不完美擬態真那麼不完美嗎？藉由比較性分析探討不完美擬態的演化機制
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| photo: Olympic Naturual History Photos |
摘要
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 (Batesian) mimics of stinging Hymenoptera. However, although some hoverfly species are considered excellent mimics, other species bear only a superficial resemblance to their models 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 perception; second, no species of syrphid maps out in multidimensional space as being intermediate in appearance between several different hymenopteran model species, as the multimodel hypothesis requires; and third, we find no evidence for a negative relationship between mimetic fidelity and abundance, which calls into question the kin-selection hypothesis. By contrast, a strong positive relationship between mimetic fidelity and body size supports the relaxed-selection hypothesis, suggesting that reduced predation pressure on less profitable prey species limits the selection for mimetic perfection.
不完美擬態 (imperfect mimicry) 指的是一個擬態物種（以人類觀點）猛一看很像A物種，但仔細端詳後又發現其實不太像，有時又會覺得很像B物種，但又覺得有差異，對人類來說就覺得「擬態的不完美」。這個看似對立於「完美擬態」的現象看似成立，其實存在著很多爭議，像是為什麼要有點不像？不像的部分對擬態也有貢獻？其實擬態兩個以上的物種？關於這些問題的研究產生很多對立的結果，到目前為止並未有一致的結論。
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| Fig. 1 食蚜蠅的斑紋分析，圓圈代表model，三角形代表mimics，白色代表蜜蜂型，灰色代表胡蜂型。 結果顯示兩型間的相似方向並不重疊。 |
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| Fig. 4 斑紋相似度與身體尺寸有強烈的正相關（詳細統計結果見原文） |
這篇文章藉由比較性的方法來討論不完美擬態的演化來源。太細節的部分就姑且不談，作者群透過比較食蚜蠅 (hoverfly) 的型態（尺寸、斑紋與不同的接受者）與系統發生學的方式，發現以下幾個有趣的證據：
1. 透過型態的測量與鳥類的觀點來看，以人類的角度來判斷物種是否不完美擬態太過簡化這個現象；
2. 經過分析後發現，本來認為可能是擬態兩個擬態模型的食蚜蠅，其實沒有任何一個物種的斑紋實際上同時涉入兩種以上的擬態環，顯示「擬態多物種假說」值得存疑；
3. 沒有任何證據顯示擬態的精確度與物種豐度有負相關（物種數量越少，擬態的越精確），這可能與親屬選汰 (kin selection) 有關；
4. 擬態的精確度與身體尺寸有強烈的正相關，支持緩和選汰假說 (relaxed-selection hypothesis)，顯示獲益較少的物種受到較少的捕食壓力，影響擬態精確度的選汰。
這個研究以食蚜蠅為例子是因為那實在是不完美擬態的典範，然而這篇文章並非完全解答這個現象，至少就食蚜蠅的總物種數來說，文中討論的物種數比例上不算多，有可能陷入見樹不見林的風險。不完美擬態也廣泛的發生在各種其他類群的昆蟲上，這個研究的論點是基於「不完美擬態是到目前為止的終點」，但更多觀察到的現象讓我們覺得「不完美擬態可能只是完美擬態的過程」，代表不同的類群不同的演化故事，帶來的結論可能大相逕庭。
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