Beginning with Darwin, biologists have struggled with one of the key unsolved mysteries in evolutionary biology: why are male and female gametes of vastly different size in nearly all plants and animals. This phenomenon, called ?anisogamy?, underlies the evolutionary processes of sex ratio and sex differences in behavior and morphology through sexual selection. Sexual reproduction initially began with isogamy: gametes of equal-size. However, anisogamy now characterizes nearly all organisms. What forces gave rise to the evolution of anisogamy?
The first accepted theoretical approach to the problem was crafted by Parker, Baker and Smith?s (the PBS model), which generates a basic solution (isogamy/anisogamy), depending on the form of the zygote survival function and on the gamete survival requirements. However, recent work has formalized different theoretical frameworks. One idea suggests that anisogamy evolved because a larger egg has an increased chance of being fertilized since it offers a large target for sperm. Another approach suggests that small sperm prevent transmission of cytoplasmic symbionts and/or organelles.