The Origins of Form

Ancient genes, recycled and repurposed, control embryonic development in organisms of striking diversity.

sticklebacks

Major difference in the skeletal patterns of closely related populations reflects a difference in how a single tool-kit gene is used. Two forms of the three-spined stickleback fish (Gasterosteus aculeatus), which differ in the size of their pelvic fins, have repeatedly evolved in freshwater lakes. Long pelvic spines protect the open-water form from attack by other fish. Yet in the bottom-dwelling form (lower image), whose principal predator is dragonfly larvae rather than other fish, the tool-kit gene controlling the formation of the pelvic fin has been selectively turned off. Photo by Mike Shapiro and David Kingsley, Stanford University and Howard Hughes Medical Institute; Kingsley lab links: kingsley.stanford.edu & www.hhmi.org/research /investigators/kingsley.html; Stanford Center of Excellence in Genomic Science: cegs.stanford.edu

The implications of that insight are particularly significant for understanding human evolution. We humans have long supposed that we hold some unique position in the animal kingdom. Surely we must be the most genetically well-endowed species. Yet the reality, as molecular biologists now know from sequencing the genomes of our own and other species, is that the genes of human beings are very similar in number and kind to the genes of the chimpanzee and of the mouse—in fact, of all other vertebrates. No one should expect to account for the evolution of bipedalism, language, speech, or other human traits by finding novel genes. A more likely explanation will come from understanding how our “old” genes, shared with other primates, mammals, vertebrates, and more distant animal relatives, have found new applications.

Darwin knew very well the difficulty people would have in picturing how complex structures or “contrivances” arose. In fact, as scholars such as the late Stephen Jay Gould and Randy Moore, a biologist at the University of Minnesota in Minneapolis, have pointed out, Darwin’s choice of the term “contrivances,” which appears fifteen times in Origin, was a deliberate one, used for rhetorical effect. It evoked a term the Reverend William Paley used in his 1802 book, Natural Theology. Paley saw the fashioning of “contrivances” in nature for specific purposes as revelations of God’s design:

Contrivance must have had a contriver; design, a designer. . . . It is only by the display of contrivance, that the existence, the agency, the wisdom of the Deity, could be testified to his rational creatures.

Paley’s argument is the essence of the idea of “intelligent design,” now being touted as a new “alternative” to evolutionary science. Darwin admired Paley’s book, and declared that he had virtually committed parts of it to memory. He then structured much of his argument in Origin as a direct refutation of Paley. Where Paley compared the design of the eye with the design of the telescope, Darwin explained how such contrivances arose by natural selection, without the intervention of a divine contriver.

But Darwin’s explanation, no matter how brilliant, was founded on the extrapolation of natural selection over vast periods of time. He had no access to fundamental knowledge about the development of eyes or their detailed evolutionary history. The new knowledge of tool-kit genes makes it clear how such complex structures are built. Evo-devo makes it possible to connect this everyday, observable, and experimentally accessible process to the long-term process of evolutionary change. Evo-devo shows how complex forms and structures evolve, not only in ways that lead from one species to the next, but also in ways, such as the making of body plans, that have shaped the major differences in the higher taxonomic ranks.

The major tenet of the modern evolutionary synthesis is that the evolution of forms above the species level (“macroevolution”) can be extrapolated from processes operating at the level of populations, within species (“microevolution”). For those who have doubted that the modern evolutionary synthesis could explain macroevolution, the new insights from evo-devo should resolve the question. Through the lens of evolutionary developmental biology, biologists can finally see beyond external forms into the very processes that forge them, completing the picture of how the endless forms of nature have been, and are being, evolved.

 

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