性 (生物學)

性（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的配子。^[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），如爬蟲類和甲殼動物的溫度-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來識別。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），例如稱為石蓴的綠藻。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]

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]

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（雄蕊和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]

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、植物和動物）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的紅藻門化石，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）及一些真社會性的黃蜂、花蜂類、螞蟻和白蟻。^[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）和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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延伸閱讀

Arnqvist G, Rowe L. Sexual conflict. Princeton University Press. 2005. ISBN 978-0-691-12217-5.
布魯斯·艾伯茨, Johnson A, Lewis J, Raff M, Roberts K, Walter P. Molecular Biology of the Cell 4th. New York: Garland Science. 2002. ISBN 978-0-8153-3218-3.
Ellis H. Psychology of Sex. London: W. Heinemann Medical Books. 1933. N.B.: One of many books by this pioneering authority on aspects of human sexuality.
Gilbert SF. Developmental Biology 6th. Sinauer Associates, Inc. 2000. ISBN 978-0-87893-243-6.
Maynard-Smith J. The Evolution of Sex. Cambridge University Press. 1978. ISBN 978-0-521-29302-0.