Anisogamy
Anisogamy is a form of sexual reproduction that involves the union or fusion of two gametes that differ in size and/or form. The smaller gamete is male, a microgamete or sperm cell, whereas the larger gamete is female, a larger macrogamete or typically an egg cell. Anisogamy is predominant among multicellular organisms. In both plants and animals, gamete size difference is the fundamental difference between females and males.
Anisogamy most likely evolved from isogamy. Since the biological definition of male and female is based on gamete size, the evolution of anisogamy is viewed as the evolutionary origin of male and female sexes. Anisogamy is an outcome of
both natural selection and sexual selection, and led the sexes to evolve different primary and secondary sex characteristics including sex differences in behavior.
Geoff Parker, Robin Baker, and Vic Smith were the first to provide a mathematical model for the evolution of anisogamy that was consistent with modern evolutionary theory. Their theory was widely accepted but there are alternative hypotheses about the evolution of anisogamy.
Etymology
Anisogamy comes from the ancient Greek negative prefix a-, the Greek adjective isos and the Greek verb gameo, eventually meaning "non-equal reproduction" obviously referring to the enormous differences between male and female gametes in size and abilities. The first known use of the term "anisogamous" was in the year 1891.Definition
Anisogamy is the form of sexual reproduction that involves the union or fusion of two gametes which differ in size and/or form. The smaller gamete is considered to be male, whereas the larger gamete is regarded as female.There are several types of anisogamy. Both gametes may be flagellated and therefore motile. Alternatively, as in flowering plants, conifers and gnetophytes, neither of the gametes are flagellated. In these groups, the male gametes are non-motile cells within pollen grains, and are delivered to the egg cells by means of pollen tubes. In the red alga Polysiphonia, non-motile eggs are fertilized by non-motile sperm.
The form of anisogamy that occurs in animals, including humans, is oogamy, where a large, non-motile egg is fertilized by a small, motile sperm. The egg is optimized for longevity, whereas the small sperm is optimized for motility and speed. The size and resources of the egg cell allow for the production of pheromones, which attract the swimming sperm cells.
Sexual dimorphism
Anisogamy is a core element of sexual dimorphism that helps to explain phenotypic differences between sexes. Researchers estimate that over 99.99% of eukaryotes reproduce sexually. Most do so by way of male and female sexes, both of which are optimized for reproductive potential. Due to their differently sized and shaped gametes, both males and females have developed physiological and behavioral differences that optimize the individual's fecundity. Since most egg laying females typically must bear the offspring and have a more limited reproductive cycle, this typically makes females a limiting factor in the reproductive success rate of males in a species. This process is also true for females selecting males, and assuming that males and females are selecting for different traits in partners, would result in phenotypic differences between the sexes over many generations. This hypothesis, known as the Bateman's Principle, is used to understand the evolutionary pressures put on males and females due to anisogamy. Although this assumption has criticism, it is a generally accepted model for sexual selection within anisogamous species. The selection for different traits depending on sex within the same species is known as sex-specific selection, and accounts for the differing phenotypes found between the sexes of the same species. This sex-specific selection between sexes over time also leads to the development of secondary sex characteristics, which assist males and females in reproductive success.In most species, both sexes choose mates based on the available phenotypes of potential mates. These phenotypes are species-specific, resulting in varying strategies for successful sexual reproduction. For example, large males are sexually selected for in elephant seals because their large size helps the male fight off other males, but small males are sexually selected for in spiders for they can mate with the female more quickly while avoiding sexual cannibalism. However, despite the large range of sexually selected phenotypes, most anisogamous species follow a set of predictable desirable traits and selective behaviors based on general reproductive success models.
