左图的这只飞蝇是一个谜。
按道理说，它应当进化出像黄蜂一样的外表，就像右图看到的一样，从而能够更好地避开那些饥饿的小鸟。
但是许多食蚜蝇科的成员（图中的这两只都是）却仅仅是依稀相似于那种带刺的昆虫。
科学家曾推断，这些不精确的模仿是因为它们在同时拷贝多个物种的外貌，或者人类所看到的这些缺陷是鸟儿无法辨识的。
为了验证这些以及其他解释，研究人员分析了38种食蚜蝇，以及10种蜜蜂和黄蜂。
在对昆虫的身体尺寸及颜色，还有它们的丰度及糊弄人类和鸟类的本事进行了统计学分析之后，研究人员放弃了大多数已有的解释。
实际上，加拿大卡尔顿大学的昆虫学家Heather D.Penney和同事在3月21日出版的《自然》杂志上报告说，答案归根结底为昆虫的大小——大的食蚜蝇有最棒的模拟，而小的、像家蝇大小的食蚜蝇则最差。
研究小组推断，由于鸟类倾向于吃掉那些最大的飞蝇，因此小的飞蝇仅仅是因为缺乏足够的压力而无法进化出完美的伪装。（生物谷 bioon.com）
doi:10.1038/nature10961
PMC:
PMID:
A comparative analysis of the evolution of imperfect mimicry
Heather D. Penney, Christopher Hassal,Jeffrey H. Skevington,Kevin R. Abbott，Thomas N. Sherratt
Although exceptional examples of adaptation are frequently celebrated, some outcomes of natural selection seem far from perfect. For example, many hoverflies (Diptera: Syrphidae) are harmless (Batesian1) mimics of stinging Hymenoptera2. However, although some hoverfly species are considered excellent mimics, other species bear only a superficial resemblance to their models3 and it is unclear why this is so. To evaluate hypotheses that have been put forward to explain interspecific variation in the mimetic fidelity of Palearctic Syrphidae we use a comparative approach. We show that the most plausible explanation is that predators impose less selection for mimetic fidelity on smaller hoverfly species because they are less profitable prey items. In particular, our findings, in combination with previous results, allow us to reject several key hypotheses for imperfect mimicry: first, human ratings of mimetic fidelity are positively correlated with both morphometric measures and avian rankings, indicating that variation in mimetic fidelity is not simply an illusion based on human perception4; second, no species of syrphid maps out in multidimensional space as being intermediate in appearance between several different hymenopteran model species, as the multimodel hypothesis5 requires; and third, we find no evidence for a negative relationship between mimetic fidelity and abundance, which calls into question the kin-selection6 hypothesis. By contrast, a strong positive relationship between mimetic fidelity and body size supports the relaxed-selection hypothesis7, 8, suggesting that reduced predation pressure on less profitable prey species limits the selection for mimetic perfection.
