天擇在自然環境中如何正向篩選警戒訊號？

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photo from Fig. 1
標題：Warning signals are under positive frequency-dependent selection in nature.

 摘要

 Positive frequency-dependent selection (FDS) is a selection regime where the fitness of a phenotype increases with its frequency, and it is thought to underlie important adaptive strategies resting on signaling and communication. However, whether and how positive FDS truly operates in nature remains unknown, which hampers our understanding of signal diversity. Here, we test for positive FDS operating on the warning color patterns of chemically defended butterflies forming multiple coexisting mimicry assemblages in the Amazon. Using malleable prey models placed in localities showing differences in the relative frequencies of warningly colored prey, we demonstrate that the efficiency of a warning signal increases steadily with its local frequency in the natural community, up to a threshold where protection stabilizes. The shape of this relationship is consistent with the direct effect of the local abundance of each warning signal on the corresponding avoidance knowledge of the local predator community. This relationship, which differs from purifying selection acting on each mimetic pattern, indicates that predator knowledge, integrated over the entire community, is saturated only for the most common warning signals. In contrast, among the well-established warning signals present in local prey assemblages, most are incompletely known to local predators and enjoy incomplete protection. This incomplete predator knowledge should generate strong benefits to life history traits that enhance warning efficiency by increasing the effective frequency of prey visible to predators. Strategies such as gregariousness or niche convergence between comimics may therefore readily evolve through their effects on predator knowledge and warning efficiency.
photo from Fig. S1

面對擬態這個自然現象時，科學家都會說「因為同享這個斑紋的物種被捕食者避免，所以擬態斑紋是天擇後留下來的結果」，也就是說，天擇偏好牠們，所以留下來了，可是其實只有很片面的證據支持，有的時候是實驗室內的行為實驗，有的時候是區域性的生態觀察，所以「天擇偏好擬態的性狀」這件事其實是結果，科學家對天擇如何偏好這樣的性狀知道的還很少。

 秘魯北方的數個區域的毒蝶(Heliconius)提供研究這個假說的環境。作者利用1997-2013年的採集資料判斷出幾個區域，這幾個區域的毒蝶（見上圖一）佔優勢的擬態型都不盡相同，而且每個區域佔優勢的型每年出現的頻率都很穩定。作者測試如果有其他的擬態型出現在當地的話，應該會被捕食者所篩選的假設，利用製作非常精巧的人工假餌（印製精確，經過測試反射光譜與套用捕食者模型後，知道看起來可能的模樣），在各地區設置6-8公里的穿越線，放置480-600個假餌（120/型，4-5型/區域）。

photo from fig. 2
結果顯示如果不是當地佔優勢的擬態型，被攻擊的頻率就是會比較高（圖二）。但有趣的是不佔優勢的擬態型不會無限制的被攻擊，而是會被攻擊到與當地不佔優勢的擬態型的比例相當。這也就是說當地捕食者的篩選不是只針對某個型，而是基於全面性的「知識」判斷能不能吃。而那些看起來跟優勢型相像的也某種程度上受到保護，這可能會因為捕食者的「知識」不足而有機會發展成為優勢族群。

 這整個實驗展示天擇如何在擬態上展現「正向選汰」的機制，實驗本身不難，珍貴的是那1997-2013的採集資料，若非有這些基礎資料，連判斷擬態型的區域都極為困難，更別提當地的各型所佔的比例，也凸顯長期樣區的物種記錄基礎研究的重要性。

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