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Not everything is an adaptation

Although living things sport some amazing adaptations, many characteristics of species are not adaptations at all.

It's tempting to look for adaptative explanations for everything, from the shape of a flower petal, to the way your dog turns in a circle before it lies down to sleep, to your neighbor's strawberry blond hair. We could make up an adaptive explanation, but there are other explanations to consider:

  1. The result of history. Why does the base sequence GGC code for the amino acid glycine in a protein, as opposed to some other amino acid? Because that's the way it happened to start out — and that's the way we inherited it from our common ancestor. There is nothing special about the relationship between GGC and glycine. It's just a historical accident that stuck around.

  2. Just a by-product. Why is blood red? It's a by-product of the chemistry of blood, which causes it to reflect red light. The chemistry of blood may be an adaptation, but blood's redness is not itself an adaptation.

  3. An outdated adaptation. It might be an adaptation for a past environment and not the current one. For example, scientists have hypothesized that the large, hard-shelled fruit of the calabash tree is actually an adaptation for seed distribution by large mammals such as the gomphothere. But these early relatives of elephants went extinct over 10,000 years ago! If the hypothesis is correct, these fruit characteristics can no longer be considered adaptations for seed distribution.

Gomphothere Calabash fruit
An artist's representation of a gomphothere (extinct). A modern-day calabash fruit.

  1. The result of genetic drift. Some biologists can get quite passionate about how much genetic variation is adaptive and is maintained by natural selection and how much is neutral and is maintained by genetic drift.

Gomphothere picture courtesy of the Florida Museum of Natural History; Calabash fruit photo courtesy of Sara Herbert, Virginia Commonwealth University.