At some point in evolution (almost 6 million years ago), humans began walking on two legs -- an adaptation that enabled them to hunt, flee and use their hands to make primitive tools. Bipedalism was an adaptation and a beneficial one, which is why it was passed on through natural selection; the walkers had survival advantages and produced more offspring that inherited the ability to walk upright. But adaptations are traits, distinct from the natural selection that drives them.
Natural selection is simply the tendency of beneficial traits to increase in frequency in a population. This occurs when the trait is beneficial (increasing the organism’s chance of survival, mating and reproducing), and heritable (it can be passed down through generations). A clear example is the peacock’s iridescent rump plumage. The tail feathers, which are 4 to 5 feet long, actually hinder the male's ability to flee predators, but they attract females that prefer the most elaborately adorned males as mates. Thus prehistoric longer-tailed peacocks mated more frequently than shorter-tailed peacocks, sired more offspring and the trait was passed on to the point that males in the entire peafowl species now have extravagant plumage. The color of the tail feathers evolved over time as well, and tells us that peahens favored brightly colored plumage.
An adaptation is the characteristic itself that increases an organism's chances of surviving, mating and reproducing. The peacock’s tail is such an adaptation; so is the snake’s hinged jaw, which enables it to eat larger prey like rodents and frogs, which may be larger than the snake’s head.
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Other examples of beneficial traits include protective coloration, the ability to use a new food source (e.g., lactose tolerance), or a change in size or shape that enables a species to adapt more successfully to an environment.
How They Relate
Natural selection and adaptation are distinct from one another. Natural selection is the mechanism that drives the evolution of adaptations. The adaptation is the characteristic, while natural selection is the mechanism that ensures that characteristic is passed on and becomes commonplace.
The ancient lungfish, which appeared about 417 million years ago, was able to survive droughts in ways that other fish could not. A few fish might have had a superior ability to breathe surface air in a shallow pool, a characteristic that was passed on because they survived and reproduced, ultimately leading to the adaptation of lungs.
Mutation Selection Theory
Mutation selection theory holds that adaptations are sudden and random. This theory would hold that, all of a sudden, a longer-tailed peacock appeared and for no apparent purpose, as did a snake with an articulated jaw. Humans with six fingers appear often enough (and presumably did so in prehistoric populations). But a mutation may be beneficial, deleterious or neutral. Only beneficial mutations are passed on through natural selection. Presumably, a sixth finger has proven to offer no benefit to humans, as it remains a mutation rather than a trait.