Having gametes that differ greatly in size; i.e., ova and sperm. This results in genders; by biological definition, females who produce large ova, and males who produce small sperm.

Evolution selects for anisogamy over isogamy because having a female produce large, nutritive gametes (ova) enhances survival of the resulting zygote after fertilization. Males are then selected for producing large quantities of small, fast, motile gametes (sperm) that have the best probability of fertilizing ova.