A biological continuum

[Curtis Clark]

Date sent: 29-MAR-1995
Stephen Darbyshire wrote:
 >Mammals and birds are the only major groups of eukaryotic organisms
 >that reproduce exclusively by sexual means (a rather small proportion
 >of eukaryotes), and even they are not free from phylogenetic
 >reticulation.

I believe that my herpetologist colleagues view such things as partheno-
genesis in lizards as the exception, rather than the rule.  A group
does not need to reproduce exclusively by sexual means in order for
sexual reproduction to have a major influence on its evolution.
And certainly the fact that cladistics works, and in many cases works
well, argues that reticulation, while perhaps important in certain
cases, is not an overriding factor in evolution.

 >As J. K. Veldkamp has pointed out: All plants are hybrids, but some are
 >greater bastards than others.

 >There is a continuum between taxon pairs where the taxa are discretely
 >distinct and pairs where the taxa are continuously variable.

One of my research specializations is homoploid hybrid speciation in
flowering plants.  It is abundantly clear that hybrid speciation does not
of itself blur the boundaries between taxa.  In fact hybrid species are
very often as distinct as their parents, simply because the selection
factors that allowed them to form in the first place act to constrain
their morphology.  It is true that introgressive hybridization can
produce some strange intermediates, but introgression does not result
in new species (although it can "inject" new features in existing ones),
and it is often of limited scope in relation to the total ranges of the
parents.

Just because intermediates exist, that doesn't mean that they are important,
either statistically or biologically.  I still contend that we blind ourselves
to the actual variation that exists by assuming a continuum based on the
evidence of a few intermediates.

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Curtis Clark                                       Voice: (909) 869-4062
Biological Sciences Department                     FAX:   (909) 869-4396
California State Polytechnic University, Pomona
Pomona CA 91768-4032                               jcclark at csupomona.edu
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