Environmental pressure, like what Malthus described, will determine which traits are more favorable than others. Traits associated with a better response to the pressure are adaptive and they become legitimate adaptations when they increase so much in frequency that a majority of the population has the trait. Adaptations take hold when successive generations of individuals with the adaptive trait have increased reproductive success or fitness.
For example, giraffes that have long necks may be the only ones that can reach and eat the leaves on the highest tree branches during times of drought. These long-necked individuals will have offspring that have long necks that are also successful during times of drought, and so on.
Adaptations can arise from unexpected places. For example, having a long neck for eating out-of-reach leaves could, in a new situation, be adaptive for something completely different like keeping the head above water in a flood. In this hypothetical case, having a long neck for feeding is an exaptation for swimming. It is hypothesized that the human larynx is an exaptation since it may have been co-opted for making speech sounds after it had already dropped in the throat for other reasons.
Certainly, the state of lacking a trait can be an adaptation as well. The loss of hind legs in whales led to their streamlined bodies for swimming. It is not yet known whether the loss of the ape and human tail offered an advantage or was simply the result of happenstance. After all, having a tail is beneficial for balancing on tree branches, and as a consequence of being tailless, apes do not move about the same way that monkeys with tails do.
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