[Taxacom] On genera

John Grehan calabar.john at gmail.com
Sun Jul 21 14:41:26 CDT 2013


I find myself losing track of what the problem is. If I look at the
distribution of unique derived characters and find that there are more
shared between humans and orangutans than humans and chimps I might
hypothesize that humans and orangutans are more closely related to each
other than either is to the chimpanzee, that humans and orangutans share a
more recent common ancestor, that the ancestor may be expected to exhibit
all of those features uniquely present in humans and orangutans, that any
fossil entity that also shares these features may be a member of the
ancestor or one of its descendants. I would be interested to see how
Richard would approach the human nearest relative question in his way so I
can better understand what his argument is about.

John Grehan


On Sun, Jul 21, 2013 at 1:33 PM, Richard Zander <Richard.Zander at mobot.org>wrote:

> Jason:
>
> 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."
>
> Richard
>
> -----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
> monophyly?..."
>
> 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.
>
> Best
>
> Jason
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