Reproduction is the creation of a living being from the material of previously living beings. It is a process common to all living entities—to plants, animals, microorganisms, and fungi. One central definition of life itself is the ability to make more life—to reproduce.
There are several processes by which living organisms reproduce themselves. Many organisms, especially simpler organisms, can reproduce in many different ways. More complex organisms, such as mammals, can only reproduce sexually.
Reproductive processes are divided into two main categories distinguished by whether or not an organism can reproduce itself by itself. Asexual reproduction occurs when organisms reproduce themselves without any genetic contribution from another organism. The offspring produced through asexual reproduction are genetically identical to the parent. Sexual reproduction occurs when the genetic material of two different organisms fuses to form offspring. These new organisms are genetically different from their parents.
Modes of asexual reproduction include fission, budding, the production of gemmules, regeneration, fragmentation, the formation of rhizomes and stolons, and parthenogenesis. New organisms produced asexually are clones, formed from existing cells that have a full complement of chromosomes. The cells involved in most modes of asexual reproduction have divided by mitosis, a mode of cell division in which the original cell's complement of two sets of chromosomes appears in the new cell.
Bacteria and yeasts often reproduce by fission, or by cell mitosis where one cell copies its chromosomes and divides in half. Yeasts and hydras reproduce by sprouting "buds" or protrusions, which break off and form new organisms. Sponges produce small internal buds called gemmules, which, when released into the environment, produce new individuals. Planarians can produce new individuals when broken into fragments in a process called fragmentation. Each fragment grows into a new adult. Starfish can produce new starfish from pieces broken away from a single individual in a process called regeneration. Some plant species such as lilies and grasses can reproduce asexually through the production of rhizomes, corms, tubers, and bulbs (extensions or budding of root material). Strawberries can produce new plants by means of stolons or stems that branch away from the main plant and take root in the ground.
Some fishes, some insects, and some frogs and lizards can reproduce via unfertilized eggs in a process called parthenogenesis. Eggs, which are produced through a process called meiosis, contain only half the number of chromosomes as a normal cell. These eggs develop into adults, though sometimes, as in honeybees, the unfertilized eggs can develop only into males or drones. Some insects, such as aphids, reproduce through parthenogenesis when conditions are good, permitting a rapid increase in population.
The advantages of asexual forms of reproduction are that an individual's entire genetic code is reproduced without change through a fairly simple process of cell division that uses less energy, requires less time, and does not require a partner. Asexual reproduction is rapid and can produce many offspring in a short time, enabling organisms to colonize a new environment or increase populations when conditions are good. Its disadvantages are that the genetic pool of species reproducing asexually stays the same. If a bad mutation appears or conditions change, the organisms are unable to produce genetic variety that might enable better survival through the natural selection of advantageous genes.
Many organisms that can reproduce asexually can also reproduce sexually. Bacteria and other single-celled organisms can also reproduce via a process called conjugation in which the genetic material of two individual cells is reshuffled to produce a third, new individual with a different set of genes than either of its parents. Plants, insects (such as aphids), and other animals (such as starfish), also reproduce sexually by producing sex cells, or gametes, which have only the same number of chromosomes as a normal cell. Gametes are produced in a process called meiosis, in which the pairs of chromosomes that typically inhabit each cell are split in half with each half being distributed to a new cell. Cells with half the normal number of chromosomes are called haploid cells. Sperm and eggs are the human versions of gametes. When two haploid cells merge, as they do in sexual reproduction, the result is a cell with two sets of chromosomes, just like most other cells.
Sexual reproduction is defined as including those processes that result in offspring whose genetic makeup differs from either parent. Sexual reproduction is often slower and more difficult than asexual reproduction and thus does not benefit small populations under difficult circumstances. Sexual reproduction does, however, have the advantage of producing genetic variety and renewal in populations, permitting bad genes to be eliminated and useful genes to be propagated by natural selection.
