天擇或是性擇，那是個問題：警戒性可促進箭毒蛙的聲學與物種多樣性的演化

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Fig. 1 箭毒蛙的親緣關係與將各種特徵映象至樹上，以推測其演趨勢
標題：Aposematism increases acoustic diversification and speciation in poison frogs.
摘要

Multimodal signals facilitate communication with conspecifics during court- ship, but they can also alert eavesdropper predators. Hence, signallers face two pressures: enticing partners to mate and avoiding detection by enemies. Undefended organisms with limited escape abilities are expected to mini- mize predator recognition over mate attraction by limiting or modifying their signalling. Alternatively, organisms with anti-predator mechanisms such as aposematism (i.e. unprofitability signalled by warning cues) might elaborate mating signals as a consequence of reduced predation.We hypoth- esize that calls diversified in association with aposematism. To test this, we assembled a large acoustic signal database for a diurnal lineage of apose- matic and cryptic/non-defended taxa, the poison frogs. First, we showed that aposematic and non-aposematic species share similar extinction rates, and aposematic lineages diversify more and rarely revert to the non- aposematic phenotype. We then characterized mating calls based on morphological (spectral), behavioural/physiological (temporal) and environmental traits. Of these, only spectral and temporal features were associated with aposematism. We propose that with the evolution of anti- predator defences, reduced predation facilitated the diversification of vocal signals, which then became elaborated or showy via sexual selection.

一個物種身上的特徵變化的主力不外乎天擇與性擇，但是有時候我們觀察一個特徵，或許能很輕易的說出「就是為了躲避天敵」或是「就是為了求偶」，但常常忽略兩股力量間的拉扯，譬如雄孔雀的羽毛為了求偶擁有極度漂亮的羽毛與誇張的展示行為，但相對的被天敵發現的機會就提高，而且逃脫能力也降低了，這樣的性狀就會在兩股力量間有個權衡(trade-off)，以得到利益的最大值。另一個角度來看，一個物種如果有警戒性，意即本身不需要接受太多來自天敵的壓力，這個物種用來求偶的行為會不會比起要躲避天敵的物種來的誇張呢？這對於性擇與種化又有什麼影響？

這篇研究的作者群以12S-16S rDNA建構172種箭毒蛙的親緣關係，測試警戒性與隱蔽性求偶行為與種化的演化趨勢。作者群假設隱蔽的物種透過另外發展比較低調的方式求偶，而比較警戒的物種可能將求偶的行為與警戒性結合，呈現越警戒越有求偶優勢的趨勢。透過一連串的形質測量，包含測試體表的光學反射評估隱蔽與警戒性，蒐集16657筆青蛙的求偶叫聲並將其分類以及比較體表植物鹼的分泌能力等，再將這些資料綜合進行一連串的分析，用以推測作者群的假設。

Fig. 2 箭毒蛙的種化分析
Fig. 3 系統發生學上對植物鹼的分泌與顯眼度的回歸分析
最主要的結果如最上圖。首先，作者群發現無論是警戒的或隱蔽的物種，其物種滅絕率是相似的，但出現警戒性的支系分化出的物種多樣性比較高，而且顯少出現返回到非警戒性特徵的趨勢。將求偶叫聲以型態（光學形質）、行為/生理（時間相關形質）與環境特徵作分類的話，發現光學與時間相關形質與警戒性有關。這顯示為了抵抗捕食者演化出來的特徵，也促進聲音利用的演化，進而成為影響性擇與種化。（本研究做了非常多的分析，細節請參閱內文）



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