Data from: Factors leading to the evolution and maintenance of a male ornament in territorial species

Male ornamentation is assumed to have evolved primarily from selection by female mate choice. Yet this is only one possible reason for ornament evolution. Ornaments might also be useful in aggressive competition by improving opponent assessment between males, or they might function to enhance signal detection by making males more conspicuous in the environment. We tested both these ideas in territorial Anolis lizards in which female choice is either absent or secondary to males competing for territories that overlap female home ranges. Male tail crests only evolved in species in which territory neighbors were distant, consistent with the signal detection hypothesis. Once the tail crest had evolved, however, it seems to have become a signal in itself, with variation in the frequency and size of tail crests within species correlating with variables predicted by the aggressive competition hypothesis. Our study presents an apparent example of a male ornament in which the selection pressure leading to variation among species in ornament expression is different from the selection pressure acting on variation within species. The Anolis tail crest is therefore likely to be an exaptation. We caution that conclusions made on the evolution of male ornaments are dependent on the phylogenetic perspective adopted by a study. Studies restricted to single species are useful for identifying selection pressures in contemporary settings (i.e., the current utility of traits), but may lead to erroneous conclusions on the factors that initially lead to the origin of traits.

雄性装饰性状的演化通常被认为主要源于雌性配偶选择的选择压力，但这仅为装饰性状演化的潜在动因之一。装饰性状还可能在雄性间的攻击性竞争中发挥作用，帮助双方更精准地评估对手；或是通过提升雄性在环境中的显眼程度，强化信号检测效率。
我们以领地性安乐蜥（Anolis lizards）为研究对象检验了这两种假说。该类物种的雌性配偶选择要么缺失，要么让位于雄性为争夺与雌性活动范围重叠的领地而展开的竞争。研究发现，雄性尾冠仅在领地邻域间距较大的物种中演化出现，这一结果与信号检测假说相符。然而，尾冠演化形成后，其自身逐渐成为一种独立信号：物种内部尾冠的出现频率与大小变异，与攻击性竞争假说所预测的变量呈现显著相关。
本研究揭示了一个典型案例：驱动雄性装饰性状物种间表达差异的选择压力，与作用于物种内部性状变异的选择压力并不相同。因此，安乐蜥的尾冠很可能是一种预适应性状（exaptation）。
我们需要提醒的是，针对雄性装饰性状演化得出的结论，取决于研究采用的系统发育视角。仅针对单一物种开展的研究，虽有助于识别当代环境下的选择压力（即性状的当前功能），但可能会在解释性状起源的初始动因时得出错误结论。