This article is about biological sexes: male, female, etc.. For alternate uses, such as sexual intercourse, see Sex (disambiguation).
Sexuality Portal Sex refers to the male and female duality of biology and
reproduction. Unlike organisms that only have the ability to reproduce asexually, many species have the ability to produce offspring through meiosis and
fertilization. Often, individuals of the two sexes attract one another and communicate their readiness to procreate through biological changes, or, in social species, through courtship behaviours.
An organism''''s sex is defined by its biological role in reproduction, not according to its sexual or other behavior. The female sex is defined as the one which produces the larger gamete and which typically bears the offspring. In contrast, the male sex has a smaller gamete and rarely bears offspring. In some animals and many plants sex may be assigned to specific structures rather than the entire organism. Earthworms, for example, are normally hermaphrodites.
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 gametes is called fertilization. The condition of having types of gametes that are externally similar—particularly in size—is isogamy; having gametes that are somewhat dissimilar is anisogamy. The condition of having greatly dissimilar gametes—particularly a large, immotile cell and a much smaller, motile one—is oogamy. By convention, the larger gamete cell 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
source:wikipedia