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Scott Hartman wrote:
Sorry I'm a day late, but I had to jump in here; macroevolution is NOT
simply "large scale changes" (and microevolution is not always less
phenotypic change than macroevolutionary events).
Well, yes and no. Macroevolution can be large scale phenotypic changes. It
doesn't *have* to be, but it *can* be. Since macroevolution can refer to
any evolutionary change at or above the level of species, it also can go as
high as "classes" or "phyla". Yes, we're back to outdated Linnean ranks and
typology. But the modern neo-Darwinian synthesis of evolution posits that
the mechanism behind micro- and macroevolution is identical, and that the
only difference between the two is time and scale (with the latter usually a
product of the former). That's the rationale I was using. Body plans (and
mosaic evolution) are therefore within the domain of macroevolution.
The distinction arose out of the days post Darwin when many biologists
thought that natural selection could not alone account for speciation, let
alone large scale changes. Of course, back in the day of asking questions
like how one "class" of vertebrates evolved from another "class", this
meant that large phenotypic changes were indeed the domain of
Yes, this is the typological concept of macroevolution. I was using a more
modern synthetic version.
But today biologists generally accept that large changes in body plan
result from the accumulation of many small changes over time; there is
scant evidence for even a weak version of
Goldschmidt's "Hopeful Monster" form of phenotypic saltation.
Aside from homeotic events. Take the different number of segments in
centipedes, for example. Segments appear to be added or subtracted as a
whole, not gradually. Same for numbers of teeth in the jaw, number of
remiges on a wing, number of phalanges in a digit, etc.
Other such changes may involve shape, not number. The shape of the
articular surfaces of caudal vertebrae in titanosaurs is a good example.
Titanosaurs across the board display opisthocoelous, procoelous,
amphicoelous and biconvex centra. The most parsimonious explanation behind
this diversity in caudal morphology is that the front and back surfaces
could switch between one and the other conditions (e.g., amphicoelous or
opisthocoelous directly from procoelous), without always having to pass
through intermediate morphologies (e.g., weakly procoelous, amphiplatyan