The closing words of Darwin’s Origin of Species are probably the best known passage in all of biology: “There is a grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved”.
Darwin deliberately contrasted predictable cycles with the endless change of biological systems. How does novelty arise in evolution? Genetic variation alone seems insufficient. How do the nearly-identical genomes of bonobos, chimpanzees and humans give rise to such different organisms? The answer, as Darwin himself suspected, lies in the dynamics of development of organisms from embryos to adults.
In Endless Forms Most Beautiful (Norton, 2005), University of Wisconsin biologist Sean Carroll explains the mechanisms of animal development with breathtaking clarity. Some of the key discoveries come from his own work on developmental differentiation of the striking patterns of butterfly wings.
Carroll emphasizes three hallmarks of evolutionary innovation: (1) evolution works by modifying structures and processes that are already present, not by creation de novo; (2) structures and processes in organisms are multifunctional and partially overlapping, opening the possibility for differentiation through specializing or reorganizing the division of labor; and (3) organisms are modular, opening the possibility of changing the number of modules or the functions of individual modules. The underpinnings of modularity are modular geography of embryos, and modular genetic switches which allow evolutionary change to occur in one part of the organism, independent of other parts. He writes:
We have seen that insects, pterosaurs, birds or bats did not invent wing genes, butterflies a spot gene, or humans a bipedalism or speech gene. Rather, innovation in all these groups has been a matter of modifying existing structures and teaching old genes new tricks.
The key to innovation at the genetic level is the multifunctionality of tool kit genes. The multifunctionality of tool kit genes stems from their deployment at different times and places through batteries of genetic switches. In this manner, a protein such as Distal-less can act at one time to promote limb formation, and at another to promote eyespot development. The protein made each time is identical, so the difference in function is due to its action on different switches in these different contexts.
At an anatomical level, multifunctionality and redundancy are keys to understanding the evolutionary transitions in structures . . .The history of these structures also illustrates how “endless forms” evolve through cycles of invention and expansion. New structures open up new ways of living. The insect wing led to the evolution of dragonflies and mayflies, butterflies and beetles, fleas and flies, and more. The expansion of these groups was catalyzed in turn by a cycle of innovation and expansion by making modifications to the wings or body plan . . .
Why are existing body parts and genes the more frequent pathway to innovation? This is a matter of probability. Variation in existing structures and genes is more likely to arise than are new structures or genes, and this variation is therefore more abundant for selection to act upon.
The tool kit genes central to evolutionary innovation have been conserved through about 500 million years of animal evolution, and are found across the animal kingdom. Paradoxically, the fundamental mechanisms of evolutionary innovation have been strongly stabilized over eons of time.