Reproduction is the process by which an organism produces offspring, an ability that is a unique characteristic of living things. There are two kinds of reproduction: asexual and sexual.
In asexual reproduction, parts of an organism split off to form new individuals, a process found in some animals but which is more common in plants: for example, the fission of single-celled plants; the budding of yeasts; the fragmentation of filamentous algae; spore production in bacteria, algae, and fungi, and the production of vegetative organs in flowering plants (bulbs, rhizomes, and tubers).
In sexual reproduction, special (haploid) cells containing half the normal number of chromosomes, called gametes, are produced: in animals, sperm by males in the testes and ova by females in the ovaries; in plants, pollen by males in the stamens and ovules by females in the plant ovary. The joining of gametes (fertilization) produces a (diploid) cell with the normal number of chromosomes, the zygote, which grows to produce an individual with genes inherited from both parents (see heredity). Fertilization may take place inside the female (internal fertilization) or outside (external fertilization). Internal fertilization demands that sperm be introduced into the female – insemination by copulation – and is advantageous because the young spend the most vulnerable early stages of their life-histories protected inside the mother. See also reproductive system.
At the molecular level, the most important aspect of reproduction is the ability of the chromosome to duplicate itself (see nucleic acid). The production of haploid cells is made possible by a process called meiosis and is necessary to prevent doubling of the chromosome number with each generation in sexually reproducing individuals. The advantage of sexual reproduction is that the bringing together of genes derived from two individuals produces variation in each generation enabling populations to change and thus adapt themselves to changing environmental conditions. See also evolution and natural selection.