有性生殖是生殖的一种类型，它导致了后代加强基因多样化。它可以用两个进程刻画。第一个是减数分裂，涉及将染色体个数减半。第二个是受精，它使得两个配偶子融合，并恢复原来的染色体个数。在减数分裂时，每对染色体通常交叉以达到基因重组。

性的演变是现代演化生物学的重大谜团。最早的有性繁殖的生物的化石证据是来自狭带纪的真核细胞，距今约12到10亿年。有性生殖是绝大多数可见生命体的繁殖形式，包括几乎所有的动物和植物。细菌接合（bacterial conjugation），也就是两个细菌之间的DNA转移，有时被错误地视为有性生殖，因为机理其实很相似。

Sexual reproduction is a form of reproduction where two morphologically distinct types of specialized reproductive cells called gametes fuse together, involving a female's large ovum (or egg) and a male's smaller sperm. Each gamete contains half the number of chromosomes of normal cells. They are created by a specialized type of cell division, which only occurs in eukaryotic cells, known as meiosis. The two gametes fuse during fertilization to produce DNA replication and the creation of a single-celled zygote which includes genetic material from both gametes. In a process called genetic recombination, genetic material (DNA) joins up so that homologous chromosome sequences are aligned with each other, and this is followed by exchange of genetic information. Two rounds of cell division then produce four daughter cells with half the number of chromosomes from each original parent cell, and the same number of chromosomes as both parents, though self-fertilization can occur. For instance, in human reproduction each human cell contains 46 chromosomes, 23 pairs, except gamete cells, which only contain 23 chromosomes, so the child will have 23 chromosomes from each parent genetically recombined into 23 pairs. Cell division initiates the development of a new individual organism in multicellular organisms, including animals and plants, for the vast majority of whom this is the primary method of reproduction. A species is defined as a taxonomic rank. A species is often defined as the largest group of organisms where two hybrids are capable of reproducing fertile offspring, typically using sexual reproduction, although the species problem encompasses a series of difficult related questions that often come up when biologists define the word species.