Anisogamy

Anisogamy is sexual reproduction involving two classes of gametes that differ in size. The larger gamete defines the female reproductive role, and the smaller gamete the male role. In humans, an ovum and a sperm illustrate the distinction. Mobility is a separate character: the definition is not simply that one gamete moves and the other does not.

A resource-budget model makes the evolutionary tension visible. With total investment $B$ and mass $m$ per gamete, the number produced is $B/m$. Smaller gametes can be more numerous, whereas a larger combined contribution can improve survival of the resulting zygote. Models ask when these competing effects favor distinct size classes rather than a single intermediate size; the answer depends on their assumptions about encounters and biological fitness.^[ref]