There has been some discussion that suggests that organisms fall into
"natural" categories, dog type, horse type, etc.
I used to feel (10 years ago) that we woud be able to use DNA or protein
sequence data to identify these basic types of organisms. I am less
confident of that position now. Prompted by S. Fawl, I spent a month or
two learning how to download cytochrome c sequence data from Genbank and
to manipulate the sequence for tree-drawing. What I found was interesting
for me. Using the cyc data, the genetic difference between donkeys and
horses is only slightly less than the distance between dogs and the
Southern Elephant Whale (about 0.9 vs 1.0). The distance between humans
and humans and chimp is 0 and the distance between humans and resus monkey
is about 0.9. By the way, the Arabian Camel has the same sequence as the
grey whale.
While these trees are not robust at every branch point and length, the
general pattern of few long, deep branches separating organisms is seen
throughout the literature. We may observe limits to change at the
morphological level, but limits to change at the genetic level do not
seem to be there.
Jim Behnke, Asbury College, Wilmore, KY 40390 jimbeh@ms.uky.edu