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

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Genetic Triggers for Developmental Change
These changes in timing, known collectively as heterochrony, have proved to be numerous and significant. But Gould knew very well that the ultimate explanation for heterochrony would be found not in metaphorical radio knobs but in the genes whose effects those knobs represented. Around the time that Ontogeny and Phylogeny was published, biologists began to isolate genes involved in development for the first time. Since then they’ve gotten a much better look at how these genes send signals that trigger other genes, and how they induce embryonic cells to proliferate, die off, crawl to new locations or stick together.

Developing Drosophila embryo expresses the hairy gene (dark bands) over the course of its first four hours of life. Four stages of this expression are shown in the sequence above. Different species turn on the gene at slightly different times. In frame D, an arrow marks the furrow that will eventually separate the head from the rest of the body.

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.

• Drosophila embryo images courtesy of Junhyong Kim, University of Pennsylvania.

An interesting note on Hox genes.