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Abstract
A popular theory has proposed that anisogamy originated through disruptive selection
acting on an ancestral isogamous population, though recent work has emphasized the
importance of other factors in its evolution. We re-examine the disruptive selection
theory, starting from an isogamous population with two mating types and taking into
account the functional relationship, g(m), between the fitness of a gamete and its
size, m, as well as the relationship, f(S), between the fitness of a zygote and its
size, S. Evolutionary game theory is used to determine the existence and continuous
stability of isogamous and anisogamous strategies for the two mating types under various
models for the two functions g(m) and f(S). In the ancestral unicellular state, these
two functions are likely to have been similar; this leads to isogamy whether they
are sigmoidal or concave, though in the latter case allowance must be made for a minimal
gamete size. The development of multicellularity may leave g(m) relatively unchanged
while f(S) moves to the right, leading to the evolution of anisogamy. Thus, the disruptive
selection theory provides a powerful explanation of the origin of anisogamy, though
other selective forces may have been involved in the subsequent specialization of
micro- and macrogametes.