  Sexual reproduction is not always synonymous with the existence of two
morphologically different sexes; isogamous species produce sex cells of equal
size, typically falling into multiple distinct self-incompatible classes,
termed mating types. A long-standing open question in evolutionary biology is:
what governs the number of these mating types across species? Simple
theoretical arguments imply an advantage to rare types, suggesting the number
of types should grow consistently, however, empirical observations are very
different. While some isogamous species exhibit thousands of mating types, such
species are exceedingly rare, and most have fewer than ten. In this paper, we
present a mathematical analysis to quantify the role of fitness variation -
characterised by different mortality rates - in determining the number mating
types emerging in simple evolutionary models. We predict that the number of
mating types decreases as the variance of mortality increases.
