性 (生物學)

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(英語:Sex)是一種特徵,其可決定有性生殖生物體產生雄性或是雌性配子[1][2][3]於有性生殖過程中的雄性和雌性配子會融合,而形成受精卵,之後受精卵發育成為一繼承父母雙方特徵的後代。傳統上產生較小、更具移動性配子(精子)的生物體被稱為雄性,而產生較大、非移動性配子(卵細胞)的生物體被稱為雌性。[4]而可同時產生兩種配子的生物體則為雌雄同體[3][5]

於非雌雄同體物種,其中個體的性別是透過幾個生物性別決定係統中的一種來決定。大多數哺乳動物物種都具有XY性別決定系統,其中雄性通常攜帶一X和一Y染色體 (XY),雌性通常攜帶兩條X染色體 (XX)。其他的動物染色體性別決定系統包含有鳥類的ZW性別決定系統和昆蟲的X0性別決定系統。另有與前述不同的環境性別決定系統英语Environmental sex determination,如爬蟲類甲殼動物溫度-性別決定系統[6]

物種的雄性和雌性可能是外觀上相似(性單態性)或是有外觀上的差異(兩性異形)。大多數鳥類和哺乳動物是兩性異形物種,個體性別通常透過觀察其性別特徵識別性選擇(或稱擇偶)可加速兩性之間的差異進化。

"雄性"和"雌性"通常不適用於無性分化的物種,其個體是同構的(性單態性),且配子是同配生殖的(大小和形狀難以區分),例如稱為石蓴綠藻。但有些物種個體之間存在一些功能差異(例如真菌),[7]其繁殖方式為有性生殖中的交配類型英语mating type[8]

蕈類繁殖是真菌有性生殖(交配類型)中的一種。

性系統

性系統是一物種中的生物體,其內部雄性和雌性功能的配置。[9]

交配中的食蚜蠅科

動物

大約95%的動物物種有其獨立的雄性和雌性個體,稱為雌雄異體。大約有5%的動物物種是雌雄同體。[9]這種低百分比歸因於將種類眾多的昆蟲包含在內,昆蟲中並無雌雄同體存在。[10]大約99%的脊椎動物是雌雄異體,所餘1%的雌雄同體中,幾乎全是魚類。[11]

植物

大多數植物具有兩種性別,[12](p. 212)或是雌雄同體(雄蕊雌蕊都在同一朵花上),或是單性同株英语Monoecy[13][14]在雌雄異株的物種中,雄性和雌性生長於不同株上。[15]約有5%的開花植物是雌雄異株的,由5,000之多的獨立品種演化而來。[16]雌雄異株在裸子植物中很常見,約佔65%,但大多數針葉樹是單性同株。[17]

通常開花植物的花朵上同時有雄性及雌性性器官(雌雄同體)。
松樹及其他針葉樹的毬花,圖左含雌性性器官,圖右含雄性性器官。

性的進化

不同形式的異配生殖:
A)細胞遷移異配生殖,B) 卵式生殖英语oogamy (卵子細胞與精子細胞), C) 非細胞遷移異配生殖 (卵子細胞與精原體)。
不同形式的同配生殖:
A) 細胞遷移同配生殖,B) 非細胞遷移同配生殖,C) 結合

人們普遍認為異配生殖是由同配生殖演化而來,[18]且異配生殖在不同的真核生物群體中(如原生生物藻類植物動物)各自獨立進化過幾次。[10]異配生殖的演化與雄性和雌性的起源是同一件事。[19]這也是邁向兩性異形的第一步,[20]且影響到各種性別差異的演化。[21]

然而異配生殖的演化並沒留下任何化石證據,[22]迄2006年尚無遺傳證據證明性別和交配類間的演化連結。[23]目前還不清楚是異配生殖首先導致雌雄同體的進化,或是雌雄異體的進化。[12](p. 213)

但一件來自12億年前,稱為Bangiomorpha pubescens紅藻門化石,為雄性和雌性生殖的分化提供最古老的化石記錄,顯示性別在真核生物中很早就已進化。[24]

最初的性形式是體外受精。而我們所知的繁殖形式 - 體內受精,其演化發生在後,[25]是脊椎動物在陸地上出現後才成為主流。[26]

性別比例

本節摘自性別比

性別比是整個群體中雄性與雌性所佔的比例。費雪原理解釋說,在有性繁殖的物種中出於進化原因,性別比例通常約為1:1。[27][28]然而有許多物種會週期性或永久性偏離此種均勻比例。例子包括單性生殖物種、週期性交配的生物體(如蚜蟲)及一些真社會性黃蜂花蜂類螞蟻白蟻[29]

人類學家和人口統計學家對人類性別比特別感興趣。在人類社會中,出生時的性別比可能會因母親生產時的年齡[30]以及性別選擇性墮胎英语Sex-selective abortion殺嬰等因素而造成相當大的偏差。接觸農藥和其他環境污染物也可能是重要的影響因素。[31]截至2014年,全球出生性別比估計為107男孩相對於100女孩(1,000名男孩:934名女孩)。[32]

參見





參考文獻

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