I've read, I believe that was by Richard Dawkins, maybe in the "selfish gene", that sexual reproduction is a stable evolutionary strategy, or that sexes are adaptative peaks that arises easily.
Seems that is really isogametes - equal gametes, both are "half" egg and sperm, with no specialization for reaching the other gamete or to just wait and select a certain gamete that reaches - that are a hard equilibrium. If a genetic condition produces gametes that instead of simply "float" randomly unworried to met other, rather have some strategy of reaching another gamete quicker that others that do not do it, that's a great advantage. This strategy may be simply producing more and tinier gametes, to start. At the same time, with some gametes being reached easily by those specializing in seek for another, there would be a advantage for just relaxing and staying in, wainting to be reached. But, better than just being "lazy", is making gametes that grant a good development for the formed zygote. This can be simply producing a smaller number, but better, more nutritious gametes. And then the peaks are formed, and any advantage that make the pre-sperm or pre-egg more efficient in being in their respective adaptations will spread very rapidly; the more isogametic-like a gamete is, the more disvantage he's in.
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