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WHITHER MOLECULAR PHYLOGENETIC SYSTEMATICS? |
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WHITHER MOLECULAR PHYLOGENETIC SYSTEMATICS? A
long discussion raged on the listserver Taxacom in early to mid 2005 about
whether the orangutan or the chimpanzee was mankind's little brother.
Morphological evidence apparently favored the man-orangutan while molecular
evidence favored man-chimp. The discussions are archived at http://listserv.nhm.ku.edu/archives/taxacom.html. Given
the statistically strongly supported molecular tree of (((man chimp) gorilla)
orangutan, gibbon), most discussants, including myself, poo-pooed the
morphological evidence as contradictory and doubtless convergent. After
that discussion, I've had second thoughts, and here is my slightly edited
comment on contradictory morphological and molecular results, and the
immediate future of molecular phylogenetic analysis: I've been
looking in the evo-devo literature recently (e.g. the journal Evolution
and Development, and in fact there is some (not much, but some) evidence
that evolution commonly involves large gene complexes regulated (e.g. turned
off and on) by non-coding regulatory genes. This makes
for a sloppy evolutionary tree because the silenced gene complexes can travel
the lineage byways for up to 10 million years before being degraded. The problem is that some major morphological
complexes are greatly split by molecular systematics. Although
statistically the molecular split is well supported, that split statistically
contravenes Dollo's Law that it is not to be expected by chance alone that
two taxa sharing many complex morphological characters should separately
exactly re-evolve (by gradual accumulation of traits). Molecular systematics, in my opinion,
tracks reproductive splits assuming a Biological Species Concept (iffy for
plants for which there is much evidence of gene-permeable species barriers),
which mainly also does reflect evolutionary changes (to the extent it matches
expectations from morphological analysis), but instances of apparent massive
homoplasy can be mediated by identical or little changed shared gene
complexes turned on by selection on regulatory genes. This
re-activation of a gene complex is the only explanation of such homoplasy,
and it builds on Darwinian thinking.
The human-chimp relationship is statistically supported by molecular
studies as a sister group, but we share too many critical morphological
traits with the patristically more distant orangutan not to suspect some kind
or degree of direct shared genetic past, a deep homology. If demonstrably shared evolution is more
important to systematics and classification than tree-like segregation of
gradually accumulated changes in introns and junk DNA, and if evolution may
not be recovered in at least some salient cases of apparent improbable
convergence by molecular systematics, then careful examination of morphology
is a major clue to evolutionary relationships. So, dust off
your 1970's phenetic techniques! Morphology will rule again! |
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