Sexual reproduction

Sexual reproduction

Hoverflies mating

Sexual reproduction is a prevalent system for producing new individuals within various species. Individuals of sexually reproducing species produce special kinds of cells called gametes, whose function is specifically to fuse with one unlike gamete and hence form a new individual. This fusion of two unlike gametes is called fertilization. By convention, where one type of gamete cell is physically larger than the other, it is associated with female sex. Thus an individual that produces exclusively large gametes (ova in humans) is said to be female, and one that produces exclusively small gametes (spermatozoa in humans) is said to be male. An individual that produces both types of gametes is called hermaphrodite (a name applicable also to people with one testis and one ovary). In some species hermaphrodites can self-fertilize, in others they can achieve fertilization with females, males or both. Some species, like the Japanese Ash, Fraxinus lanuginosa, only have males and hermaphrodites, a rare reproductive system called androdioecy‎.

What is considered defining of sexual reproduction is the difference between the gametes and the binary nature of fertilization. Multiplicity of gamete types within a species would still be considered a form of sexual reproduction. However, of more than 1.5 million living species,^[1] recorded up to about the year 2000, "no third sex cell — and so no third sex — has appeared in multicellular animals."^[2][3][4] Why sexual reproduction has an exclusively binary gamete system is not yet known. A few rare species that push the boundaries of the definitions are the subject of active research for light they may shed on the mechanisms of the evolution of sex. For example, the most toxic insect,^[5] the harvester ant Pogonomyrmex, has two kinds of female and two kinds of male. One hypothesis is that the species is a hybrid, evolved from two closely related preceding species.

Fossil records indicate that sexual reproduction has been occurring for at least one billion years.^[6] However, the reason for the initial evolution of sex, and the reason it has survived to the present are still matters of debate, there are many plausible theories. It appears that the ability to reproduce sexually has evolved independently in various species on many occasions. There are cases where it has also been lost. The flatworm, Dugesia tigrina, and a few other species can reproduce either sexually or asexually depending on various conditions.^[7]