|
WHITHER MOLECULAR PHYLOGENETIC SYSTEMATICS? |
|
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! |
|
|