Anisogamy
[8] 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.
The form of anisogamy that occurs in animals, including humans, is oogamy, where a large, non-motile egg (ovum) is fertilized by a small, motile sperm (spermatozoon).
[15] Anisogamy is a core element of sexual dimorphism that helps to explain phenotypic differences between sexes.
This hypothesis, known as the Bateman's Principle, is used to understand the evolutionary pressures put on males and females due to anisogamy.
The egg cell produced is not only large, but sometimes even immobile, requiring contact with the more mobile sperm to instigate fertilization.
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,[16] results in the development of traits to ease reproductive success rather than individual survival, such as the inflated size of a termite queen.
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.
These sperm cells are created through spermatogenesis, a form of gametogenesis that focuses on developing the most possible gametes per sexual encounter.
[21] Spermatogenesis occurs in the testis, a male specific organ that also produces hormones that trigger the development of secondary sex characteristics.
[19] 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.
[22] 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.
[27] Anisogamy first became a major topic in the biological sciences when Charles Darwin wrote about sexual selection.
[29] According to John Avise anisogamy probably originated around the same time sexual reproduction and multicellularity occurred,[30] over 1 billion years ago.
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.
[37] 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.
[48] A valuable model system to the study of the evolution of anisogamy is the volvocine algae, which group of chlorophytes 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.
[49] Marine algae have been closely studied to understand the trajectories of such diversified reproductive systems,[50] evolution of sex and mating types,[51] as well as the adaptiveness and stability of anisogamy.
