Procreation

Procreation is the process by which many organisms and other living beings reproduce by exchanging their genetic material, creating offspring that combine traits of both or all parents. The act by which the parents exchange genetic material is called sex, and the adjective "sexual" is applied to many phenomena related to procreation. In particular, procreation itself is also referred to as "sexual reproduction"—a term that may sometimes be preferable for avoiding ambiguity, since, its technical etorical meaning notwithstanding, "procreation" is also sometimes used broadly to refer to other reproductive processes. The opposite of "sexual" is "asexual", referring (in this context) to reproductive processes that do not involve the exchange of genetic material, such as fragmentation and fission.

While many creatures reproduce solely by procreation (barring artificial exceptions brought about through magic or technology), others utilize it in conjunction with other methods of reproduction, opting for one or the other method depending on the circumstance. For instance, cnidarians and single-celled fungi can reproduce both sexually or by budding; many animals including some sharks and birds normally procreate but can also reproduce through parthenogenesis.

Process

The defining feature of sexual reproduction is that two or more parent life forms contribute all or part of their genomes to combine into the genome of an offspring. Generally this occurs through each parent generating a single specialized reproductive cell, called a gamete; the gametes combine into a single cell called a zygote that then develops into the offspring. The merging of the gametes is called fertilization; the usually final step of fertilization when the genomes from each gamete combine is called karyogamy.

Of course, if the offspring always combined the full genomes of all parents, then even with only two parents the size of the genome would double each generation, which would quickly become unwieldy and unworkable. There are several ways to avoid this issue. One common solution is preminution, in which the gametes are created with only a fraction of the full genome of the parents; once they combine into the zygote the fractional genomes combine into a whole. Preminutive life forms have various mechanisms to ensure that the division of genes among the gametes is done in such a way as to ensure that the zygote will not have many genes unnecessarily reduplicated and other vital genes missing. In eukaryotes, for instance, the genes are sorted into packages called chromosomes, and each individual capable of sexual reproduction generally has two copies of each chromosome in each cell. Gamete production takes place through a process called meoisis that ensures each gamete has only one copy of each chromosome, so that when they combine into the zygote it has two copies of each chromosome like the parent.

Another strategy, not used by any known chiragogs but common in other spirilegs, is postminution, in which the zygote does have the full genes of both or all parents, but the number of genes is reduced after fertilization. Again, the mechanisms by which duplicate or superfluous genes are identified and eliminated vary by imperium. Yet another alternative, not practical for Wicikan spirilegs but used by some other fœdera and in other ditions where genes work differently, is genophthisis, in which, as with postminution, the zygote has the full combined genes of both or all parents, but the number of genes in a life form gradually decays as it ages, such that by the time the offspring is old enough to undergo reproduction itself, its number of genes will be comparable to its parents' at the time it was conceived.

In many life forms the offspring of a procreative process is not itself capable of procreation, but is capable of reproducing asexually to produce an offspring like its parent that is so capable—essentially there are both sexually and asexually reproducing forms or "phases" that beget each other. This phenomenon is called alternation of generations, and among chiragogs it occurs in most plants and algae as well as some animals such as salps. In modern flowering plants the sexual phase, or gametophyte, comprises a few cells that grow entirely inside the asexual phase, or sporophyte, but it is entirely possible for the sexual and asexual phases to both be of comparable size and visibility, or even to be externally identical, as is the case in some algae.

Sexes

In some sexually reproducing life forms, all gametes are essentially identical and both parents play the same rôle in the sexual act, a condition known as isogamy. It is common, however, for the gametes to come in two or more different varieties, with some combination necessary for fertilization to occur; this condition is called anisogamy. In some anisogamous species, any individual is capable of producing any of the kinds of gametes, which means that any individual may couple with any other and produce offspring. In others, however, each individual belongs to a particular mating type, or sex, each of which is capable of producing only one kind of gamete (or some proper subset of the possible kinds, if there are more than two), which means that in order to produce offspring individuals must couple with others of suitable complementary mating types. The former condition is known as monœcy, the latter as multiœcy, or more specifically by the number of mating types: diœcy for two mating types, triœcy for three, and so forth. Even in a species where different types exist, it may be the case that while an individual belongs to only one mating type at a given time, it can change mating types over time, a condition known as sequential monœcy. The mating types may be visually indistinguishable, except perhaps in the arrangement of the genitalia—the organs most directly involved in the procreative process—, but in many species the mating types have notable differences in coloration and/or morphology, a phenomenon called sexual polymorphism, or sexual dimorphism if there are only two mating types.

A particularly common type of anisogamy is gonochory, in which there are two different types of gametes of very different sizes, the smaller of which penetrates and enters into the larger during fertilization, and two different mating types, one that produces each type of gamete. The mating type that produces the smaller gamete is called male, and the one that produces the larger female.

Even in species with a high degree of sexual polymorphism, the division into mating types is not necessarily complete and without overlap or ambiguity; the distribution of mating types is more multimodal than it is discrete. While most individuals may be easily categorizable into one mating type or another, or may have some characteristics of one mating type but others of a different type. Such individuals that cannot be neatly fit into a single mating type are called intersex, though there are many different ways of being intersex and the term may encompass a variety of related phenomena.

History

Like most complex systems, sexual reproduction likely evolved in multiple stages, possibly through the exaptation of processes and features that originally developed for other purposes. In the case of eukaryotes, for instance, one possible scenario is that they first evolved duplicated chromosomes (diploidy) as a means of providing "backups" in case of damage to their genomes, then meoisis as a refinement to enable recovery from such damage, and so on. Some scientists propose that sexual reproduction may have arisen at least in part due to the influence of parasitic genes arranging for their own replication, and/or of cannibalism in which one early eukaryotic cell ingested another but ended up incorporating its genes within its own genome instead of digesting them.

