Evolution and Development for the 21st Century:
Stephen Jay Gould (1 of 2)

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.

Stephen Jay Gould

Evo Meets Devo Again
More than anyone else, the Harvard paleontologist Stephen Jay Gould (left) drew attention back to embryos as evolutionary time capsules. In his landmark 1977 book Ontogeny and Phylogeny, Gould documented the history of scientific research that had led to so much confusion. But he also demonstrated that the wealth of cases could be organized by some simple principles. Imagine that the timing of development is controlled by two knobs like you’d find on a radio. One controls the rate at which an organism grows. The other controls the rate at which it changes shape over time. Random mutation may end up changing the settings of each knob, thereby speeding up or slowing down the rate at which a species’ embryos develop. These kinds of adjustments can alter the entire body of an organism, or individual organs.

Different timing in development of salamander and 
Adult salamander and axolotl
If evolution had slowed the rate of shape change of a salamander, but kept everything else the same, we would have ended up with the axolotl.

• 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.

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