The most perfect thing, explained

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Science  23 Jun 2017:
Vol. 356, Issue 6344, pp. 1234-1235
DOI: 10.1126/science.aan2517

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Every bird egg serves the same function: to protect and nourish the offspring within while it grows from two cells to a fully formed chick. Yet this identical function is served by a striking diversity of egg shapes. Explanations for both the origin and function of this diversity have remained little more than anecdotal (1). On page 1249 of this issue, Stoddard et al. (2) marry biophysics and ecology to provide a general theory that explains how and why diverse egg shapes arose. Based on a mathematical model, the authors predict that simple changes in the forces experienced by the shell membrane as the egg develops in the female's oviduct are sufficient to generate the observed egg-shape diversity across all birds. The selection pressure that best explains its evolution comes from the characteristic we most associate with birds: flight.