Evolution &LimitsToBreeding

Sun, 16 Nov 1997 06:39:35 -0500 (EST)


On Nov. 12 you posted a note in which you "suggest[ed] a way to explain your
limits to breeding and how evolution can get around these limits."

You continued:

"Morphology is determined by at least two genetic systems. There is the
genetic program for the developing embryo. This is controlled by the HOX
genes. But then there is a more subtle effect of the genetic alleles of the
various traits. The HOX genes control the entire developmental program.
They are the master switches. Gilbert writes:"

"'How, then, can one modify one Bauplan to create another Bauplan? The
first way would be to modify the earliest stages of development....It appears
then, that mutations that create new Bauplan could do so by altering the
earliest stages of development.' " ~Scott F. Gilbert, Developmental Biology
(Sunder land, Mass.: Sunder land, Sinauer Associates, Inc., 1991),p. 831-832.

Here are my comments: I take it you are suggesting two things: 1) a way to
make major changes--from one Bauplan to another, and 2) how to make small
changes within a species or variety, e.g., from a chihuahua to a St. Bernard.

Let's look at the first one. You called on Gilbert for a mechanism of how a
Bauplan can be changed. Let's agree on what a Bauplan is. Valentine defined
it this way: "At the upper level of the taxonomic hierarchy, phyla- or
class-level clades are characterized by their possession of particular
assemblages of homologous architectural and structural features...it is to
such assemblages that the term Bauplan is applied." (Valentine, 1986.) I
agree with this definition and it is the one Wallace Arthur uses in his new
book, *The Origin of Body Plans* (Cambridge, 1997).

(Incidentally, I rely heavily on Arthur. The book is really current with hot
research in the field. I think you would find his book excellent background
in the area of evolutionary developmental biology.)

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).

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.

Gilbert went on to say:

"In this perspective, evolution is the result of hereditary changes affecting
development. This is the case whether the mutation is one that changes the
reptilian embryo into a bird or one that changes the color of Drosophila

According to Fisher's Principle, phenotypic change from a reptilian embryo to
a bird is very large, and the probability of the mutation(s) producing it
being selectively advantageous is low. That is, it's not likely to happen.
On the other end, changing the color of Drosophila eyes is the result of
small mutation(s), and is more likely to be advantageous, and thus more
likely to survive. This suggests that speciation, resulting from small
mutations (microevolution) occurs further downstream from the formation of
the body plan. 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.

Hope this proves to be useful.