Key Terms in Evolutionary Biology of Sex, Gender, and Sexuality: Anisogamy

= Anisogamy =

Definition
Anisogamy, sometimes refered to as heterogamy, is a form of sexual reproduction in which two gametes of different sizes and/or forms fuse together.1Within anisogamous reproduction, the smaller gamete is almost always the male gamete, while the larger one is almost always the female gamete. The male gametes are sperm cells and the female gametes are egg cells.

Example(s) of use in context
There are several different combinations of gametes that can result in anisogamous reproduction. The first type would be if both the larger female gamete and the smaller male gamete are motile, or flagellated. Other times, neither the female or male gametes are motile, which can occur in some plant life. Lastly, one gamete can be motile while the other is not; this would occur when a flagellated sperm cell fertilizes a non-flagellated egg cell. [See Figure 1.]

In the context of our class and the evolutionary biology framework in which we have been studying reproduction, we have looked at anisogamy as it occurs in animals. This type of anisogamy, where a large and non-motile egg cell (also called an ovum) is fertilized by a much smaller, motile sperm cell (also called a spermatozoon) is called oogamous reproduction. The large ovum is designed for longevity, while the smaller sperm cell is designed for optimum speed and mobility.2

Scientific background &amp; history
It has been popularly theorized that anisogamous reproduction evolved from the disruptive selective pressures on isogamous reproduction, wherein both gametes are morphologically the same and, rather than being differentiated by female and male, are instead denoted as “+” or “-” gametes.3

Evolutionary game theory has been used as a way to explain the existence and continuous stability of anisogamous strategies of reproduction. This theory says that in ancestral unicellular organisms, isogamy was sufficient and efficient. The theory holds, though, that the development of multicellular organisms may have left the relationship between the fitness of a gamete and its size mostly unchanged, but the relationship between the fitness of a zygote and its size led to the evolution of anisogamy.

More recently, a theory emphasizing the importance of the physical constraints of the process of the fusion of gametes has emerged, that these constraints would be enough to cause strong disruptive selection. This theory provides an explanation of the necessary development of anisogamous reproduction, though it should be noted that there may have been other selective forces at play in the specialized development of micro and macrogametes.4

Debates &amp; controversial applications
In the context of our coursework, it is important to look at the ways in which anisogamy has been applied to theories about the roles of male and female organisms, specifically humans, in the reproduction and raising of offspring.5 Some evolutionary biologists have asserted that because women possess egg cells, gametes that are larger and more limited resources, and men posses sperm cells, gametes which are smaller and multitudinous in number, there are inherent differences in parental investment based solely on the anisogamous nature of human reproduction.

Based on these differences, some evolutionary biologists have stated that women, with their large and limited gametes, must be more selective about the men who they choose as mates to "spend" their ova on. Men, on the other hand, with their large supply of tiny gametes, are focused on spreading their seed to as many women as possible. This has led to theories that the natural human reproduction model is not, as is commonly practiced, monogamous relationships, but rather polygyny or polygamy, or a harem-like relationship structure; this theory is called the polygyny threshold model.6

Author
Danica Marie Hecht

Figure 1
7