homology and ancestry

John Grehan jrg13 at PSU.EDU
Wed Sep 22 23:18:01 CDT 1999

Peter Hovenkamp wrote
>Thus I arrive at the understanding that in Panbiog "homology" refers to
>"the same feature found in different distributions, as a result of descent
>from a common ancestor" (this is, mutatis mutandis, the concept of homology
>as formulated by Tom DiBenedetto, following Eldredge and Cracraft).

Perhaps this is getting a bit complicated being discussed in fragments. If one
is dealing with two different taxa, for example, they each have their own
biological ancestor. The biogeographic homology indicates that they
share with each other the same ancestral distribution range. Thus, for two
families, each with genera and species endemic to eastern South America and
western Africa, they both share the same biogeographic homology in terms
of a uniquely shared character associated with earth history. In this case
the common
feature is the Atlantic basin (geomorphology) or more precisely, the
ridge (tectonics) - this being assigned by the minimal spanning tree criterion.

 As a homology statement the distributions can be hypothesized to be
associated with that feature - i.e. they both had ancestral distribution
ranges that crossed
the geographic sector in which the spreading ridge and tectonic basin
developed. They
are biogeographically homologous in terms of distributinal origin, and not
homologous with
a distribution such as a taxon in west Africa, India, and Java.

Note that the homology is hypothesized, just as homologies are hypothesized for
biological characters, but in biogeographic analysis the homology is a
spatial rather
than biological character.

>This raises the following points:
>1. Do distributions "speciate"? They need to, if the concept of common
>ancestry  is to be applied.

I suppose one might use this perspective. I have no problem with it.

>2. Are the features of distributions heritable?

In my opninion, yes. Taxa formed vicariously inherit their distribution from
the ancestral range.

>If there is only the slightest doubt about either of these points, I tend
>to agree with Tom that using the word "homology" is just conceptual inflation.

So by the above criteria the word homology for panbiogeography is not just
a conceptual inflation.

John Grehan

More information about the Taxacom mailing list