小雨蛙鳴叫聲地理變異之成因探討與理論檢測

Abstract

動物行為會受到遺傳距離及環境的影響而產生差異，而亦有許多研究指出，物種間的交互作用使個體產生行為特徵上的改變。當親緣關係相近的兩種動物共域時，為了減少錯誤配對的機會，擇汰會加強牠們特徵或行為的差異，此現象稱為繁殖性狀替換（reproductive character displacement）。兩棲類的鳴叫行為直接影響到牠們的適存度，且研究指出兩棲類容易受到地理屏障的影響，因此鳴叫特徵適合用來研究動物行為在地理上的變異。當鳴叫特徵差異分別與遺傳距離及地理距離高度相關時，則鳴叫特徵的變異可能是受到基因漂變（genetic drift）的影響。然而當特徵差異只與地理距離相關時，則可能是受到文化漂變（cultural drift）的影響。除了漂變之外，自然選汰（ecological selection）也可能造成鳴叫特徵的變異，在這個理論的預測下，鳴叫特徵會與環境因子相關。本研究使用小雨蛙（Microhyla fissipes）與黑蒙西氏小雨蛙（M. heymonsi）做為研究材料，小雨蛙廣泛分布在台灣全島的低地平原，在西部的台中及花蓮以南與黑蒙西氏小雨蛙共域，這兩種小雨蛙使用相似的棲地，且發出人耳難以辨認的宣告叫聲。而在過去的調查中，我們也發現小雨蛙的鳴叫聲在各地間有些微的差異。因此本實驗欲檢測以下問題：(1)小雨蛙的鳴叫聲變異，是否是因為與黑蒙西氏小雨的蛙種間交互作用－繁殖性狀替換所造成？(2)基因漂變、文化漂變與自然選汰，何者較能解釋小雨蛙種內的鳴叫聲地理變異？我們蒐集了13個地點，共233隻動物，分析鳴叫聲特性，並與溫度及體型做回歸殘差來校正鳴叫特徵。利用粒線體的COI序列來進行族群結構的建立，計算遺傳分化程度及遺傳距離後，進行遺傳距離、地理距離及氣候差異鳴叫特徵差異的相關性。結果顯示，兩物種共域時並不會影響其鳴叫特徵，顯示這兩種小雨蛙間並不存在繁殖性狀替換。鳴叫特徵的差異與遺傳距離呈現顯著的正相關，與地理距離也顯著正相關，顯示鳴叫特徵的地理變異符合基因漂變的假說。另外，鳴叫特徵的差異與年均溫、年均濕度及年降雨量都沒有顯著相關，表示自然選汰並不是造成地理變異的原因。本研究的結果證實基因漂變是最有可能造成小雨蛙鳴叫聲地理變異的原因，而非文化漂變或選汰。Diversification of signals can provide insight into evolutionary processes of communication system. The forces undergo signal divergence included interaction between species, ecological force, genetic drift, or the recently proposed hypothesis of cultural drift. When closely related species are geographically overlapping, selection would favor differentiated in sexual traits through reproductive character displacement (RCD) in order to prevent from hybridization. Ecological factors such as temperature and humidity would also cause signal divergence in different selective regions. Furthermore, stochastic processes could not be excluded in the evolution of signal diversity. In this study, I tested alternative hypotheses including reproductive character displacement, ecological selection, genetic drift, and cultural drift to figure out the reason for geographic acoustic variation of Microhyla fissipes in Taiwan. I recorded calls from 13 populations, among which 8 populations are sympatrically distributed with the closely related M. heymonsi which produces advertisement calls very difficult for human to distinguish and uses almost the same niche. My results showed that there is no significant tuning of calls in sympatric populations, indicating that RCD does not occur between these two species. Population structure constructed by COI demonstrated that M. fissipes in Taiwan can be divided into four clades: northwest, southwest, south and east, while acoustic signals were significantly different among the clades. Acoustic variation was significantly correlated with both geographic distance and genetic distance. On the contrary, the correlations between acoustic distance and climatic factors were not significant. I concluded that geographic variation in advertisement call of Taiwan M. fissipes was mainly caused by genetic drift instead of cultural drift. Sexual selection and ecological selection did not affect the advertisement calls of M. fissipes. My study confirmed that RCD did not occur between M. fissipes and M. heymonsi and provided a better understanding of signal evolution.