Page "Anisogamy" Paragraph 8

So, it is fertilization risk and the cooperation of different mating types to deal with it that caused the evolution of anisogamy.

Sperm evolve to increase fertilization events and eggs evolve to compensate the nutrient loss.

Additionally, sperm can evolve even smaller when eggs become larger to ensure zygote nutrient, and eggs can evolve even larger when sperm become smaller to reduce fertilization risks.

This cooperation may be the reason why many organisms have extremely small sperm and extremely large eggs.