How do evolutionary novelties arise? The conventional explanation is that the first step is the chance formation of a genetic mutation, which results in a new phenotype, which, if it is favored by selection, may be fixed in a population. No one sensible can seriously argue with this idea—it happens. I'm not going to argue with it at all.
However, there are also additional mechanisms for generating novelties, mechanisms that extend the power of evolutionary biology without contradicting our conventional understanding of it. A paper by A. Richard Palmer in Science describes the evidence for an alternative mode of evolution, genetic assimilation, that can be easily read as a radical, non-Darwinian, and even Lamarckian pattern of evolution (Sennoma at Malice Aforethought has expressed concern about this), but it is nothing of the kind; there is no hocus-pocus, no violation of the Weissmann barrier, no sudden, unexplained leaps of cause-and-effect. Comprehending it only requires a proper appreciation of the importance of environmental influences on development and an understanding that the genome does not constitute a descriptive program of the organism.
Continue reading "Symmetry breaking and genetic assimilation" (on Pharyngula)
5 Comments
PvM · 5 November 2004
Genetic Assimilation in the Laboratory from Scott Gilbert's Devbio companion
Pete Dunkelberg · 5 November 2004
That reminds me of My Fair XX, and just now something lighter might hit the spot.
Salvador T. Cordova · 9 November 2004
Hi PZ,
I enjoy reading many of your postings. I have Mary Jane West-Eberhard's book as well. It was a tedious read.
Symmetry breaking and genetic assimilation is defintiely a compelling example of "genes being followers".
What I found astonishing is bilateral morphological symmetry with the symmetry broken in the developmental process.
For example:
page 335, Nature's Destiny by Denton:
A curious aspect of the development of the namatode and one that would never have been predicted is that although the organism is bilaterally symmetrical--that is, its left and right halves are mirror images of each other--the equivalent organs and cells on the right- and lefthand sides of the body of the larva are not derived from equivalent cells in the embryo In other words, identical components on the right and left sides of the body are generated in different ways from different and nonsymemetrically placed progenitor cells in the early embryo and have therefore lineage patterns which are in some cases completely dissimilar. This is like making the right and left headlight on an automobile in completely different ways and utilizing completely different process.
Even individual cells of the same cell type in any one organ, such as, say, the muscle cells, gland cells, or nerve cells of the pharynx, are also derived from different lineages. For example, one particular cell progenitor of the pharynx gives rise to muscle cells, interneurons, gland cells, and epithelial cells. Another progenitor gives rise to to muscle and gland cells.
timi · 9 November 2004
"This is like making the right and left headlight on an automobile in completely different ways and utilizing completely different process."
I think it's more like making ceramic mugs out of clay taken from different locations. What it tells us is that many of the ealier progenitor cells are not irreversibly committed to one particular fate but can be triggered to develop in particular way by multiple signals.
Great White Wonder · 9 November 2004