taxa and centers of origin

[John Grehan]

>Let's say in one point on earth a new species arose in some unique,
>peculiar, extremely localized "centre of origin".

Its own range is its immeadiate center of origin, so ok.

>Let's suppose that this species has a very poor dispersing power.
>
>It would be worth to calculate it through, but I think even under those
>circumstances (centre of origin, low dispersion) the species would quickly
>fill out the whole space suitable as habitat, until coming to some geological
>or ecological limit line.

Maybe, maybe not. Biologists often represent dispersal ability as a
quality that is internal to the organism (ie is a spatiotemporally unrestricted
quality), but biogeography may suggest that dispersal ability is a
contingent factor - it depends on particular organism-environment
interactions.

>What I want to say is the following: Let dispersion be as slow as it may be, it
>will still be very much faster than tectonic events at a geological time scale.

Again, maybe or maybe not. There are taxa wioth restricted distributions
that correlate with Mesozoic geology. Trouble with the idea of spreading
out to the limits of habitat is that the limits are defined by the distribution
itself, so can be circular in reasoning.

>Or in other words: Maybe Croizat's idea of wide-spread ancestors does not
>contradict the assumption of small centres of origin.

It does not contradict the idea that a widespread species may have had a
formelyr restricted range. Croizat's view is, however, that the "center of
origin" for a series of related vicariant taxa is more usually the combined
range of those taxa than a restricted part of it.


. At the point of time when the ancestor is
>widespread, he is so due to the result of dispersion which has already
>happened before.

Yes. Dispersal is the process that leads to widespread ranges in times
of mobilism.

>But the time between origin and reaching maximal distribution might be
>very small when compared with the geological time scale.

Yes it might
>
>But, what is orthogenetics?

Orthogenesis is the concept of evolution taking place in a concerted
fashion without natural selection being required to establish a character
through differential reproductive success. Orthogenetic generation of
characters will result in evolution so long as the characters are either
neutral with respect to reproductive success, or increase reproductive
success.

John Grehan