Before proceeding, I would like to emphasize the distinction between adaptation and adaptability. In the realm of human action, we commonly use the word “adaptation” to mean behavioral change. For example, we “adapt to the cold” by lighting fires and wearing additional clothes. However, in evolutionary terms, “adaptation” refers to Darwinian natural selection of genetic changes appropriate to particular conditions. In this case, “adaptation to the cold” might refer to hereditary changes in the subcutaneous tissue, hair structure, and general anatomy that would help an organism stay warm in cold conditions. An example would be the large body and thick coat of the woolly mammoth during the last ice age.

An adaptation is frequently defined as a genetically fixed trait, such as the structure of a head. Because the anatomy is genetically fixed, little modification is possible unless a genetic mutation produces a different fixed form. (This is why the Maya people resorted to pressing and binding the malleable skulls of their children: their genetics did not naturally create and form a conical skull shape despite the perceived cultural benefit.) When a genetic mutation happens, the organism cannot return to the original condition, so the evolutionary result is inflexible.