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Re: Molecular felids



Tom Rothwell (rothwell@amnh.org) wrote:

<The first problem generated by the manuscript is that it the authors assume
the monophyletic status of the modern cats. Figure 2 depicts a convenient
divergence for each lineage beginning with a single felid ancestor who lived
approximately 10 million years ago. There is no evidence that this is indeed
true. During the time period depicted by this figure, at least 27 genera of
extinct felids have been identified to date, enumerating many extinct species.
Many of these extinct cats are potential ancestors for various lineages of
felids alive today. (Not to mention thefact that much of the diversity during
this time period was saber-toothed.)>

  The figure is a generation from the genetic sequences of the living cats
compared to those of other feliform carnivorans, including hyenas, mongooses,
etc. The lack fo genetic data of any fossil cat prevent their inclusion.
However, the genes indicate the cats were monophyletic; additionally,
morphological studies indicate this would be true even if we were to plop a few
fossil cats into the tree. Machairodont sabretooths seem to share their own
common ancestor to the exclusion of modern cats, and many other more primitive
"cats" like *Pronailurus* would simply be basal to these forms. The tree is
thus not an assumption of phylogeny, but as the authors state a consideration
of the relatedness of their sequenced genomes. Divergence estimates take more
explanation, but are largely mapped using known gene mutation rates, and
averageing them onto the tree. Such that, whatever number of fossil cats would
have been in the tree (and given the ghost lineage for *Panthera* and
*Neofelis*, there would be a lot), they would still have a monophyletic
relationship to the exlusion of the other taxa on the tree, and their
divergence would seem to have been in the Miocene.

<A second problem concerns the ocelot, or neotropical lineage. Johnson et al.
state that their data suggests that the six or so species of this South
American lineage radiated 8.0 to 2.9 million years ago, before the Isthmus of
Panama formed. This scenario gives us two possibilities. Either the ancestor
somehow found its way onto South America when it was an island continent (any
fossil evidence for this event?), or all species of the lineage somehow found
their way onto the same continent after the land bridge formed (3 million years
ago?) If the molecular clock for this lineage needs to be adjusted, perhaps the
time periods mentioned in the manuscript are suspect as well?>

  It's just as likely that there have been intromittent isthmuses between the
Americas, during which it is likely also that the jaguar made it to the
Americas. That is, the cats must have gotten there somehow. The divergence
estimates are not concrete, and Johnson et al. would admit to this, but
estimates based on sampled gene mutation rates and the inferred appearance of
the cat. It is possible, for example, that the *Leopardus* lineage derived
prior to migration into the Americas, and I do not think Johnson has ruled this
out.

  It should also be of note to consider that modern cats are no longer shruken
lions, but that lions, apparently on the earliest diverging branch, are the
huge kitties they are, through the effects of peramorphic size increase.

  Cheers,

Jaime A. Headden

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)


                
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