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Anisogamy is a form of sexual reproduction that involves the union or fusion of two that differ in size and/or form. The smaller gamete is male, a microgamete or sperm, 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 and .
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 . Anisogamy is an outcome of both natural selection and sexual selection, and led the sexes to 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.
There are several types of anisogamy. Both gametes may be flagellum and therefore motility. Alternatively, as in , Coniferophyta and Gnetophyta, 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 . In the red algae Polysiphonia, non-motile eggs are fertilized by non-motile sperm.
The form of anisogamy that occurs in , including , is oogamy, where a large, non-motile egg (ovum) is fertilized by a small, motile sperm (spermatozoon). 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 , which attract the swimming sperm cells. (2025). 9780674031166, Harvard University Press. ISBN 9780674031166
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.
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.Davies, N. B., Krebs, J. R., & West, S. A. (2012). An introduction to behavioural ecology. Oxford: Wiley-Blackwell.
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 (2025). 9780128151457 ISBN 9780128151457 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.
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 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.
Mathematical models seeking to account for the evolution of anisogamy were published as early as 1932, but the first model consistent with evolutionary theory was that published by Geoff Parker, Robin Baker and Vic Smith in 1972.
The three main theories for the evolution of anisogamy are gamete competition, gamete limitation, and intracellular conflicts, but the last of these three is not well supported by current evidence. (2025). 9780123725684 ISBN 9780123725684 Both gamete competition and gamete limitation assume that anisogamy originated through disruptive selection acting on an ancestral isogamous population with external fertilization, due to a trade-off between larger gamete number and gamete size (which in turn affects zygote survival), because the total resource one individual can invest in reproduction is assumed to be fixed.
The first formal, mathematical theory proposed to explain the evolution of anisogamy was based on gamete limitation: this model assumed that natural selection would lead to gamete sizes that result in the largest population-wide number of successful fertilizations. If it is assumed that a certain amount of resources provided by the gametes are needed for the survival of the resulting zygote, and that there is a trade-off between the size and number of gametes, then this optimum was shown to be one where both small (male) and large (female) gametes are produced. However, these early models assume that natural selection acts mainly at the population level, something that is today known to be a very problematic assumption. (2025). 9781400820108 ISBN 9781400820108
The first mathematical model to explain the evolution of anisogamy via individual level selection, and one that became widely accepted was the theory of gamete or sperm competition. Here, selection happens at the individual level: those individuals that produce more (but smaller) gametes also gain a larger proportion of fertilizations simply because they produce a larger number of gametes that 'seek out' those of the larger type. However, because zygotes formed from larger gametes have better survival prospects, this process can again lead to the divergence of gametes sizes into large and small (female and male) gametes. The end result is one where it seems that the numerous, small gametes compete for the large gametes that are tasked with providing maximal resources for the offspring.
Some recent theoretical work has challenged the gamete competition theory, by showing that gamete limitation by itself can lead to the divergence of gamete sizes even under selection at the individual level. While this is possible, it has also been shown that gamete competition and gamete limitation are the ends of a continuum of selective pressures, and they can act separately or together depending on the conditions. These selection pressures also act in the same direction (to increase gamete numbers at the expense of size) and at the same level (individual selection). Theory also suggests that gamete limitation could only have been the dominant force of selection for the evolutionary origin of the sexes under quite limited circumstances, and the presence on average of just one competitor can makes the 'selfish' evolutionary force of gamete competition stronger than the 'cooperative' force of gamete limitation even if gamete limitation is very acute (approaching 100% of eggs remaining unfertilized).
There is then a relatively sound theory base for understanding this fundamental transition from isogamy to anisogamy in the evolution of reproduction, which is predicted to be associated with the transition to multicellularity. In fact, Hanschen et al. (2018) demonstrate that anisogamy evolved from isogamous multicellular ancestors and that anisogamy would subsequently drive secondary sexual dimorphism. Some comparative empirical evidence for the gamete competition theories exists, (2025). 9780521880954 ISBN 9780521880954 although it is difficult to use this evidence to fully tease apart the competition and limitation theories because their testable predictions are similar. It has also been claimed that some of the organisms used in such comparative studies do not fit the theoretical assumptions well.
“Inflated isogamy” hypothesis
In 2022, Yasui proposed the “inflated isogamy” hypothesis, offering a new perspective on the evolution of anisogamy beyond the size-disruptive selection assumed in the Parker–Baker–Smith (PBS) model. In unicellular organisms, the basic state of 'smart isogamy' is observed. In this state, each parent (size 2R) splits the minimum amount of resources required for survival (R) by producing four isogametes, each of size 0.5R, through meiosis. One of these isogametes then fuses with another (0.5R + 0.5R = R). If one parent reduces its gamete size (i.e., attempts to become male), the resulting zygote fails to reach the required size R and dies. Moreover, unicellular organisms can produce only up to four gametes. However, with the evolution of multicellularity, resource production capacity increased, allowing gamete size in both sexes to "inflate" to 1R, forming zygotes of 2R. This created a temporary state in which the zygote received more resources than strictly necessary for survival. At this point, one sex adopted a "cheating" strategy by reducing its gamete size to 0.5R and thereby lowering its own investment, gaining an advantage by producing many small gametes (i.e., sperm) and fertilizing multiple partners—note that multicellularity enabled the production of more than four, even five or more, gametes. This marked the origin of males. Initially, the opposite sex (which became female) did not change its investment from the inflated isogamy state and accepted this "cheating" without gaining or losing from it, resulting in a state of commensalism in which only males benefitted. However, as males began to supply numerous small gametes, the fertilization rate increased, and females also gained an advantage by reducing the risk of unfertilized eggs. Thus, the relationship between males and females evolved into mutualism, where each compensated for the other’s weaknesses—females providing nutrition but lacking mobility, and males offering mobility but lacking nutritional contribution. Eventually, this mutualistic relationship stabilized into a distinct dimorphism: large, immobile eggs and tiny, motile sperm—a stable form of anisogamy with two separate sexes.
A valuable model system to the study of the evolution of anisogamy is the volvocine algae, which group of Chlorophyta is quite unique for its extant species exhibit a diversity of mating systems (isogamy and anisogamy) in addition to its extremes in both unicellularity and multicellularity with a diversity of forms in species of intermediate ranges of sizes. Marine algae have been closely studied to understand the trajectories of such diversified reproductive systems, evolution of sex and mating types, as well as the adaptiveness and stability of anisogamy.
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