暖化與海拔梯度下雌雄體型二型性之變化

Abstract

種內變異是物種因應環境變化產生演化反應的先決條件，而種內變異的分布也會受演化進程影響。雌雄體型二型性呈現種內的體型變異，由於雌雄體型差異會影響生殖策略，雌雄體型差異的變化可能造成生殖演化上的改變。本研究根據雌雄體型二型性形成原因的假說進行推論，探討不同平均體型之尺蛾群聚間雌雄體型二型性的變異。解釋雌雄二型性變化的假說包括：雌雄對同一擇汰壓力有不同反應的演化限制假說，以及強調雌雄承受不同擇汰壓力的繁殖力擇汰假說和性擇假說。演化限制假說認為性別差異 (如累加遺傳變異度) 會影響個體在相似擇汰壓力下的體型反應，造成體型二型性，本研究據此檢驗雌雄體型二型性之群聚其雌雄二性的體型變異度是否反應潛在的累加遺傳變異度。繁殖力擇汰假說和性擇假說強調特定性別承受較大的擇汰壓力，而性別間有高度但又不完全相等的遺傳關聯性。繁殖力擇汰假說解釋雌性較大的情況，背後機制為體型較大個體有較高的繁殖力，所以當族群主要受到雌性繁殖力擇汰時，雌性體型變異較大，而雄性體型會因遺傳關聯產生較雌性小的反應。性擇假說可解釋雄性較大的情況，如體型大的雄性在競爭配偶時有優勢，當族群主要受到雄性性擇壓力時，雄性體型變異較大，而雌性體型因遺傳關聯產生較小的反應。本研究利用氣候暖化和海拔提供的平均體型梯度來探討不同平均體型之尺蛾雌雄體型二型性的變異，分析在1965年和2007年於馬來西亞神山 (此期間估計升溫攝氏0.7度) 跨海拔調查收集來的尺蛾體型資料，並檢驗上述假說。演化限制假說預期尺蛾在暖化下體型變小的2007年，面臨相同的的環境篩選 (暖化) 壓力下，雌性較大的群聚會呈現較小的雌性體型變異度，而雄性較大的群聚雄性會有較小的體型變異度。而繁殖力擇汰假說認為雌性較大的群聚平均體型主要會隨著雌性體型所變動，而雄性的變動會小於雌性的變動，因此預期這些群聚在平均體型較小時 (如暖化後與低海拔族群)，雌雄體型二型性的差異程度會減小。同樣的，性擇假說認為雄性較大的群聚平均體型主要會隨著雄性體型所變動，而雌性變動會小於雄性的變動，因此預期在平均體型較小時 (如暖化後與低海拔族群)，雌雄體型二型性的差異程度會減小。結果顯示，兩性的體型變異度並未隨暖化或海拔有顯著差異，不支持演化限制假說。雄性較大群聚的雌雄體型二型性也未受暖化 (年份) 和海拔影響，不支持性擇假說。但是在雌性較大的群聚中，雌雄體型二型性隨暖化而變小，支持繁殖力擇汰假說，並且根據主成分分析結果，雌雄體型二型性的變化主要是雌性體型變化所造成。此結果顯示這些尺蛾群聚體型結構對環境的反應主要來自於雌性的變化，以及物種性狀的改變可能只是部分個體的反應，而其他個體因為遺傳的關聯性連帶產生反應，這樣的結果也暗示著種內變異對演化進程的影響。

Intraspecific variation is a determinant of species evolutionary response to environmental change. As a source of intraspecific variation, sexual size dimorphism (SSD), associated with reproductive strategy, may reveal the evolutionary process in play. In this study, I investigated the body size allometry for SSD for geometrid moth assemblages of different average body sizes based on three hypotheses for SSD: (i) evolutionary constraints, (ii) fecundity selection, and (iii) sexual selection. The evolutionary constraint hypothesis suggests sex-specific responses to similar selection pressures as a cause of SSD. On the other hand, the fecundity selection and sexual selection hypotheses suggest the contribution of sex-specific selections to SSD. The fecundity selection hypothesis suggests that in female-biased SSD (FBSSD), males show an evolutionary response that is correlated with but weaker than female response to fecundity selection, hence a more variable female body size. In contrast, the sexual selection hypothesis suggests that in male-biased SSD (MBSSD), sexual selection on males produces a weaker evolutionary change in females, hence a more variable male body size. The well-documented observations of body size response to climate warming and geographic gradients provide a natural variation for examining the body size allometry for SSD. In this study, geometrid moth body size data retrieved from altitudinal surveys on Mount Kinabalu in 1965 and 2007 (0.7°C warming during this period) were analyzed to test the hypotheses for SSD allometry. Given that the decades of warming generally reduced moth body size, the evolutionary constraints hypothesis would predict that the male in FBSSD and the female in MBSSD in 2007 should have a greater body size variation over the other sex. The sex-specific selection hypotheses predict a less pronounced SSD in both FBSSD and MBSSD for assemblages with smaller average body size, which were found at lower altitudes and after decades of warming. The results show no difference in the sex difference in body size variation or SSD in MBSSD across altitudes and decades of warming. However, the less pronounced FBSSD after decades of climate warming supports the prediction based on the natural selection hypothesis. Moreover, the SSD variation is mainly caused by female size change according to the result of principal component analysis. The results imply that the response of body size structure to environmental changes can be driven primarily by female responses to selection pressures. Overall, this study reveals that trait changes in a species could be attributed to the evolutionary response of some individuals (e.g., females) along with genetically-correlated others (e.g., males), which implicates the potential influences of intraspecific variation on evolutionary processes.