Guy Hoelzer (hoelzer at unr.edu) wrote:
: > tdib said:
: > the two phylogenies are the same or they are not. Why would you need to
: > prove anything in order to combine the two data-sets?
: The means or variances of any two data sets can
: differ due to sampling error or because they actually represent different
: populations (i.e., different phylogenies). Phylogenetic data are no
: different. They, too, are samples of taxa, of the extant variation within
: those taxa, of characters, etc. Furthermore, the extant taxa and variation
: is a sample of historic taxa and variation. Therefore, when you have two
: data sets, even when they contain samples from identical sets of taxa, they
: may differ either due to sampling error or because they are actually
: samples of different phylogenies (e.g., sequences from two different genes
: in the same set of individuals may have different phylogenetic histories
: (see Pamilo & Nei 1988)).
So far I'm with you.
: Therefore, it is important to know whether the
: data sets really contain conflicting phylogenetic signals prior to
: combining the data.
This is where I start wondering.... Why the "therefore"? What difference
does it make WHY the phylogenies are different at this point in the analysis?
: It has been argued that combining data sets with conflicting phylogenies,
: caused by the use of different characters, is still a useful way to get at
: the phylogenetic relationships of the whole taxa, rather than just of the
: set of characters in a particular data set. The basic idea is that the
: areas of conflict will become noise and the remaining phylogenetic signal
: will better represent the organismal relationships.
I would agree with this. It seems to be one of the more basic concepts in
systematics.
: Others argue that it is inappropriate to combine statistically
: different data sets.
Why?
: In this
: case, one should examine the set of distinct trees available for the taxa
: under study. The differences among them might be informative
of what?
: and the similarities are likely to indicate real patterns in the
: history of the whole taxa.
The similarities would certainly show up in a combined analysis, no?
: BTW, subsets of what is collected as a single
: data set can also contain significantly distinct phylogenetic signals;
: so, the question of combining data sets is identical to the question of
: searching for conflicting signals within any one data set.
But would those who advocate _not_ combining data-sets actually break up a
single data set simply because partitions of it may support different
topologies? If not, then why would it be justified to "combine"
conflicting data in an originally singular data-set, but not to combine
two conflicting data-sets? If they do break up originally singular
data-sets, where does it stop? What stops the deconstruction before it
reaches single contradictory characters?
: To state my answer to the question posed above (Why would you need to prove
: anything in order to combine the two data-sets?) more directly; you don't
: need to prove anything, but you might be missing out on interesting and
: important information.
I dont see the relevance of knowing whether the conflicts are caused by
sampling error or by different character trees until you arrive at a final
assessment of the overall phylogeny, something I have a hard time
imagining emerging from anything but a total evidence analysis of all the
relevant taxa (IOW, just go ahead and combine them, add in all other
evidence, and _then_ see how things can be interpreted).
--
Tom diBenedetto
Fish Division
University of Michigan Museum of Zoology
