Gene Expression

Genetic conflict in fish

Ancient and continuing Darwinian selection on insulin-like growth factor II in placental fishes:

…We found that IGF2 is subject to positive Darwinian selection coincident with the evolution of placentation in fishes, with particularly strong selection among lineages that have evolved placentation recently. Positive selection is also detected along ancient lineages of placental livebearing fishes, suggesting that selection on IGF2 function is ongoing in placental species. Our observations provide a rare example of natural selection acting in synchrony at the phenotypic and molecular level. These results also constitute the first direct evidence of parent-offspring conflict driving gene evolution.

I’ve discussed genomic imprinting before. Though not restricted to mammals, much of the work has been done on this taxon simply because gestation increases the window during which such conflicts could occur and the relatively high investment the mother makes into the development of the embryo/fetus. John Timmer at Ars Technica covered this paper in detail several weeks ago.

But I think this study is also interesting in regards to its big picture implications. In his book The Ancestor’s Tale Richard Dawkins threw in his lot with paleontologist Simon Conway Morris in agreeing that the power of selection results in broad convergent trends across taxa which can overcome contingency and phylogenetic constraint. This is in explicit contrast to Stephen J. Gould & fellow travelers, who suggest that if one could rewind the clock the small initial chance differences could result in a radically different proliferation of the tree of life. This study shows that when “placental” structures emerge in widely disparate taxa, the same selective pressures drive the forms of genetic conflict which seem so counterintuitive to evolutionary biologists when they were first encountered. Of course, this does not mean that Dawkins & Conway are right and that Gould was wrong, evolutionary biology is the science of variation, and selection does not operate over infinite populations and times. My own perspective is not to constantly adduce whether one philosophical meta-scientific perspective is correct or not, but rather evaluate the dynamics on a case-by-case basis. As it is over time one may converge upon a overall tendency in terms of the tug of war between deterministic selection and stochastic processes, but the real enterprise of science is to character the full distribution.


  1. #1 TGGP
    July 28, 2007

    I’m reading the ancestors tale right now (I got it at the same time as World on Fire which I finished in a sitting and the Shia Revival based on your recommendation) and just passed through the part where placental, marsupial and monotremes converged. I thought all fish had eggs, though they are fertilized outside the mother.

  2. #2 windy
    July 30, 2007

    I thought all fish had eggs, though they are fertilized outside the mother.

    Even the live-bearers do have eggs (internally fertilized, of course). The simplest type of live-bearing is to retain the eggs in the body until they hatch without providing any additional nutrients (‘ovoviviparity’). The fish Reznick & co studied have evolved placenta-like structures, but the embryos attached to the placenta still look a lot like “normal” fish eggs with yolk:

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