[Article] 在被擬態者具高豐度的情況下，可能允許貝氏擬態的緩慢演化

原文標題：High-model abundance may permit the gradual evolution of Batesian mimicry: an experimental test

Kikuchi DW & Pfennig DW. 2009. High-model abundance may permit the gradual evolution of Batesian mimicry: an experimental test. Proc. R. Soc. B. in press. [page]

[摘要]

In Batesian mimicry, a harmless species (the ‘mimic’) resembles a dangerous species (the ‘model’) and is thus protected from predators. It is often assumed that the mimetic phenotype evolves from a cryptic phenotype, but it is unclear how a population can transition through intermediate phenotypes; such intermediates may receive neither the benefits of crypsis nor mimicry. Here, we ask if selection against intermediates weakens with increasing model abundance. We also ask if mimicry has evolved from cryptic phenotypes in a mimetic clade. We first present an ancestral character-state reconstruction showing that mimicry of a coral snake (Micrurus fulvius) by the scarlet kingsnake (Lampropeltis elapsoides) evolved from a cryptic phenotype. We then evaluate predation rates on intermediate phenotypes relative to cryptic and mimetic phenotypes under conditions of both high- and low-model abundances. Our results indicate that where coral snakes are rare, intermediate phenotypes are attacked more often than cryptic and mimetic phenotypes, indicating the presence of an adaptive valley. However, where coral snakes are abundant, intermediate phenotypes are not attacked more frequently, resulting in an adaptive landscape without a valley. Thus, high-model abundance may facilitate the evolution of Batesian mimicry.

這篇文章主要在談貝氏擬態的擬態型演化理論。在擬態生物學中牽扯到斑紋變化的主要有兩個理論，一個是Fisher (1930)所提出的two-step theory，另一個則是Fisher (1927)基於天擇說與穆氏擬態的特徵漸變理論，也就是特徵的變化是由環境逐漸淘汰所形成的，但這個理論在擬態生物學中之所以不被認為是一個常態，主要原因是擬態所遭受的天擇壓力十分強大，只要性狀有變異就可能遭受淘汰，因此若是一個物種無論從cryptic，或是從另一個擬態群要改變斑紋加入另一個擬態群，勢必要經歷的是斑紋從不相似到相似，而這個過程中會被捕食者所選擇。

珊瑚蛇擬態被認為是貝氏擬態的經典例子，在貝氏擬態的演化歷程中，被預測的是斑紋會快速的演變，也就是Fisher(1930)所提出的two-step theory較符合貝氏擬態的演化歷程，但此篇研究提供了另一個證據，也就是在貝氏擬態的前提下，仍然可以逐漸的改變顏色組合逐漸逼近model，但是必須在model豐度高的時候才成立，這篇研究經由phylogeny與野外的實驗來測試這件事情，結果都支持這個擬態群的顏色組變化是逐漸變化的，也就是不太像的mimic在變化時受到model豐度的保護，讓捕食者只要有一點像那個不好吃的東西就不吃，而在model豐度較低的地方，捕食者沒有那麼高頻度的遇到model，mimic只要稍有不像就可能遭受攻擊，phylogeny的結果則顯示整體的演化方向是cryptic->intermideately->precisely。

這篇文章提供一個擬態斑紋的演化機制，並且在某種程度上支持了frequency-dependent的論點，並且與這個研究群先前以族群遺傳做出來的結果一致，顯示擬態斑紋的變化可受到族群量的直接影響。