How are intermediate forms of adaptations produced by natural selection?
Intermediate forms are one of the key predictions of the theory of evolution, which stipulates that species evolved through a gradual process of natural selection acting on small variations. It follows that if a kind of animal C is evolved from a different kind of animal A, there will have been intermediate forms B1, B2, B3 etc between A and C. Adaptation is brought about by cumulative natural selection, the repeated sifting of mutations by natural selection. Small changes, favored by selection, can be the stepping-stone to further changes. Complex traits usually evolve through viable intermediates. For many traits, it initially seems unlikely that intermediates would be viable. A trait's current utility is not always indicative of its past utility. It can evolve for one purpose, and be used later for another. A trait evolved for its current utility is an adaptation; one that evolved for another utility is an exaptation.
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