Mismatch theory is a concept in evolutionary biology that refers to fluidity in fitness criteria.
The essence of mismatch theory is that organisms possess traits (including behavioral, emotional, and biological) that have been passed down through generations, preserved by natural selection because of their adaptive function in a given environment. However, the given environment of the evolutionary period can be quite unlike the current environment. Therefore, traits that were at one time adaptive in a certain environment, are now "mismatched" to the environment that the trait is currently present in. This can present a number of problems for the organism in question. One example is the taste of foods high in fat and sugar to humans. In Pleistocene environments, sugars and fats were relatively uncommon in the human diet. In the modern Western diet, however, foods with such properties are relatively easy to acquire. This can be problematic since an abundance of such foods combined with the human adaptation to prefer them can, and often does, contribute to obesity and chronic metabolic syndrome.
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