The anisogamy, or heterogamy, is the union of two gametes, called anisogametes, which differ in size and structure. It is the opposite term to isogamy, which is the union of two similar gametes, called isogametes..
Depending on the type of organism, anisogametes have different characteristics. For example, both gametes can be flagellate, as in some algae, or amoeboid, as occurs in some protozoa.
In oogamy, which is the variant of anisogamy in more complex organisms, a small, mobile gamete, often referred to as a sperm, or pollen, fertilizes a much larger, immobile gamete, called an ovum..
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The evolution of diploid multicellular organisms created the necessary conditions for the development of anisogamy, which is believed to have been evolutionarily preceded by isogamy. Gametes serve only for sexual reproduction. Anisogamy allowed specializations related to this type of reproduction.
The progression of anisogamy towards its most extreme form, that is, oogamy, led to the differentiation of the male and female sexes, with all that this implies in terms of dimorphism, selection and sexual roles..
As multicellular organisms grew larger, they often became rare. Sedentary habits prevailed in plants and numerous groups of marine animals. This created difficulties for the male and female gametes to meet..
Males specialized in producing very high numbers of small gametes (microgametes) capable of locating and fertilizing female gametes. The females specialized in producing a reduced number of large and immobile gametes (macrogametes), provided with nutritive matter for the development of the embryo.
The excess of male gametes with respect to the female inherent in anisogamy generates a strong competition between sperm, or between pollen, which favors the selection, both among male gametes and in the individuals that produce them, of those attributes that favor fertilization.
Sexual selection is the process that favors the evolution of advantageous attributes for the mating and production of offspring by males and females. Sexual selection is responsible for the existence of the characteristics that distinguish the sexes. In general, the greater the dimorphism between the sexes, the greater the sexual selection.
In males, sexual selection determines that male gametes have characteristics that increase their probability of fertilizing, or that anatomical and behavioral attributes appear that favor access to females by increasing their ability to successfully court them, or to fight other males for they.
In females, sexual selection determines the evolution of attributes that help them choose males that will allow them to produce offspring of good genetic quality, or that possess territories or provide nutritional resources that favor breeding..
In many animal species, when looking for a mate, females are selective, while males are not. This is mainly attributed to the fact that females produce a limited number of eggs that they must invest well. In contrast, males produce virtually unlimited amounts of sperm.
In species with parental care, the issue is not just a question of "expensive" eggs and "cheap" sperm: females also tend to invest more in the offspring than males. Mammalian lactation, carried out exclusively by females, is an example of this. Females also risk their lives for their young.
If a female mates with a male carrying defective genes, and her children do not reach maturity because of it, she loses her reproductive effort. Instead, males can mate with large numbers of females, passing their genes on to future generations, regardless of the failure of some of their offspring..
If males invest little in gametes and in raising their offspring, they can use the saved energy to compete with other males, and try to mate with as many females as possible, thus maximizing their reproductive capacity. This explains male sexual promiscuity in many species..
Males of many species of rodents produce "copulatory plugs." The sperm of these males solidifies inside the female reproductive tract, preventing copulation by other males. As a countermeasure, in some species, males are able to pierce plugs left by other males..
In species in which it is common for the sperm of several males to compete to fertilize the ovules of the same female, the males tend to have larger testicles and accessory glands, thus producing more concentrated and abundant sperm.
Females have evolved sophisticated mechanisms that facilitate or prevent fertilization by the sperm of different males. For this they use muscle contractions, ciliary currents and various behaviors. Chickens, for example, may voluntarily expel sperm from low-ranking roosters.
Charles Darwin considered the existence of gametes to be one of the most enigmatic aspects of living beings. A century and a half later, the raison d'être of gametes is still under debate.
Isogamy is common in single-celled organisms. In contrast, anisogamy is prevalent in multicellular plants and animals. It has been argued that size dimorphism at the gamete level allows increasing body volume and complexity.
The most widely accepted models to explain anisogamy invoke disruptive selection: small gametes would be favored because they can be produced in large numbers; large gametes would be favored because they allow a larger zygote size, which would increase the probability of embryo survival.
Recently, some authors have questioned that anisogamy is an important factor in the evolution of sexual roles. According to them, these roles could have arisen at random, or as a result of differences in the life histories of males and females..
However, the current broad majority consensus is that chance may not have caused consistent differences between the sexes, and that differences in male and female life histories are the result of selection ultimately determined by anisogamy..
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