Re: Evolution &LimitsToBreeding

Karen G. Jensen (
Wed, 19 Nov 1997 19:03:33 -0600

Sun, 16 Nov 1997 17:10:53 -0600 Glenn Morton wrote:

> I was saying that alterations to
>the embryonic development program are the means by which major morphological
>change occurs (albeit rarely) and that the limit to change that Karen Jenson
>was suggesting is due to a limit to the shuffling of the alleles which is
>not per se evolutionary change.
A good case can be made for alterations in embryonic development
influencing speciation. It is not change of bauplan, but quite dramatic.

For example, the differing patterns of stripes on different zebra species:
"Timing of embryonic events apparently controls the stripe pattern in some
zebras. The stripes on the lower back of the zebra Equus burchelli are
widely and irregularly spaced, as the result of differential growth rates
of the embryo after the stripe pattern is established. The stripe pattern
in Equus grevyi is not established until after that differential growth is
completed, and consequently the stripes in the adult of this species are
more equally spaced.
(Figure 5 [shows the two developmental sequences])." (see Alberch, 1985,
Systematic Zoology 34:46-58).

This may explain some fossil sequences also:
"Allometry, or differential growth, has been proposed to explain the
differences for example, between several species of fossil titanotheres
(Figure 6 [shows a size and head-shape series]). The change from one
species to another is simply increased overall size, and proportionally
faster growth of the horn and certain other facial features. (see Futuyma
1986, Evolutionary Biology p. 368)
(GRI's Origins 20(2)60-82, p.74, 1993).

This is more than shuffling of alleles. I think you would call it
"evolutionary change". But I call it speciation, not major morphologic
change in the sense of evolutionary transformation.