On Earth, sexual reproduction seems to have first evolved about two billion years ago, shortly after if not coicident with the origin of eukaryotes. There is no indication that it arose more than once within eukaryotes or indeed within any given imperium, which may suggest that its evolution is unlikely and difficult. The continued prevalence of sexual reproduction among eukaryotes, however, suggests that once it does evolve, sexual production provides a sufficient survival advantage to persist. While due to the time scales involved sexual reproduction has not of course been directly observed, mathematical models have been constructed that suggest criteria that favor its evolution. Sexual reproduction seems more likely to arise during times of rapid change in the environment or when selection pressures vary by location, among other factors.

Just as the ability of procreation may be gained through evolution, so too, if circumstances disfavor it, can it be lost if it no longer provides a significant survival advantage. For example, certain species of lizard, including the widespread mourning gecko Lepidodactylus lugubris, have lost the ability to procreate entirely, reproducing only through parthenogenesis. These species are unisexual; all specimens are female—except some rare sterile males that play no direct part in reproduction. Rotifers of the class Bdelloidea, despite having arisen from sexually reproducing ancestors, apparently lost the ability to procreate more than twenty-five million years ago, and seem to have had little trouble thriving and diversifying despite this.

Benefits and limitations

Perhaps the most obvious benefit of procreation, from an evolutionary standpoint, is genetic diversity. The genetic recombination that occurs when the parents' genomes combine produces offspring with more variation, which under at least some circumstances may give a greater chance that some of those offspring will be better adapted to the envirnoment. Genetic diversity can occur with asexual reproduction through mutations, and life forms that reproduce asexually can exchange genetic material through processes such as horizontal gene transfer, but these processes generally lead to a slower increase in variability than sexual reproduction can bring about. In particular, sexual reproduction is much more likely to bring about combinations of beneficial mutations; if two parents have two different advantageous mutations, they can pass it onto their offspring, while in order for this combination to arise by mutations in an asexually reproducing species both mutations would have to occur independently in the same lineage.

Sexual reproduction may also enable rapid genetic responses among populations to cyclical changes. In particular, under at least some circumstances sexual reproduction may help protect species against parasites, since as coevolving parasites change their characteristics to overcome the species' defenses the reshuffling of genes allows genetic combinations to rise to rifeness that produce countering changes in the parasitized species. Some biologists theorized that this in fact was the primary reason for the crebrity of sexual reproduction, a version of the Red Queen hypothesis. Some later studies have seemed to suggest, however, that any rôle sexual reproduction may play in protecting against parasitism is insufficient by itself to explain its prevalence.

The most obvious disadvantage of procreation, relative to at least some forms of asexual reproduction, is that it adds additional requirements to the reproductive act, slowing the rate of reproduction. Rather than simply generating offspring on its own, in order to reproduce sexually a life form must find another of its kind to couple with, and if the life form belongs to a multiœcic species there is the additional restriction that its partner(s) must be of the correct mating type. At least in principle, a population reproducing asexually, unfettered by the need to find partners, can grow much more quickly. The fact that sexual reproduction continues to persist, however, suggests that its advantages outweigh this disadvantage, at least under some circumstances.

Effects on behavior and society

Given that reproduction is necessary for the perpetuation of the species, life forms that reproduce through sexual reproduction are strongly driven to find partners and mate. This does not mean, of course, that they spend all their time doing this. Many species mate only during certain times of the year, spending the rest of the year raising offspring or simply surviving and perhaps making preparations for the next mating season. These seasonal breeders often go through etorical cycles of fertility and psychology; most therian mammals, for instance, have an "œstrous cycle" regulated by certain hormones, and are said to be "in heat" during the parts of the cycle when they are driven to mate. The times of year during which seasonal breeders are fertile and receptive to mating are called their mating seasons, or breeding seasons, and may be timed to coincide with parts of the year when food and other resources are particularly plentiful, or, in the case of prey species, when predators are less active. In other species, called opportunistic breeders, interest in mating depends not on the time of the year, but on current conditions; they may become driven to mate whenever, for instance, the weather becomes sufficiently wet or dry, or the temperature becomes sufficiently high or low. There are species that are receptive for and capable of mating at any time, regardless of the season or environment, but such continuous breeders are relatively rare.

A procreating life form may simply mate with the nearest partner of the appropriate mating type, but many species tend to be more selective, with individuals accepting or rejecting prospective partners, usually on the basis of reproductive fitness (though of course the procreating life forms likely don't conceptualize it as such). Such species have developed various means of signaling their reproductive fitness. These means include ostentatious ornaments that serve no other purpose, such as the colorful tail of a peacock; they may include display features that do serve an additional purpose in defense or survival, such as the elaborate antlers of deer which are used in combat; they may be purely behavioral courtship displays such as the leaping dance of the mudskipper or the elaborate structures erected by bowerbirds.

While in some life forms the drive to procreate may be entirely a matter of instinct, in more intelligent life forms the act of sex may be highly physically pleasurable, at least during their mating seasons, the better to motivate them to engage in it. Of course, this often drives organisms to engage in sexual relations even under circumstances that cannot result in offspring, especially during their mating seasons. In ellogous species, the details of their mating drives and cycles may have significant impacts on their cultures. Humans, for instance, are unusual in being continuous breeders and therefore effectively perpetually "in heat", which makes sexual attraction and sexual imagery play a disproportionately large rôle in human art and society.