Two Sexes

Abstract

Males and females of a particular species may differ in many ways. Such differences may include hormone levels, genotypes, and various anatomical and physiological characteristics. However, the only consistent difference across all species having two sexes is gamete size; females have larger, stationary eggs, while males have smaller, mobile sperm or pollen. Models suggest that having two sizes of gametes, especially when the larger is stationary and sends out signals, has been selected because the ability to find each other is high. Although a few species remain isogamous (having gametes of the same size), even these have differences between mating types that permit recognition. In addition, a few species have more than one mating type, potentially increasing the encounter rate of a mating partner. These conditions are, however, rare. While the ability to produce male and female gametes is most often found in different individuals, hermaphrodites can contain both types of sex cells within one body. Regardless, self-fertilization is uncommon. After male and female gametes come together, they undergo a series of physiological processes, enabling them to recognize one another and ultimately combine their genetic material.