An adaptation is a positive characteristic of an organism that has been favored by natural selection. The concept is central to biology, particularly in evolutionary biology. The term adaptation is also sometimes used as a synonym for natural selection, but most biologists discourage this usage.
There is a great difference between adaptation and acclimation. Adaptation occurs over many generations; it is a gradual process caused by natural selection. Acclimatization generally occurs within a single lifetime and copes with issues that are less threatening. For example, if a human was to move to a higher altitude, respiration and physical exertion would become a problem, but after spending time in high altitude conditions one may acclimate to the pressure and function and no longer notice the change. This ability to acclimate is an adaptation, but not the acclimation itself.
http://en.wikipedia.org/wiki/Evolution
In biology, evolution is the change in the inherited traits of a population from generation to generation. These traits are the expression of genes that are copied and passed on to offspring during reproduction. Mutations in these genes can produce new or altered traits, resulting in heritable differences (genetic variation) between organisms. New traits can also come from transfer of genes between populations, as in migration, or between species, in horizontal gene transfer. Evolution occurs when these heritable differences become more common or rare in a population, either non-randomly through natural selection or randomly through genetic drift.
Natural selection is a process that causes heritable traits that are helpful for survival and reproduction to become more common, and harmful traits to become more rare. This occurs because organisms with advantageous traits pass on more copies of these heritable traits to the next generation. Over many generations, adaptations occur through a combination of successive, small, random changes in traits, and natural selection of those variants best-suited for their environment. In contrast, genetic drift produces random changes in the frequency of traits in a population. Genetic drift arises from the role chance plays in whether a given individual will survive and reproduce.
Adaptations are structures or behaviors that enhance a specific function, causing organisms to become better at surviving and reproducing. They are produced by a combination of the continuous production of small, random changes in traits, followed by natural selection of the variants best-suited for their environment. This process can cause either the gain of a new feature, or the loss of an ancestral feature. An example that shows both types of change is bacterial adaptation to antibiotic selection, with mutations causing antibiotic resistance by either modifying the target of the drug, or removing the transporters that allow the drug into the cell.
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EDIT:
Proof of Evolution: The Human Immune System.
Every time the immune system adapts to fight off a new type of virus or bacteria, it is fundamentally different than it was before.
For example, when you were a small child, you probably got chicken pox. Then your body developed an immunity, that is to say that your immune system adapted to the virus thus preventing you from ever contracting the disease again.
This immunity you now have is fundamentally different from your previous immunities in that this disease will no longer affect you.
There you go. Evolution proved. Pencils down, class dismissed.