Female phenotypes
For internal fertilizers, female investment is high in reproduction since they typically expend more energy throughout a single reproductive event. This can be seen as early as oogenesis, for the female sacrifices gamete number for gamete size to better increase the survival chances of the potential zygote; a process more energetically demanding than spermatogenesis in males. Oogenesis occurs in the ovary, a female-specific organ that also produces hormones to prepare other female-specific organs for the changes necessary in the reproductive organs to facilitate egg delivery in external fertilizers, and zygote development in internal fertilizers. The egg cell produced is not only large, but sometimes even immobile, requiring contact with the more mobile sperm to instigate fertilization.Since this process is very energy-demanding and time-consuming for the female, mate choice is often integrated into the female's behavior. Females will often be very selective of the males they choose to reproduce with, for the phenotype of the male can be indicative of the male's physical health and heritable traits. Females employ mate choice to pressure males into displaying their desirable traits to females through courtship, and if successful, the male gets to reproduce. This encourages males and females of specific species to invest in courtship behaviors as well as traits that can display physical health to a potential mate. This process, known as sexual selection, results in the development of traits to ease reproductive success rather than individual survival, such as the inflated size of a termite queen. It is also important for females to select against potential mates that may have a sexually transmitted infection, for the disease could not only hurt the female's reproductive ability, but also damage the resulting offspring.
Although not uncommon in males, females are more associated with parental care. Since females are on a more limited reproductive schedule than males, a female often invests more in protecting the offspring to sexual maturity than the male. Like mate choice, the level of parental care varies greatly between species, and is often dependent on the number of offspring produced per sexual encounter.
In many species, including ones from all major vertebrate groups females can utilize sperm storage, a process by which the female can store excess sperm from a mate, and fertilize her eggs long after the reproductive event if mating opportunities drop or quality of mates decreases. By being able to save sperm from more desirable mates, the female gains more control over its own reproductive success, thus allowing for the female to be more selective of males as well as making the timing of fertilization potentially more frequent if males are scarce.
Male phenotypes
For males of all species, the sperm cells they produce are optimized for ensuring fertilization of the female egg. These sperm cells are created through spermatogenesis, a form of gametogenesis that focuses on developing the most possible gametes per sexual encounter. Spermatogenesis occurs in the testis, a male specific organ that also produces hormones that trigger the development of secondary sex characteristics. Since the male's gametes are energetically cheap and abundant in every ejaculation, a male can greatly increase his sexual success by mating far more frequently than the female. Sperm, unlike egg cells, are also mobile, allowing for the sperm to swim towards the egg through the female's sexual organs. Sperm competition is also a major factor in the development of sperm cells. Only one sperm can fertilize an egg, and since females can potentially mate with more than one male before fertilization occurs, producing sperm cells that are faster, more abundant, and more viable than that produced by other males can give a male reproductive advantage.Since females are often the limiting factor in a species reproductive success, males are often expected by the females to search and compete for the female, known as intraspecific competition. This can be seen in organisms such as bean beetles, as the male that searches for females more frequently is often more successful at finding mates and reproducing. In species undergoing this form of selection, a fit male would be one that is fast, has more refined sensory organs, and spatial awareness.
Some secondary sex characteristics are not only meant for attracting mates, but also for competing with other males for copulation opportunities. Some structures, such as antlers in deer, can provide benefits to the male's reproductive success by providing a weapon to prevent rival males from achieving reproductive success. However, other structures such as the large colorful tail feathers found in male peacocks, are a result of Fisherian runaway as well as several more species specific factors. Due to females selecting for specific traits in males, over time, these traits are exaggerated to the point where they could hinder the male's survivability. However, since these traits greatly benefit sexual selection, their usefulness in providing more mating opportunities overrides the possibility that the trait could lead to a shortening of its lifespan through predation or starvation. These desirable traits extend beyond physical body parts, and often extend into courtship behavior and nuptial gifts as well.
Although some behaviors in males are meant to work within the parameters of female choice, some male traits work against it. Strong enough males, in some cases, can force themselves upon a female, forcing fertilization and overriding female choice. Since this can often be dangerous for the female, an evolutionary arms race between the sexes is often an outcome.