Evolution and Development for the 21st Century: Stephen Jay Gould
With the fall of Ernst Haeckel's Biogenetic Law in the 1920s, the evolutionary study of embryos receded into the intellectual backwaters for decades. Haeckel's notion that ontogeny recapitulates phylogeny was deeply flawed, but it was at least straightforward. The few researchers who tried to carry on the study of embryos and evolution proposed a confusing jumble of different kinds of evolutionary change for which they invented a jumble of hideously confusing names such as paedomorphosis, proterogenesis, and phyloembryogenesis. Most embryologists chose instead to focus on understanding how embryos develop a formidable question in itself without thinking much about the evolutionary implications of their work. Meanwhile, evolutionary biologists concentrated much of their efforts on the blossoming field of genetics.
Evo meets devo again
Genetic triggers for developmental change
At the dawn of this new scientific age, Gould predicted that heterochrony and similar evolutionary changes would not be directed by the genes that actually build various body parts. Instead, the genes that regulate other genes would hold the key to the evolution of embryos. His prediction has now been borne out. In 2000, for example, Junhyong Kim and his fellow Yale biologists compared the timing at which a crucial developmental gene (see photos, right) became active in the fruit fly, Drosophila melanogaster, and two closely related species, D. simulans and D. pseudoobscura. They found that the gene started to make its proteins 24 minutes later in D. pseudoobscura than D. melanogaster. Meanwhile, D. simulans gets a head start: its gene becomes active 14 minutes earlier. And that change led to differences in their anatomy even though the developmental gene itself is identical in all three species.
As scientists have begun to isolate these regulatory genes, they've been shocked at how powerful they are and how long they've been in power over the course of evolution.
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Gould image courtesy of Jon Chase/Harvard News Office, © 1997 President and Fellows of Harvard College; Salamander image (Pseudotriton ruber ruber) © 2002 John White; Axolotl (Ambystoma mexicanum) image © 2003 Jessica Miller; Drosophila embryo images courtesy of Junhyong Kim, University of Pennsylvania
Understanding Evolution © 2016 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California