  The vast majority of multi-cellular organisms are anisogamous, meaning that
male and female sex cells differ in size. It remains an open question how this
asymmetric state evolved, presumably from the symmetric isogamous state where
all gametes are roughly the same size (drawn from the same distribution). Here,
we use tools from the study of nonlinear dynamical systems to develop a simple
mathematical model for this phenomenon. Using theoretical analysis and
numerical simulation, we demonstrate that competition between individuals that
is linked to the mean gamete size will almost inevitably result in a stable
anisogamous equilibrium, and thus isogamy may naturally lead to anisogamy.
