orthogenesis and natural selection

[John Grehan]

>Appealing to orthogenesis for the _loss_ of a gland has an interesting
>corallary. If orthogenesis is directed to losing the gland, then you
>can hardly appeal to it for the original development of the same gland.

Why not?

>In addition, if natural selection resulted in the development of the
>gland against an orthogenetic trend opposing that development, then
>natural selection must be even more powerful than most people suppose
>it to be.

I don't think that could happen. Natural selection can increase (or decrease)
 the frequency of a character, but if that character were not produced 
in the first place there is nothing for selection to work on (i.e. an
orthogenetic trend opposing that development is to say that the character
is not present in the first place).

>
>	I don't see how you can have it both ways. If orthogenesis is
>the explanation for the loss of a feature, then natural selection (or
>a _third_ mechanism??) must have produced that feature. And vice versa.

As above, orthogenetic trends may explain both the spread or loss of a
character. Selection is another mechanism with similar effect. My point was
that there are
two possibilities to consider. Whether one or both interacted in the
status of the gland in question is an open question here.

If orthogenetic processes such as molecular drive are prevalent, selection
processes are interacting with variation that has a life of its own so to
speak
(rather than the generation of variation being purely random). Such a
situation would have different implications for understanding the origin of
both adaptation and taxagenesis.

John Grehan