>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
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
Only on the assumption that these transitions are (or were) accessible
to undirected mutation and selection, which seems to me entirely at issue.