Positivism vs Realism

Thomas Pape thomas.pape at NRM.SE
Fri Dec 12 20:38:43 CST 1997

At 13.58 1997-12-12 -0800, Zander wrote:

>   Cladistic
>convergene is homoplasy that occurs for [?far] enough away that convergence
>cannot be interpreted wrongly as due to shared ancestry. Take a true
>tree ((a b) b), where character b is an advanced trait, and is evolved
>twice patristically near. "Parsimony analysis" necessarily interprets
>the true tree as ((b b)a). Patristically nearby homoplasy is verboten in
>maximum synapomorphy analysis.

Well, did you say "terribly simplistic evolutionary theory" ?? I would say
that your example is just too 'terribly simplistic' being based on only two
states. Your "patristically nearby homoplasy" is by no means "verboten" in
max parsimony analysis. If other evidence is in favour of the ((a b) b)
tree, this is what we accept. And if proper outgroup comparison will
indicate that state b is apomorphic at the level of your three taxon clade,
state b will be considered to either have evolved twice -- or to have
evolved once and been reduced (or transformed) once.

Indeed, this allows the possibility of BOTH convergence AND common ancestry.
But how should ANY other analytic method be able to distinguish between
these two possibilities?

Thomas Pape

Thomas Pape
Department of Entomology
Swedish Museum of Natural History       Voice: +46 8666 4094
Box 50007                               Fax:   +46 8666 4099
S - 104 05 Stockholm, SWEDEN            e-mail: thomas.pape at nrm.se

WWW URL:                       http://www.nrm.se/en/pape.html.en

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