Differences in physical size and the ability to fulfill the requirements of sexual selection have contributed significantly to the outcome of secondary sex characteristics in each species.

In many species, males differ from females in more ways than just the production of sperm. For example, in some insects and fish, the male is smaller than the female. In seed plants, the sporophyte sex organ of a single organism includes both the male and female parts.

In mammals, including humans, males are typically

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