Why are some creatures two genders look the same and some don't

In evolutionary terms the general thinking behind this is that sexual dimorphism is driven primarily by sexual selection - which is occurs when one sex is "limiting" to the other as a "resource" for reproducing. Although it works the other way around in some species, typically females are the "limiting sex" and this can drives males to compete with each other (which may mean they need to be large in some species that fight physically, or carry weaponry like horns). It also means that females can be choosy, and this imposes selection on males for ornamental traits (e.g. tails in peacocks). In such cases the male traits would not be beneficial to the females and may in fact be costsly. So for instance in guppies, bright colours on the males are attractive to females, but also get you noticed by a predator. For males this cost is worth it, for females the colours carry only downsides - resulting in sexual dimorphism.

In many animals female fitness is limited by factors other than access to males. So, for example fecundity often increases with body size in insects and fish. So in these groups, and especially if males compete for females in ways that don't require overt strength (e.g. by trying to be more attractive, or through sperm competition) females are often larger than males.

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