人为什么分成男人和女人?

 Why are there males and females? Why are there two sexes instead of three, or twelve, or one? On this Moment of Science we'll learn why two is the magic number. Let's start with why there is sex at all. When two organisms reproduce sexually, sperm¹ and egg combine, using DNA² from both partners to make offspring. Combining DNA has a number of advantages, like correcting defects in each parents' genes³, so the offspring are healthier.

But why are there two sexes? This means, if you want to reproduce, you can only mate with fifty percent of the population, making the perfect mate that much harder to find. If there were only one sex you could reproduce with anyone, and it would be easier to find a mate. You'd think this would be an advantage for survival.

As it turns out, lots of other stuff can happen when two cells combine to reproduce. While the DNA in the cells' nuclei⁴ might be happy to join forces, other parts of the cell might not be so pleased with the merger⁵. Specifically, a cell's mitochondria tend to violently resist merging⁶ with other mitochondria, and feuding⁷ mitochondria can turn the DNA's love feast into a microscopic⁸ battlefield.

If there were only one sex, then mitochondria in each reproductive cell would start out on equal footing. They would need to fight it out every time. With a two-sex system, one sex can always unilaterally disarm⁹, surrendering its mitochondria for the sake of easier relations between the sexes. Male sperm cells are smaller than female eggs partly because they surrendered this mitochondrial battle long ago. Female mitochondria always win.

 

1 sperm   

n.精子，精液

参考例句：

• Only one sperm fertilises an egg.只有一个精子使卵子受精。
• In human reproduction,one female egg is usually fertilized by one sperm.在人体生殖过程中，一个精子使一个卵子受精。

2 DNA   

(缩)deoxyribonucleic acid 脱氧核糖核酸

参考例句：

• DNA is stored in the nucleus of a cell.脱氧核糖核酸储存于细胞的细胞核里。
• Gene mutations are alterations in the DNA code.基因突变是指DNA密码的改变。

3 genes   

n.基因( gene的名词复数 )

参考例句：

• You have good genes from your parents, so you should live a long time. 你从父母那儿获得优良的基因，所以能够活得很长。 来自《简明英汉词典》
• Differences will help to reveal the functions of the genes. 它们间的差异将会帮助我们揭开基因多种功能。 来自英汉非文学 - 生命科学 - 生物技术的世纪

4 nuclei   

n.核

参考例句：

• To free electrons, something has to make them whirl fast enough to break away from their nuclei. 为了释放电子，必须使电子高速旋转而足以摆脱原子核的束缚。
• Energy is released by the fission of atomic nuclei. 能量是由原子核分裂释放出来的。

5 merger   

n.企业合并，并吞

参考例句：

• Acceptance of the offer is the first step to a merger.对这项提议的赞同是合并的第一步。
• Shareholders will be voting on the merger of the companies.股东们将投票表决公司合并问题。

6 merging   

合并（分类）

参考例句：

• Many companies continued to grow by merging with or buying competing firms. 许多公司通过合并或收买竞争对手的公司而不断扩大。 来自英汉非文学 - 政府文件
• To sequence by repeated splitting and merging. 用反复分开和合并的方法进行的排序。

7 feuding   

vi.长期不和（feud的现在分词形式）

参考例句：

• Riccardo and Cafiero had been feuding so openly that the whole town knew about it. 里卡多和卡菲埃罗一直公开地闹别扭，全城的人都知道此事。 来自辞典例句
• The two families have been feuding with each other for many generations. 这两个家族有好多代的世仇了。 来自互联网

8 microscopic   

adj.微小的，细微的，极小的，显微的

参考例句：

• It's impossible to read his microscopic handwriting.不可能看清他那极小的书写字迹。
• A plant's lungs are the microscopic pores in its leaves.植物的肺就是其叶片上微细的气孔。

9 disarm   

v.解除武装，回复平常的编制，缓和

参考例句：

• The world has waited 12 years for Iraq to disarm. 全世界等待伊拉克解除武装已有12年之久。
• He has rejected every peaceful opportunity offered to him to disarm.他已经拒绝了所有能和平缴械的机会。