>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.
[...]
>There is no evidence that such changes are prohibited by any genetic or
>morphological constraints.
On the contrary, there is abundant evidence of genetic and developmental
constraints, derived from large-scale mutagenesis experiments in such
model systems as Drosophila or zebrafish. Indeed, it is precisely this
evidence (or, rather, lack of evidence, for the heritable [viable]
modification of body plan features) that has led to widespread speculation
about non-uniformities in evolutionary processes -- e.g., to hypotheses that
the genomes and developmental architectures of early metazoans were somehow
"labile," allowing for rapid morphological change on a scale we can no
longer observe experimentally.
For a good discussion of genetic and developmental constraints on
macroevolution, see Brian K. Hall, "Bauplane, Phylotypic Stages, and
Constraint: Why There Are So Few Types of Animals," _Evolutionary
Biology_ 29 (1997): 215-253, ed. M.K. Hecht (New York: Plenum). Wallace
Arthur's recent book, _The Origin of Animal Body Plans_ (Cambridge
University Press, 1997), also contains an extended discussion of the
requirement for heritable [viable] early-acting mutations, of a sort we
do not now observe, to generate novel body plans and structures.
>The fossil record in fact provides substantial evidence for just
>such transitions.
Only on the assumption that these transitions are (or were) accessible
to undirected mutation and selection, which seems to me entirely at issue.
Paul Nelson