[Taxacom] On genera

Richard Zander Richard.Zander at mobot.org
Sun Jul 21 12:33:24 CDT 2013


Let's focus on one thing at a time: "The ancestor in a phylogeny is hypothetical." No, it isn't a hypothesis. A hypothesis has some data or observations of some sort to support it. A shared ancestor is there by cladistic definition, as in "There must be a shared ancestor, since of any three taxa two are more closely related." Related, in phylogenetics, means distance on a cladogram.

Shared ancestral nodes are contrivances, not hypotheses. They are put there to make sure all cladograms have the potential to be fully resolved. If an ancestral taxon has, say, two daughter taxa, then one of those taxa will be more closely "related" in a cladogram to the ancestral taxon (as sister groups) by chance alone. The only way that one might postulate an extinct shared ancestor is if there were data supporting such, for instance, a group of specialized taxa in isolated recent environments that share some distinctive set of traits that might be ascribed to a more generalized and widespread but now extinct ancestral taxon. 

Do not put your trust in anagenetic change. Always look for stasis first, because if it is there (and fits theory by biogeography, relative age of habitat, relatively generalized morphology, maybe even fossils), then postulating unknown, unnamed, and ad hoc shared taxa is not parsimonious.

You indicate that my "hunch" may be countered by demonstrable phylogenetic relationships. Piffle. My "scientific hypothesis" can never be countered by imaginary shared ancestors. The stochastic element generating the shared ancestral node is provided by the false idea that characters themselves evolve (as opposed to an organism). Thus, any two of three taxa that share traits must have "evolved" through those traits away from the third. Proof, of course, is that the cladogram shows "phylogenetic distance."


-----Original Message-----
From: taxacom-bounces at mailman.nhm.ku.edu [mailto:taxacom-bounces at mailman.nhm.ku.edu] On Behalf Of JF Mate
Sent: Thursday, July 18, 2013 2:06 PM
To: Taxacom
Subject: Re: [Taxacom] On genera

Thanks Richard, sorry for the delay. It´s not very clear what you mean by:

"... cladistics, because it will not name shared ancestors, cannot infer

The ancestor in a phylogeny is hypothetical. With the exception of
incipient species you will never encounter the ancestor. It is gone,
evolved into the other sister taxon. Is this what you mean?

"Suppose we have a bunch of taxa terminal on a clade. Two are most terminal
with shared synapomorphies. The next taxon down, however, clearly, derived
from the same ancestral taxon as the two most terminal taxa. Asserting that
the two most terminal taxa are monophletic splits the ancestral taxon...
Lots of subgenera commonly have some one wide-ranging species of
generalized morphology with some closely related species specialized into
more recent habitats. A theory could be developed if one was not a cladist
that these are all daughter species of the more generalized species."

You appear to suggest that phylogenetics doesn´t make use of plesiomorphic
characters and that it is a bad thing. But lack of change is hardly
informative about relationships isn´t it. Stasis cannot record anything by
definition since it is unchanging. Sure, it can tell you something about
the selective pressures on the taxa but nothing about relationships at that
level. In any case not useing plesiomorphies doesn´t impede I have to be
clear that all data has a phylogenetic signal, the problem is if it is at
the level of interest. On the other hand a phylogeny would not be very
useful if you imposed a given direction on the character state changes.
After all you analyze the data to build a cladogram and in the process test
your hunch. If your hunch is part of the analysis why bother?

"In the first place, I advance a scientific theory, descent with
modification from a species at hand with a clear and known evolutionary
process, specialization into particular habitats. Otherwise, inventing ad
hoc, unnamed and unnameable, invisible explanations for a natural
phenomenon is a step back. The only reason to advance an unidentified
ancestral species is when all the taxa involved are specialized into
particular habitats."

A phylogeny narrows the possibilities in regards to evolutionary direction
but it is hardly ad hoc. You may have a hunch as to how evolution in a
certain group has proceeded (generalized to specialized for example) but if
the relationships are at odds with this scenario you can´t simply dismiss
it or the method as someone´s invention. Either the phylogeny is wrong
(why, do you have outside data that support this) or your hunch is wrong.
Either way you need more data.


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