Re: Limits of Kinds

Keith B Miller (
Thu, 20 Nov 1997 22:52:55 -0600 (CST)

The whole discussion on limits to change seems misdirected. People talk as
though logical limits to change in a _particular direction_ (eg size) thus
imply limits to evolutionary change in the sense of refuting common
descent. This argument is invalid. What is really at issue is whether
organisms can continue to respond to changing selective pressures. Can a
fish respond to environmental pressures to improve locomotion in shallow
muddy bottoms? Or can they improve their ability to absorb oxygen from the
air under low oxygen conditions or forced periods of subaerial exposure?
Can these changes lead to eventual adaptation to a terrestrial environment?
There is no evidence that such changes are prohibited by any genetic or
morphological constraints. The fossil record in fact provides substantial
evidence for just such transitions. Improved understanding of the
regulatory genes responsible for limb formation provide insight into which
gene changes would have been responsible.

Organisms evolve within the constraints provided by their existing geneitic
and morphological characteristics, and by physical law. Organisms cannot
evolve in any arbitrary direction, but only in ways which are open to them.
This is what is referrred to as contingency. The morphology of an
organism is determined by its past history.


Keith B. Miller
Department of Geology
Kansas State University
Manhattan, KS 66506