Link: The problem with 'basal' in phylogenetic trees

Stacey Smith summarizes the problems with using the term “basal” when discussing phylogenetic positions of organisms: “The ancestors are not among us”.

Basal is a term that refers to the position of a node near the root of a phylogenetic tree. It’s a perfectly good term, it just doesn’t describe the branches that come from that node. To understand why, I find the following paragraph makes the problem with “basal” especially clear:

Every branching in a (phylogenetic) tree is rotatable (see Fig. 1). Of course, the tree has a base, and there is a most basal branching and a next most basal branching, but there is no such thing as the most basal clade. Because branchings are rotatable, there are always two most basal clades (if the most basal branching is completely resolved, Fig. 1) or even more most basal clades (if the most basal branching is not completely resolved, e.g. in Polyneoptera in Fig. 1). Both branches originating from a node (i.e. the two sister groups) are of equal age and have undergone equivalent evolutionary change. Whether a group has branched off early (basal) or later in the phylogeny contains no information about this particular group, but information about both this group and its sister group, because both branched off at the same time.

That is from an editorial from 2004 by Frank Krell and Peter Cranston in the journal Systematic Entomology, “Which side of the tree is more basal? Smith points to the editorial in her post, and I found this discussion helpful.

I’ve thought a good amount about this issue as I’ve been working on an undated assemblage. Reading the literature in this light, I am amazed at how often anthropologists cover up uncertainty about the relationships within Homo with the term “early Homo”. I admit to being guilty of using this term myself, it seems like such a convenient way to forget about all the problems applying species concepts.

Reading these articles about “basal” reminded me that “basal” is no substitute. Phylogeny doesn’t give us an easy way to refer to these lineages from deep time, and that is probably for the best. It’s important to keep our trees straight.

References

Krell, F. T., & Cranston, P. S. (2004). Which side of the tree is more basal?. Systematic Entomology, 29(3), 279-281. doi:10.1111/j.0307-6970.2004.00262.x