RE: "Common ancestor" in cladistics

[Mike Milbocker <mmilbocker@psdllc.com>]

Michael Habib wrote:

"This means that the generally
assumption is that all nodes should have two branches; more (a
polytomy) are said to imply a lack of agreement/resolution."

I find the statement that a node should have only two branches or else a
lack of agreement/resolution is implied, to be troubling. (I'll set aside
for later debate the implication here that a character should have just 2
states.)I'll start by positing that each node represents at least a species,
maybe multiple very similar species, and not a particular mating pair.
Allowing this, then if a species survives long enough to undergo 2 changes
in state, not necessarily in the same character, (i.e., two branches) why
not 50 changes spread across 50 species, each different from the node
species by just 1 character change? (I'll interject here, for the sake of
accuracy, that all the species are positioned at the termini of branches,
and that the nodes represent sets of character states) I'm not aware of any
empiricle evidence that suggests that evolution occurs serially, rather than
in parallel. In fact, the opposite is supported. Consider for example,
Darwin's Finches, all living at the same time, all contributing to the next
change in state, the fact that some of these Finches were more "basal" than
others does not disadavantage them in any way to producing the next
variation. The hypothesis that only 2 branches should occur suggests to me
an assumption that extreme selection pressure is in effect. That as soon as
a superior variation occurs, the "basal" species becomes extinct. While this
may be valid for densely populated niches, it certainly should not hold for
periods just after mass extinction, where niche filling is less constrained
by competition. Trees that cross multiple extinction events should not be
expected to preserve a bifurcating structure.

Regards,

Mike Milbocker