if longer legs make you faster, and speed helps you escape predators (or be a better predator), then selection will favor long legs. But because speed is obviously always beneficial, why don’t more species have long legs? The explanation is trade-offs. Because variations almost always involve limited alternatives, natural selection has to act on competing costs and benefits. If you are long-legged and big, then you can’t be short-legged and small, which has other advantages depending on your situation. Selection will inevitably favor whichever alternative or compromise most improves your
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