NOTHING EVER SPECIATES:

Around 20 years ago, biologists expected genetics and environmental factors to produce substantial heterogeneity, giving natural selection plenty of choice, said Alex Lancaster, an evolutionary biologist at the Ronin Institute in New Jersey who wasn't involved in the new study. But, he said, more recent observations have attested to unexpected similarity across populations.

Over dinner with co-author Richard Carthew a few years ago, Mani started thinking about pinning down exactly how much and in what ways individuals can physically vary. In other words, what options exist within the constraints of embryonic and postnatal development? For a growing organism, "What's on the menu?" Mani wondered. "People like to say that science is some sort of method, but it started with just that childish, harebrained question."

Prior studies relied on a few points of comparison on an organism, but Mani, Carthew and their co-authors Vasyl Alba and James Carthew (Richard Carthew's son and a member of his lab) wanted more thorough information. They knew that, when sequencing genomes, algorithms line up sections of the genetic code for comparison. In the same way, the scientists decided to align the wings of common Drosophila melanogaster fruit flies.

Their thousands of wing photos resemble line drawings with dark veins standing out from light backgrounds. A computer program standardized these images, fitting each into a circle while preserving features such as the angles of the veins, and digitally stacked the thousands of pictures on top of one another. This gave the researchers around 30,000 points of comparison -- every pixel in that stacked image -- instead of a few dozen crude landmarks. "It's more comprehensive," said Lancaster. "It catches features that wouldn't normally be caught."

Unexpected simplicity emerged from this rich data. The scientists saw a narrow range of possible appearances for the wings, which mostly diverged in a small set of characteristics. The variation was concentrated near the hinge of the wing and showed up in a few particular spots, such as the shape of the frontmost vein. Moreover, these variable traits were linked: When one of the traits on a wing was far from the average, the other traits usually were, too. This was true no matter which genetic or environmental modifications that fly experienced, implying that these factors individually have very limited influence.

Richard Carthew had anticipated that more of the flies' developmental complexity would be captured in their physical forms. That the variation was all funneled into a short list of menu options is "quite a marvelous thing," he said. As flies grow into adults, they have "this magical ability to correct for differences and create a very robust final form."

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