Dances with fruit flies
The rapidly improving techniques of molecular biology made that possible, and evo devo--the study of how the development process creates animal bodies and changes them over time--was born. In Endless Forms , Carroll traces the development of the field from its earliest days, starting with the first revelation in 1983 of how a small set of genes, called Hox genes, controls development in fruit flies. That seemingly arcane triumph took on much greater significance when geneticists found the same genes--doing the same jobs--throughout the animal kingdom.
That discovery pointed to the deep connections between all animals and helped scientists work out the complex details of development. But it also raised a new problem: If key developmental genes have remained largely unchanged throughout evolutionary history, how can we account for all the differences, great and small, between different species? In some cases, the answer turned out to be a process of duplication and innovation. Several key points in animal evolution are associated with an increase in the number of developmental genes. The resulting redundancy in the control system, Carroll says, allowed evolution to "experiment" with new developmental programs and body forms, allowing vertebrates to develop distinctive features like skulls and jaws, for example, without hopelessly messing up other body parts.
Color study. But those changes can explain only a subset of biological innovations. A much more pervasive mechanism for evolution of body form, Carroll says, lies in yet another level of genetic control. Every cell in the body contains all of an animal's genes, but individual cells use only a fraction of those genes at any given time. The difference between a white blood cell and a muscle cell, for example, lies in where and when specific groups of genes are "turned on" during development. That process is controlled by a complex series of genetic "switches" that regulate the timing and location of gene activity with remarkable precision.
Which is where the dancing fruit flies come in. All fruit flies have the genes needed to make wing spots, including a gene for black pigment called, confusingly enough, Yellow. That gene is turned on at low levels throughout all fruit fly wings, but only male biarmipes flies have the characteristic spot. Writing recently in Nature, Carroll's research team reported finding mutations in a genetic switch for Yellow in biarmipes flies that allow a finer level of control; one part of the switch keeps gene expression low throughout the wing, while another cranks up expression at the tips, creating the characteristic spot.
It's just one small step in the twisting path of evolution, of course. But it's not hard to see how many such changes in gene switches--accompanied by even small survival advantages such as females who prefer spotted mates--could lead over time to a new species, with little change in the genes themselves. It's a principle that is found again and again throughout the animal kingdom, Carroll says, and one that should help solve one of the greatest biological mysteries of recent years.