Tazzyman Samuel J, Iwasa Yoh, Pomiankowski Andrew
CoMPLEX, , University College London, London WC1E 6BT, United Kingdom; Institute of Integrative Biology (IBZ), ETH Zürich, Zürich 8092, Switzerland; The Galton Laboratory, , Department of Genetics, Environment, and Evolution, , University College London, London WC1E 6BT, United Kingdom.
Evolution. 2014 Sep;68(9):2534-49. doi: 10.1111/evo.12450. Epub 2014 Jun 12.
Why are traits that function as secondary sexual ornaments generally exaggerated in size compared to the naturally selected optimum, and not reduced? Because they deviate from the naturally selected optimum, traits that are reduced in size will handicap their bearer, and could thus provide an honest signal of quality to a potential mate. Thus if secondary sexual ornaments evolve via the handicap process, current theory suggests that reduced ornamentation should be as frequent as exaggerated ornamentation, but this is not the case. To try to explain this discrepancy, we analyze a simple model of the handicap process. Our analysis shows that asymmetries in costs of preference or ornament with regard to exaggeration and reduction cannot fully explain the imbalance. Rather, the bias toward exaggeration can be best explained if either the signaling efficacy or the condition dependence of a trait increases with size. Under these circumstances, evolution always leads to more extreme exaggeration than reduction: although the two should occur just as frequently, exaggerated secondary sexual ornaments are likely to be further removed from the naturally selected optimum than reduced ornaments.
为什么作为第二性征装饰的性状，相比于自然选择的最优状态，通常在尺寸上会被夸大，而不是缩小呢？因为偏离了自然选择的最优状态，尺寸缩小的性状会使其携带者处于不利地位，因此可以向潜在配偶诚实地传递质量信号。所以，如果第二性征装饰是通过不利条件过程进化而来的，当前理论认为，缩小的装饰应该和夸大的装饰一样常见，但实际并非如此。为了解释这种差异，我们分析了一个简单的不利条件过程模型。我们的分析表明，在偏好成本或装饰成本方面，夸大与缩小的不对称并不能完全解释这种不平衡。相反，如果一个性状的信号传递效率或对条件的依赖性随尺寸增加，那么对夸大的偏向就能得到最好的解释。在这种情况下，进化总是导致比缩小更极端的夸大：尽管两者出现的频率应该相同，但夸大的第二性征装饰可能比缩小的装饰更偏离自然选择的最优状态。
