Sexual reproduction involving a large, female gamete and a small, male gamete
Anisogamy is a form of sexual reproduction that involves the union or fusion of two gametes that differ in size and/or form. The smaller gamete is male, a sperm cell, whereas the larger gamete is female, typically an egg cell. Anisogamy is predominant among multicellular organisms.[1] In both plants and animals, gamete size difference is the fundamental difference between females and males.[2]
Anisogamy most likely evolved from isogamy.[3] 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 sexes.[4][5] Anisogamy is an outcome of
both natural selection and sexual selection,[6] and led the sexes to different primary and secondary sex characteristics[7] including sex differences in behavior.[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.[4] Their theory was widely accepted but there are alternative hypotheses about the evolution of anisogamy.[9][1]