biogeographic homology

[John Grehan]

Fred Rickson


>I have followed the biogeographic discussion with some interest because I
>am wrestling with a plant distributional problem that seems to involve
>Africa, India, and Sri Lanka.

There are many distributions with this kind of pattern (track
Africa-India-Sri Lanka),
and it is often attributed to Mesozoic drift or Tertiary connections of one
kind or
another

 The distributions are not
>continuous, they are dots on a map in the same sense as any taxa
>distribution notation.  The lines connecting terminal taxa on a cladogram
>are not indicative of direct evolutionary connections, just help in
>arranging the final organisms into one suggestion of monophyletic groups.
>Likewise, tracks just outline where one finds organisms, not how they
>are/were connected, and supposing anything more is mostly guessing without
>corroborating data.

The interpretation placed on tracks by some practitioners is that the lines
indicated spatial relationships according to the criterion of minimun
distance. In this sense tracks are like a cladogram in that they
arrange organisms into a classificaiton, only in this case the
characters are spatial rather than biological.

With tracks it is possible to compare the spatial geometry of different
taxa. Taxa with locality connections that correspond or nest within
each other, for example, can be grouped together within the same
standard track, and from this may be derived the hypothesis that the
standard track represents a common ancestral range.

John Grehan