By: W.Benson

W.Benson — Tue, 05 Feb 2013 01:58:45 +0000

You have left out one of Darwin’s most interesting finding on pigeons. In his 1842 essay, Darwin speculated that ancestral traits of vertebrates (gill clefts, notochord, etc.) have been retained in early embryonic development because of the mathematics of variation. He suggested that useful variations would in principle appear uniformly along development. Newly evolved structures would emerge gradually and cumulatively and leave the earliest embryonic features intact. As development advanced, ‘mutations’ would continuously kick in until finally the juvenile and adult stages were reached. To test his idea, Darwin, sometimes after 1855, compared metrics of hatchlings and adults of six fancy pigeon breeds to see how much adults differed in body proportions compared to hatchlings. If the body proportions of hatchlings were as different as those of adults, differentiation would have occurred entirely in the embryo. If hatchlings were similar and adults unlike, differentiation would be mostly post-hatching. Although Darwin did not explicitly state his expectation, his ‘constant variation hypothesis’ would suggest that much or most differentiation would have taken place prior to hatching, and nestlings and juveniles would display little more than finishing touches. However, Darwin found something very different. In five of the six breeds, hatchlings were very alike in body proportions compared to the adults of the same varieties. In short, most of the variation available to artificial selection, and by analogy to natural selection, was derived from genes expressed very late in development. The selective usefulness of variation appearing late in development helps clarify embryonic conservatism and, if you stop to think about it, why ‘weak recapitulation’ of the type championed by Ernst Haeckel might occur.

Comments on: How Pigeons Cured My Case of YAGS

By: W.Benson