Abstract
The chapter focuses on the possibility that an externally fertilizing organism's sexual selection is intense but approximately symmetrical across sexes. The result of sperm limitation leads to increased variation in the proportion of a female's eggs that are fertilized. Hence, the increased variation in female reproductive success relative to taxa with internal fertilization results in a selection for enhanced fertilization success for both males and females. As a consequence, sexual dimorphism in both primary and secondary sexual characteristics is reduced or absent. This hypothesis leads to the notion that anisogamy and copulation evolved because of sperm limitation rather than sperm competition. However, an adaptation, in this scenario, benefits both males and females. Males benefit because the average success of males increases, and males who are the best at copulation gain greatly. Females also benefit greatly because fertilization ceases to be a limiting factor, and choice becomes an option. The sperm-limitation hypothesis implies not that sperm competition is absent among broadcast spawners but that it has reduced importance and may be balanced by egg competition.
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