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Re: "Common ancestor" in cladistics

You make a strong argument, and I agree: there are definitely scenarios that can create real polytomies. These are called 'hard' polytomies. However, almost all authors assume that polytomies in their trees are 'soft', and trees with branches that have polytomies are not considered completely resolved. My point was that this is a general assumption made in most cladistic analyses, not that it was biologically true in all cases. After all, the treatment of common ancestors is not exactly the truth either, since it is not impossible to find the precise ancestor, just incredibly unlikely. In the same way, the chances of having several character changes all arise from the exact same ancestor at exactly the same time is very unlikely (remember that time component must be there as well). Even Darwin's finches probably diverged somewhat sequentially (though the time intervals in between would have been extremely small). However, hard polytomies are probably real, and it is simply very difficult to determine when this is case.

Cheers,
--Mike Habib

On Thursday, July 29, 2004, at 07:59 AM, Mike Milbocker wrote:

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