“Return of the embryo” and “evo-devo” may sound like sci-fi kitsch, but they are central to new developments in evolutionary thought.
University of Hawaiʻi at Hilo Professor of Philosophy Ron Amundson writes about them in the Charles Darwin commemorative issue of the popular French science magazine La Recherche. His article, The Embryo Returns: Evolutionary Developmental Biology appears in the November 2008 issue.
“The basic doctrine is simple,” Amundson writes. “According to evo-devo, the only way that a species can evolve (that is, the only way they can modify their adult bodies) is to change their embryological processes. So evolution turns out to be changes in embryological development.”
It’s not really a new concept. Nineteenth century evolutionists understood heredity in terms of embryonic development. Darwin himself observed that similarities in embryonic forms of organisms persist the longest in the most closely related species. (He called it his “pet bit” for evidence of evolution.)
What embryologists lacked was data to explain these differences in development, Amundson says.
Along came classical genetics with a new concept of heredity that bypasses embryological development.
Twentieth century evolutionists combined classical genetics with other fields to design the Modern Synthesis, which focuses on the genetic makeup of interbreeding populations. The founders doubted that any important genes were shared between species, and developmental biology was dismissed as irrelevant to evolution.
Then, in the 1990s, molecular biology identified the same genes operating in different species. The first was Pax6, a sort of genetic master switch discovered to operate in eye development in fruit flies and humans. It was later found to do this job in virtually all complex organisms.
Additional examples were soon identified. Homeobox genes organize the different sequence of body segments in species as different as squid and humans, for example. FoxP2 influences both learning of songs in birds and language acquisition in humans.
UH scientists at Kewalo Marine Laboratory discovered genes that were shared by primitive acoel flatworms and vertebrates. The genes cause the development of the vertebrate anus and the acoel gonopore, a reproductive opening not connected to the gut.
Amundson points to the work of Kewalo’s Mark Martindale and his colleagues as cutting-edge evo-devo.
The discoveries gave new life to evo-devo as the “second synthesis.” It combines the evolutionary biology study of phylogeny (species’ descent from ancestral species) with the developmental biology study of ontogeny (the change in an organism’s body over its lifetime).
“Few advocates of evo-devo now challenge natural selection, and Modern Synthesis advocates have a difficult time challenging the molecular evidence for evo-devo,” Amundson concludes.
“I have a feeling, however, that the debates are not over. Evo-devo and population genetics are still unsynthesized. We still must find a way to think about natural selection as operating on populations of ontogenies, rather than simply on populations of organisms. Embryos are once again at the center of attention in evolutionary biology,” he says.
Amundson’s extensive publications include his 2005 book The Changing Role of the Embryo in Evolutionary Thought: Roots of Evo-Devo (Cambridge University Press) and a chapter on evo-devo in the 2008 Blackwell’s Companion to Philosophy of Biology.