What is "sexual reproduction"

Reproduction \Re`pro*duc"tion\ (-d?k"sh?n), n. [Cf. F. reproduction.]

1. The act or process of reproducing; the state of being reproduced; specifically (Biol.), the process by which plants and animals give rise to offspring.

   Note: There are two distinct methods of reproduction; viz.: asexual reproduction (agamogenesis) and sexual reproduction (gamogenesis). In both cases the new individual is developed from detached portions of the parent organism. In asexual reproduction (gemmation, fission, etc.), the detached portions of the organism develop into new individuals without the intervention of other living matter. In sexual reproduction, the detached portion, which is always a single cell, called the female germ cell, is acted upon by another portion of living matter, the male germ cell, usually from another organism, and in the fusion of the two (impregnation) a new cell is formed, from the development of which arises a new individual.

2. That which is reproduced.

n. reproduction involving the union or fusion of a male and a female gamete [syn: amphimixis]

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.

The evolution of sexual reproduction is a major puzzle because asexual reproduction should be able to outcompete it as every young organism created can bear its own young. This implies that an asexual population has an intrinsic capacity to grow more rapidly with each generation. This 50% cost is a fitness disadvantage of sexual reproduction. The two-fold cost of sex includes this cost and the fact that any organism can only pass on 50% of its own genes to its offspring. One definite advantage of sexual reproduction is that it prevents the accumulation of genetic mutations.

Sexual selection is a mode of natural selection in which some individuals out-reproduce others of a population because they are better at securing mates for sexual reproduction. It has been described as "a powerful evolutionary force that does not exist in asexual populations."

Prokaryotes, whose initial cell has additional or transformed genetic material, reproduce through asexual reproduction but may, in lateral gene transfer, display processes such as bacterial conjugation, transformation and transduction, which are similar to sexual reproduction although they do not lead to reproduction.