More on reticulation

[Curtis Clark]

At 03:02 PM 2/7/02, STEPHEN MANNING wrote:
>>That's your call, of course, but most botanists I know won't reject a
>>technique just because it's not infallible.
>
>I don't either, but could it be improved?  It's not my field, but I
>sometimes wonder whether a statistician could come up with, or has come up
>with, a mathematical model which would take into account reticulation in
>such a realistic way that it would often alter, based on probable
>paleopolyploidy, what is determined to be the most parsimonious
>cladogram.  If about 50% of plant species are polyploid, this seems a
>major concern especially when analyzing relationships of species within
>genera, or genera within families (as both are subject to hybridization
>events).

Back when Mike Donoghue was at San Diego State, one of his grad students
presented a paper making the case that reticulation could not be
distinguished from homoplasy, i.e., that any character incongruence could
be explained as parsimoniously by reticulation as by homoplasy. Honestly, I
didn't follow the math, but it seems intuitively clear, especially if you
accept things like horizontal gene transfer. And Verne Grant stated
somewhere near the end of the second edition of Plant Speciation that we
could never understand the phylogeny of flowering plants because of the
high degree of reticulation.

Nevertheless, there is a surprising amount of agreement between molecular
and morphological trees for large parts of the flowering plants. Some of
the problematic areas might represent reticulation, or not. But either
Grant was for the most part wrong, or the evidence has seriously misled us.

Polyploidy is a "red herring". Despite the claims of Stebbins to the
contrary, autoploidy is not uncommon, so that a paleopolyploid doesn't
always represent hybridization. Second, we now know of several methods of
homoploid hybrid speciation. And modern alloploids often involve fairly
close relatives (in the grand scheme of things); if alloploidy in the past
behaved the same way, we would not necessarily be able to detect it ten
million years later even if it wore a name tag saying "Hi, I'm reticulate."


--
Curtis Clark                  http://www.csupomona.edu/~jcclark/
Biological Sciences Department             Voice: (909) 869-4062
California State Polytechnic University      FAX: (909) 869-4078
Pomona CA 91768-4032  USA                  jcclark at csupomona.edu