體型雌雄二型性 (SSD) 是指兩性成熟體型差異的現象。解釋此現象演化的假說至今已提出：大體型的雌性具有較高的生殖力、以及大體型的雄性具有較高的生殖成功機會。除上述機制外，實證研究在近期指出，SSD的出現是由於兩性擁有不同的生長軌跡導致。然而截至今日，兩性生長軌跡的演化如何受到性別專一的擇汰壓力、與生活史權衡的調控尚未明瞭。本研究整合生長發育的觀點，解釋兩性的生長軌跡如何因應不同擇汰壓力產生分歧，進而導致體型雌雄二型性。我利用數量遺傳模式，分析雄性與雌性體型、及體型二型性的演化動態。我假設體型大小主要受個體生長速度與生長時間的調控。個體可藉加速生長、或延長生長時間增加體型，並分別有助於雌性生殖力與雄性生殖成功的機會。個體加速生長(延長生長時間)須付出的代價為提高死亡率(縮短繁殖時間)。模式說明兩性如何因應不同繁殖策略，最適化各自的生長軌跡與體型大小。本研究藉整合生長發育之觀點釐清SSD如何演化，並提出假說待後續實證研究測試。

Sexual size dimorphism (SSD) describes divergent body sizes of adult males and females in a species. Several hypotheses (e.g., fecundity selection and sexual selection) have been proposed to explain evolution of SSD. Meanwhile, it has recently been acknowledged that SSD occurs because two sexes undergo different ontogenetic growth trajectories (i.e., growth rate and duration). However, it remains unclear how sex-specific ontogenetic growth trajectories evolve under sex-specific selection and life history trade-offs. To elucidate this, I develop a new theoretical framework by extending a quantitative genetic model for sexual trait dimorphism (i.e., male ornament) in which I reinterpret the trait as body size and redefine individual fitness in a size-dependent manner. Specifically, I assume that benefits of large body size are higher mating success for males and higher fecundity for females whereas accelerated growth rate and prolonged growth duration incur costs of lower survivorship and shorter reproduction period, respectively. Model analysis illustrated how two sexes would optimize ontogenetic growth trajectories under different trade-offs due to different reproductive strategies and divergent adult body sizes. The present framework incorporating an ontogenetic perspective provides a new insight into how SSD evolves, as well as testable hypotheses for empirical research.

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