Sexual reproduction involves the fusing of two gametes. How those gametes get together occurs through a large variety of mechanisms and depends upon a number of factors, including hormones to help individuals produce gametes, the timing of fertilization, and how rich the environment is in which individuals try to survive. Many organisms, especially those living in aquatic environments such as fish, accomplish the fusion of gametes (eggs from females, sperm from males) outside of their bodies in the water. Females deposit eggs and males swim past the eggs releasing sperm into the water. Plants distribute sperm or pollen by means of a number of different vectors including insects, the wind, and birds, depositing it on the stamens of other plants of the same species. In many species, including reptiles and mammals, the male deposits sperm directly into the reproductive organs of the female during a process called sexual intercourse. The gametes fuse inside the female's body and then in some species emerge as eggs (as with birds) in a process called oviparity, or develop inside the mother's body with no connection to the mother as with guppies or snakes (called ovoviviparity), or in others as fetuses to a gestation site on the mother's body (as with marsupials), or are connected to the mother via a placenta in an organ called the uterus as with mammals.
When sperm and egg merge in sexual reproduction, they form a single cell called a zygote with genetic information from both parents. This zygote begins to split and multiply through mitosis into more cells through a process called cleavage. Cell cleavage forms a hollow ball of cells called a blastula, which begins the process of cell differentiation into three layers of cells, which will ultimately form the various organs, tissues, and structures of the fetus. These layers continue to differentiate through a process called organogenesis. One layer, the ectoderm, will form the skin, nervous system, and pituitary gland. The second, the mesoderm, will form the skeleton, muscles, circulatory system, bowels and bladder, and the reproductive organs. The third, the endoderm, forms the liver and the linings of most of the body's internal systems.
After a period of gestation inside the mother's body during which the single-celled zygote grows into a fully formed organism with gametes of its own, the fetus is born. Some infant species are almost completely able to care for themselves. Others, such as most birds and mammals, require an additional period of development and care. Most of these species require additional growth and development before they are able to reproduce in turn.
HUMAN INTERVENTION IN REPRODUCTION
Humans currently have the ability to help the reproductive process and systematically participate in the reproduction of various species. Many animals are bred selectively, including dogs, cats, birds, and fish. Many farm animals such as cows and horses are artificially inseminated with the sperm of the best examples of their breeds. Individuals experiencing difficulty reproducing can be helped by augmenting their hormones, artificially inseminating mothers, or even harvesting a mother's eggs and fertilizing them outside of her body in a process called in vitro fertilization, then implanting the fertilized blastulas into the mother's uterus to develop. Surrogate mothers are also employed to gestate fetuses formed from the gametes of two other individuals.
The human imagination has also created modes of asexual reproduction in humans such as cloning, in which another identical organism is grown from a few cells of a single individual. Also envisioned has been the self-reproduction of mechanical beings, such as robots, who would have the ability to manufacture more of themselves in an automated process. Science fiction is full of examples of reproductive technologies that have been mechanized from Aldous Huxley's bottled babies in Brave New World to the reproduction machines of The Matrix to the creation of Frankenstein or the use of humans as environments for the development of fetal aliens, as in Alien.
The ability to choose and control reproduction has been a political issue for many years. Ethical issues about how much and in what ways humans should intervene in reproduction join questions about who has the right to terminate a pregnancy and who has parental rights in children who were reproduced with the help of sperm donors or surrogate mothers. Some observers have also raised the concern that humankind's rate of reproduction is reproducing too many humans, which will cause future difficulties as the earth reaches its limits.
see also Fertility.
Mader, Sylvia S. 2005. Human Reproductive Biology. 3rd edition. Dubuque, IA: McGraw-Hill.
Miller, Stephen A., and John P. Harley. 2005. Zoology. 6th edition. Boston: McGraw-Hill.
Piñón, Ramón, Jr. 2002. The Biology of Human Reproduction. Sausalito, CA: University Science Books.