Individual's don't evolve, populations do.
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For Evolution to occur, natural selection must come before it. Natural selection is basically the selection from varied, or a pool of genes, for the best genes that contribute to survival and reproduction. For example, snails and clownfish are hermaphite with both female and male sexual reproduction organs; or rabbits become white during winter...
When natural selection take course, there must be a mutation, creating a different, and better gene for the animal. The animal then pass it on, as most animals have multiple offspring, the offsprings will either recieve that gene or become a carrier. As the offspring mate, they continue to pass on the trait. The successful ones with that particular trait will have a higher rate of survival, especially from predators and diseases, etc. So eventually, only the animals with that particular gene will survive. Note that during this process, it takes many, many offspring for natural selection and evolution to occur; so that's why individual don't evolve.
When natural selection take course, there must be a mutation, creating a different, and better gene for the animal. The animal then pass it on, as most animals have multiple offspring, the offsprings will either recieve that gene or become a carrier. As the offspring mate, they continue to pass on the trait. The successful ones with that particular trait will have a higher rate of survival, especially from predators and diseases, etc. So eventually, only the animals with that particular gene will survive. Note that during this process, it takes many, many offspring for natural selection and evolution to occur; so that's why individual don't evolve.