Asexual reproduction is advantageous in allowing beneficial combinations of characteristics to continue unchanged and in eliminating the often vulnerable stages of early embryonic growth. It is found in most plants, bacteria, and protists and the lower invertebrates. In one-celled organisms it most commonly takes the form of fission, or mitosis, the division of one individual into two new and identical individuals. The cells thus formed may remain clustered together to form filaments (as in many fungi) or colonies (as in staphylococci and Volvox). Fragmentation is the process in filamentous forms in which a piece of the parent breaks off and develops into a new individual. Sporulation, or spore formation, is another means of asexual reproduction among protozoa and many plants. A spore is a reproductive cell that produces a new organism without fertilization. In some lower animals (e.g., hydra) and in yeasts, budding is a common form of reproduction; a small protuberance on the surface of the parent cell increases in size until a wall forms to separate the new individual, or bud, from the parent. Internal buds formed by sponges are called gemmules.

Regeneration is a specialized form of asexual reproduction; by regeneration some organisms (e.g., the starfish and the salamander) can replace an injured or lost part, and many plants are capable of total regeneration—i.e., the formation of a whole individual from a single fragment such as a stem, root, leaf, or even a small slip from such an organ (see cutting; grafting). F. C. Steward showed (1958) that single phloem cells from a carrot plant, when grown on an agar medium, would form a complete carrot plant. Among animals, the lower the form, the more capable it is of total regeneration; no vertebrates have this power, although clones of mammals have been produced in the laboratory (1997) from single somatic cells. Closely allied to regeneration is vegetative reproduction, the formation of new individuals by various parts of the organism not specialized for reproduction. In some plants structures that form on the leaves give rise to young plantlets. Rhizomes, bulbs, tubers, and stolons are other forms of vegetative reproduction.

Sexual reproduction occurs in many one-celled organisms and in all multicellular plants and animals. In higher invertebrates and in all vertebrates it is the exclusive form of reproduction, except in the few cases in which parthenogenesis is also possible. Sexual reproduction is essentially cellular in nature, i.e., it involves the fertilization of one sex cell (gamete) by another, producing a new cell (called a zygote), which develops into a new organism. The union of two isogametes (structurally identical but differing physiologically) is called isogamy, or conjugation, and occurs only in some lower forms (e.g., Spirogyra and some protozoa). Heterogamy is the fusion of two clearly differing kinds of gametes, distinguished as the ovum and the sperm.

Multicellular plants alternate sexually reproducing, or gametophyte, and asexually reproducing, or sporophyte, generations. The gametophyte produces gametes, and the union of gametes results in the growth of a sporophyte; the sporophyte produces spores that give rise to a gametophyte. The prominent generation in lower plants (e.g., mosses, liverworts) and the complex fungi is the gametophyte; in the vascular plants (ferns, conifers, grasses, and flowering plants) it is the sporophyte. The less prominent generation may be an independent plant, as is the small inconspicuous gametophyte of ferns, or a reduced organism consisting of only a few cells and dependent for survival on the prominent form, like the pollen grain, which is the male gametophyte of seed plants.

Many organisms exhibit special reproductive mechanisms to ensure fertilization; among higher plants the process of pollination may involve extremely complex interaction between the flower and the pollen-bearing agent (e.g., the yucca plant and the yucca moth). Among land-dwelling animals internal fertilization (copulation) is necessary in order to provide the fluid environment essential to fertilization.

Sexual reproduction is of great significance in that, because of the fusion of two separate parental nuclei, the offspring inherit endlessly varied combinations of characteristics that provide a vast testing ground for new variations that may not only improve the species but ensure its survival. This probably explains the predominance of sexual reproduction among higher forms. Even in those microorganisms that reproduce asexually (e.g., bacteria) exchanges of hereditary material take place; in the hermaphroditic plants and animals (e.g., the earthworm) self-fertilization is almost always prevented by anatomical specializations or by differing maturation times for male and female gametes.

See also genetics, recombination, and sex.