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性 (生物学)

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(英语:Sex)是一种特征,其可决定有性生殖生物体产生雄性或是雌性配子[1][2][3]于有性生殖过程中的雄性和雌性配子会融合,而形成受精卵,之后受精卵发育成为一继承父母双方特征的后代。传统上产生较小、更具移动性配子(精子)的生物体被称为雄性,而产生较大、非移动性配子(卵细胞)的生物体被称为雌性。[4]而可同时产生两种配子的生物体则为雌雄同体[3][5]

于非雌雄同体物种,其中个体的性别是透过几个生物性别决定系统中的一种来决定。大多数哺乳动物物种都具有XY性别决定系统,其中雄性通常携带一X和一Y染色体 (XY),雌性通常携带两条X染色体 (XX)。其他的动物染色体性别决定系统包含有鸟类的ZW性别决定系统和昆虫的X0性别决定系统。另有与前述不同的环境性别决定系统英语Environmental sex determination,如爬虫类甲壳动物温度-性别决定系统[6]

物种的雄性和雌性可能是外观上相似(性单态性)或是有外观上的差异(两性异形)。大多数鸟类和哺乳动物是两性异形物种,个体性别通常透过观察其性别特征识别性选择(或称择偶)可加速两性之间的差异进化。

"雄性"和"雌性"通常不适用于无性分化的物种,其个体是同构的(性单态性),且配子是同配生殖的(大小和形状难以区分),例如称为石莼绿藻。但有些物种个体之间存在一些功能差异(例如真菌),[7]其繁殖方式为有性生殖中的交配类型英语mating type[8]

蕈类繁殖是真菌有性生殖(交配类型)中的一种。

性系统

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性系统是一物种中的生物体,其内部雄性和雌性功能的配置。[9]

交配中的食蚜蝇科

动物

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大约95%的动物物种有其独立的雄性和雌性个体,称为雌雄异体。大约有5%的动物物种是雌雄同体。[9]这种低百分比归因于将种类众多的昆虫包含在内,昆虫中并无雌雄同体存在。[10]大约99%的脊椎动物是雌雄异体,所余1%的雌雄同体中,几乎全是鱼类。[11]

植物

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大多数植物具有两种性别,[12](p. 212)或是雌雄同体(雄蕊雌蕊都在同一朵花上),或是单性同株英语Monoecy[13][14]在雌雄异株的物种中,雄性和雌性生长于不同株上。[15]约有5%的开花植物是雌雄异株的,由5,000之多的独立品种演化而来。[16]雌雄异株在裸子植物中很常见,约占65%,但大多数针叶树是单性同株。[17]

通常开花植物的花朵上同时有雄性及雌性性器官(雌雄同体)。
松树及其他针叶树的球花,图左含雌性性器官,图右含雄性性器官。

性的进化

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不同形式的异配生殖:
A)细胞迁移异配生殖,B) 卵式生殖英语oogamy (卵子细胞与精子细胞), C) 非细胞迁移异配生殖 (卵子细胞与精原体)。
不同形式的同配生殖:
A) 细胞迁移同配生殖,B) 非细胞迁移同配生殖,C) 结合

人们普遍认为异配生殖是由同配生殖演化而来,[18]且异配生殖在不同的真核生物群体中(如原生生物藻类植物动物)各自独立进化过几次。[10]异配生殖的演化与雄性和雌性的起源是同一件事。[19]这也是迈向两性异形的第一步,[20]且影响到各种性别差异的演化。[21]

然而异配生殖的演化并没留下任何化石证据,[22]迄2006年尚无遗传证据证明性别和交配类间的演化连结。[23]目前还不清楚是异配生殖首先导致雌雄同体的进化,或是雌雄异体的进化。[12](p. 213)

但一件来自12亿年前,称为Bangiomorpha pubescens红藻门化石,为雄性和雌性生殖的分化提供最古老的化石记录,显示性别在真核生物中很早就已进化。[24]

最初的性形式是体外受精。而我们所知的繁殖形式 - 体内受精,其演化发生在后,[25]是脊椎动物在陆地上出现后才成为主流。[26]

性别比例

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本节摘自性别比

性别比是整个群体中雄性与雌性所占的比例。费雪原理解释说,在有性繁殖的物种中出于进化原因,性别比例通常约为1:1。[27][28]然而有许多物种会周期性或永久性偏离此种均匀比例。例子包括单性生殖物种、周期性交配的生物体(如蚜虫)及一些真社会性黄蜂花蜂类蚂蚁白蚁[29]

人类学家和人口统计学家对人类性别比特别感兴趣。在人类社会中,出生时的性别比可能会因母亲生产时的年龄[30]以及性别选择性堕胎英语Sex-selective abortion杀婴等因素而造成相当大的偏差。接触农药和其他环境污染物也可能是重要的影响因素。[31]截至2014年,全球出生性别比估计为107男孩相对于100女孩(1,000名男孩:934名女孩)。[32]

参见

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参考文献

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  1. ^ Stevenson A, Waite M. Concise Oxford English Dictionary: Book & CD-ROM Set. OUP Oxford. 2011: 1302 [2018-03-23]. ISBN 978-0-19-960110-3. Sex: Either of the two main categories (male and female) into which humans and most other living things are divided on the basis of their reproductive functions. The fact of belonging to one of these categories. The group of all members of either sex. 
  2. ^ Mills, Alex. Biology of Sex. University of Toronto Press. 2018-01-01: 43–45 [2023-10-03]. ISBN 978-1-4875-9337-7. (原始内容存档于2023-11-07) (英语). 
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  31. ^ Davis, Devra Lee; Gottlieb, Michelle and Stampnitzky, Julie; "Reduced Ratio of Male to Female Births in Several Industrial Countries" in Journal of the American Medical Association; April 1, 1998, volume 279(13); pp. 1018-1023
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延伸阅读

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