Re: Evolution &LimitsToBreeding

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

Hi Bob,

At 06:39 AM 11/16/97, wrote:

>Can Bauplans be changed by the method Gilbert suggested?--"mutations that
>create new Bauplan could do so by altering the earliest stages of
>development"? To answer this question Arthur calls on a little known or used
>principle, which he calls the "Fisher Principle". Fisher (1930) stated that
>there is a negative correlation between a mutation's magnitude of phenotypic
>effect and its probability of being selectively advantageous. Arthur
>interpreted the principle as follows:
>"It seems reasonable to suppose that genes controlling early developmental
>decisions, such as which end of the embryo is anterior, will be subject, on
>average, to mutations with more major phenotypic effect than genes
>controlling later developmental process, such as the production of mammalian
>hair....Those genes that control key early developmental processes are
>involved in the establishment of the basic body plan. Mutations in these
>genes will usually be extremely disadvantageous, and it is conceivable that
>they are *always* so." (pp. 13-14, his emphasis).

I need to get Arthur's book.Can you give me a full reference? I would
comment that it is quite a distance from "conceivable that they are always"
harmful to they are ALWAYS harmful. It is conceivable that every Our
conceptions are not good evidential data. I would have to agree that most
such mutations are going to be very harmful, but that does not mean all.
And further I would disagree that bauplan in the sense that Gilbert used it
only applies to phyla and classes. There is a bauplan for horse vs. cow.
While the change is less between these two bauplans than for that between
classes, I don't see a reason to limit what Gilbert is saying to some
arbitrarly limit.

>If Fisher/Arthur's perspective is correct, Gilbert's proposal is not likely
>to work. Change of one body plan to another probably does not occur.

Big if. One cannot use an unproven limit to prove a limit. This is begging
the question. there is no doubt that such mutations to fundamental bauplan
are very rare but I am not sure you can rule it out because of observations
which have taken place only over the past 100 years.

I think the truth is that we do not know or understand how
>body plans were formed. To try to shoe horn their formation into the
>microevolutionary paradigm, is counterproductive IMHO, because it forestalls
>the search for alternate paradigms. We need some new concepts regarding the
>formation of body plans.

You misunderstood what I was suggesting. I was not trying to shoehorn major
morphological change into microevolution. 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.


Foundation, Fall and Flood