A biological continuum

[Alan Harvey]

At 7:18 AM 3/31/95, Stephen Darbyshire wrote:
>
>I for one would like to see the discussion continue...

and then, a few minutes later, he wrote:

>There are many aspects to
>evolution that clearly do not fit with the methodologies of
>popular cladistics, e.g.: evolution is not necessarily always
>cladistic or parsimonious.

        The tired argument that 'parsimony-based cladistic analysis is
inappropriate for studying evolutionary patterns because evolution is not
necessarily parsimonious' is, as has been noted repeatedly in the
literature, illogical.  You might as well say we can't use computers to
study phylogenetics, because evolution does not use computers.  Parsimony
is a tool that helps us identify the interpretation(s) of a given data set
that requires the fewest number of ad hoc assumptions (i.e., homoplasy).
Any alternative hypotheses, generated by alternatives to parsimony-based
analysis, will by definition require more ad hoc assumptions.  Parsimony
methods do not require evolution to be parsimonious.  Indeed, parsimony
methods provide information about the degree to which evolution is
non-parsimonious (i.e., homoplasy).
        Also, what is meant by the statement "evolution is not necessarily
always cladistic?"

>As a procedure, cladistics cannot
>detect intermediates, or any assumption violations.

        Do people often try to use cladistics to detect intermediates?  And
what methods can better detect violations of their own assumptions?

>Characters that are variable within a taxon are eliminated from an analysis
>because they are uninformative.

        Which method can better use characters that vary within a taxon to
determine relationships between taxa?

>There are just as many cases
>where cladistic analysis of different data sets for the same
>organisms result in different topologies as they result in
>similar topologies.

        Of course, it is not clear whether this is a limitation of the data
or the methodology.  Hence the total evidence controversy.
        Not that there's no room for improvement in cladistic procedures!
But I'm still waiting to hear the conceptual problems with which cladistics
has MORE trouble than do competing approaches.

Cheers,

Alan Harvey

------------------------------------

Alan W. Harvey (aharvey at amnh.org)
Assistant Curator of Invertebrates
American Museum of Natural History
Central Park West at 79th Street
New York, NY 10024
(212) 769-5638