Qualifying as an adaptation
An adaptation is a feature produced by natural selection for its current function. Based on this definition we can make specific predictions ("If X is an adaptation for a particular function, then we'd predict that...") and see if our predictions match our observations. As an example, we'll consider the hypothesis: feathers are an adaptation for bird flight. Is the evidence consistent with this hypothesis?
We could do experiments to test each of these criteria of adaptation. So far so good the feature could have been shaped by natural selection. But we also have to look at historical questions about what was going on when it arose. Did feathers arise in the context of natural selection for flight?
So perhaps the basic form of feathers is not an adaptation for flight even though it certainly serves that function now. It may be an exaptation for flight and an adaptation for some other function. Answering questions like these, has led biologists to look for alternate scenarios for the initial evolution of feathers.
This last question emphasizes the importance of understanding organisms' history through fossils such as Archaeopteryx and reconstructed phylogenies. It is not enough to know that the feature is functional right now. We want to know what was happening when it first evolved, which often involves reconstructing the phylogeny of the organisms we are interested in and determining the likely ancestral states of the characters.
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Albatross images courtesy of Gerald and Buff Corsi, Focus on Nature, Inc.; Archaeopteryx image courtesy of Dave Smith, UCMP.
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