Farewell to Species - reticulation
TDibenedetto at DCCMC.ORG
Tue Feb 1 13:58:45 CST 2000
Thomas Lammers wrote:
Submitted for mulling over: cladistics is flawed science because it comes
to a conclusion first ("all evolution is dichotomously divergent, with
ancestors becoming extinct at the appearance of a pair of daughter-taxa")
and then seeks data to support that conclusion. When the data do not
support that conclusion but suggest that other patterns are operative and
equally valid (e.g., when we get polytomies or reticulations), they are
judged to be faulty and dismissed.
I disagree with just about all of this.
First of all, "all evolution is dichotomously divergent" is neither an
assumption of cladistics, nor is it a conclusion. Cladistics can and does
proceed without assuming this (hard polytomies are an accepted and
recognized concept amongst cladists). And cladistics does not present this
as a conclusion; once again, polytomies are found in most published
cladograms, and they are interpreted as saying "there is no evidence for a
dichotomous representation in this case". Although some cladists may be
motivated to continue their research because of a personal feeling that
there just must be some evidence out there to resolve the polytomy, this
does not lead to a biased result, since no such resolution will be presented
unless the evidence is actually found.
The conclusions that cladists are seeking are not confirmations (or
refutations) of the notion that evolution is dichotomous, but rather
statements as to which taxa are more closely related to which other taxa.
I do not know what Thomas means when he says that non-dichotomous
relationships are "judged faulty or dismissed". I do not know of any authors
who advocate "dismissing" a polytomy; I dont even know how one would do
that. Are you saying that cladists arbitrarily resolve polytomies? I dont
think so,,,I see plenty of polytomies reported in the literature. And once
again, if a particular cladists feels that a polytomy is "faulty", that
there just must be some more data out there that would resolve it, then what
is the problem with that? So long as results are reported honestly, it seems
that this does nothing but provide a motivation for further research.
Cladograms are schemes of relationships, and will be resolved to the extent
that the evidence allows. One may speculate that if the cladograms of
particular groups tend, after many years of studying the group, to be well
supported and completely resolved, then perhaps evolution does tend to be
dichotomous. Or, if the cladograms of well-studied groups often have
persistent polytomies, then perhaps evolution is only sometimes dichotomous.
These are the types of generalizations about the evolutionary process that
are not the primary focus of cladistics, but are derivative issues.
Secondly, the notion that ancestors "go extinct" at the appearance of
daughter taxa is also not a necessary assumption of cladistics. It was at
one time a conceptual formulation that found some favor amongst some
cladists, but one rarely hears it these days, and in any case it seems to
have been merely a semantical argument. What is clear is that an ancestral
taxon diverges at some point, into two or more sub-lineages. Some may choose
to see this as the ancestor becoming "extinct", for purposes of applying
names to the various lineage-parts. I prefer to see the ancestor as merely
having become more complex, with the name now referring to a
higher taxon, and new names needed for the daughter branches. This is an
argument about names and defintions, not about biology or evolution.
Cladistics, in its broadest sense, is a science concerned with describing
taxic diversity with the currency of characters, and organizing that
information into a nested hierarchy. The hierarchy is an assumption, one
which is basic to all evolutionary thought. The fully resolved hierarchy is
not a necessary assumption though, but it may serve as an ideal standard.
The true tree may not be dichotomously resolved, but we, as scientists,
should always try to resolve the unresolved. Asking "which two of these
three are more closely related to eachother?" is the basic question of
systematics; the minute we stop asking that is the minute we stop doing
systematics. I dont see what the problem is, so long as we continue to
present the polytomies that resist resolution.
tdib at dccmc.org
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