Darwinian and panbiogeographic frameworks

[Don McAllister]

"Stuart G. Poss" wrote:

> Carmine Colacino wrote:
>
> > That is NOT the meaning orthogenesis had in continental Europe.
> > For some strange reason, nobody went to check out what it actually means,
> >

Here are some science dictionary definitions of orthogenesis from either side of
the pond.

The New York Collegiate Dictionary of Zoology by Pennak:
1. straightline evolution, 2 the tenancy for certain organisms to evolve
consistantly along restricted evolutionary paths over a long period of time toward
a "predetermined" goal, e.g. the fossil history of horses.

The Oxford Concise Dictionary of Zoology by Allaby:
Evolutionary trends that remain fairly constant over long periods of time and so
appear directly from ancestor organisms to their descendants.  This was once
explained as the result of some internal directing force or "need" within the
organims themselves. Such  metaphysical interpretations have been displaced by the
concepts of orthoselection and species selection. See also aristogenesis, entelech;
and nomogenesis. [Orthoselection = A primary selective pressure of a directional
kind, which results in a self-perpetuating evolutionary trend. Species selection
via the effect hypothesis, which has been advanced as an alternative explanation
for such trends.  See also Dollo's law. ] [Aristogenesis: An outmoded theory
holding that evolution proceeds along a predetermined path. (The modern view is
that natural selection does not direct evolution towards any particular kind of
organism or physiological attribute, nor is there any mysterious inner guiding
force)].

The Cambridge University Press Dictionary of Ecology, Evolution and Systematics by
Lincoln, Boxshall and Clark:
Evolution continuously in one direction over a considerable amount of time;
commonly used with the implication that the direction is determined by a factor
internal to the organism or, at least, not determined by natural selection;
aristogenesis; entelech, orthogenetic, cf. rectilinear evolution.

I can see circumstances where selection would result in evolution in one direction.

A piscivore with few gill rakers, disperses into a lake with little planktivore
competition, and steadily evolves more numerous, slender and longer gill rakers
with which plankton food can be sieved out.  This appears to have happened in a
number of lineages of fishes. But, the number of rakers seems to reach a number of
rakers beyond which no further raker additions occur.

A mollusc, in the presence of a shell-crushing predator, would tend to evolve a
thicker shell.  That thicker shell might provoke an evolutionary response in the
predator of stronger crushers, and the latter a yet thicker shell. Those trends
also will probably reach some limit.  And the mollusc can also respond by making a
shell not thicker but structurally stonger with a "better" mechanical design.

don
Don McAllister