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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?
- Heritable
If it has been shaped by natural selection, it must be genetically
encoded — since natural selection cannot act on traits that
don't get passed on to offspring. Are feathers
heritable? Yes. Baby birds grow up to have feathers like those
of their parents.
- Functional
If it has been shaped by natural selection for a particular task,
the trait must actually perform that task. Do feathers function
to enable flight? In the case of bird flight, the
answer is fairly obvious.
Birds with feathers are able to fly and birds without feathers
would not be able to.
- Adaptive
If it has been shaped by natural selection, it must increase
the fitness of
the organisms that have it — since natural selection only
increases the frequency of traits that increase fitness. Are birds
more fit with feathers than without? Birds without feathers
aren't going to leave as many offspring as those with feathers.
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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?
- Current function
Did the trait arise when the current function arose? Did feathers
arise when flying arose? The answer to this is probably no. The
closest fossil relatives of birds, two-legged dinosaurs called
theropods, appear to have sported feathers but could not fly.
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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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