Baby fish working out

An interesting paper has just come out in the Proceedings of the Royal Society by Hu and Albertson (Hu, Y. and R. C. Albertson (2017). Baby fish working out: an epigenetic source of adaptive variation in the cichlid jaw. Proceedings of the Royal Society B: Biological Sciences 284). It illustrates in a nice way the point I am trying to make in Purpose and Desire: gene selectionism has failed as a coherent theory of evolution.

The problem they tackle is the diversity of cichlid fishes. These occur in great variety in the Great Lakes of Africa, Lake Malawi in this case, and they have long been a case study in evolutionary diversification. The Great Lakes contain dozens of species that live in a variety of ways. The trouble is there is not sufficient genetic diversity to account for the diversity in form and function among these fishes.

Hu and Albertson focused on the development of the jaw structure, which is quite diverse among the cichlids. Diverse jaw structure indicates diverse ways these fish can feed, and hence the ecological niches they occupy (the problematic concept of the ecological niche will come up at some time soon). The standard way this diversity is explained is genetic diversification. Here’s the logic. Jaw form is specified by the genes that determine jaw form. As competition among fishes drives fish to exploit one habitat or another, natural selection will favor genes that determine apt jaw structure. The logic falls because there is not sufficient diversity in the genes involved in jaw formation to account for the variation in jaw morphology. Let’s not even bring up how a particular sequence of nucleotides translates into a particular jaw structure.

The remarkable thing that Hu and Albertson found was that the fish embryos “train” their jaws to a particular shape. At the stage of embryonic development, where the jaws begin to mineralize—that is transition from cartilage to bone—the embryo begins to exercise its jaws. This puts the developing jaws under mechanical strain, which affects the pattern of jaw mineralization, which ultimately translates into a particular jaw morphology.

As I wrote in The Tinkerer’s Accomplice in 2007, this is a classic example of homeostasis in action. Bones are governed by homeostasis of strain, and are structured to normalize mechanical strain throughout. By exercising the jaws in this way, these fishes are actively shaping their jaws. Indeed, they are shaping them intentionally (I am defining intentionality very broadly, of course. You can read why I think doing so is justified in The Tinkerer’s Accomplice).

And this is the point I have tried to make in Purpose and Desire. Adaptation, in this instance a form of jaw that is apt for a particular style of feeding, is driven more by intentionality than it is by possessing “apt function genes.” And if adaptation is to play any role at all in Darwinian evolution, it has to be solidly grounded in physiology, not genetics.

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