilia"?  (That's not a rhetorical question, I do want to know.)

>                       Evolutionary taxonomy can modify the
>definition as new evidence comes in.

Bug or feature?

>     Are you saying that before cladistic taxonomy no one
>understood the inheritance, or that evolutionary taxonomists undertand
>evolution less then a cladist?

Biologists have always thought along taxonomic lines.  I personally would 
probably have made the error I described, had I not been exposed to cladistic 
taxonomy.  Professionals are doubtless not as easily misled, however I believe 
that before the rise of cladism pelycosaurs were thought to have scaly skins, 
perhaps because they were "reptiles".

I'm sure nowadays a good evolutionary taxonomist would follow more or less the 
same methodology as a cladist when trying to reconstruct characteristics of an 
extinct or badly-known organism.  But if all the taxa you use are clades, and 
if the literature is full of clades too, it becomes much easier.

>     Don't confuse cladistic analysis with cladistic taxonomy...

I'll try not to.

>> What this is assuming is simply that there is a 'tree of life', in which all 
>> organisms are united by common descent, and that once brances separate they 
>> never rejoin.
>      But there IS no tree.  The tree is a conceptual aid to help
>undertand descent and interrelationship.

I'll gladly discuss what kinds of things can and can't be, but not on the list.

>> So which did come first?  The domestication of the jungle fowl _Gallus_, or 
>> anisogamy?  :)
>Let me know what anisogamy is and I'll tell you.

The difference in size between gametes, i.e. the distinction between eggs and 
sperm.  It was a joke.

>> If you see a capital letter, 
>> you know the term is expressing a theory about evolutionary history and not 
>> about any other aspect of the creature's biology. 
>      Part of the evolutionary history of "thecodonts" is the last ones
>went extinct at the end of the Triassic...

When I said "evolutionary history" I meant "ancestry".  Sorry.

                                                        Regards,

                                                                Bill Adlam