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Adaptation
An adaptation is a feature that is common in a population because it provides
some improved function. Adaptations are well fitted to their function and are
produced by natural selection.
Adaptations can take many forms: a behavior that allows better evasion of
predators, a protein that functions better at body temperature, or an
anatomical feature that allows the organism to access a valuable new
resource—all of these might be adaptations. Many of the things that impress
us most in nature are thought to be adaptations.
Mimicry of leaves by insects
is an adaptation for evading
predators. This example is a
katydid from Costa Rica.
The creosote bush is a
desert-dwelling plant that
produces toxins that prevent
other plants from growing
nearby, thus reducing
competition for nutrients
and water.
Echolocation in bats is an
adaptation for catching
insects.
So what’s not an adaptation? The answer: a lot of things. One example is
vestigial structures. A vestigial structure is a feature that was an adaptation
for the organism’s ancestor, but that evolved to be non-functional because the
organism’s environment changed.
Fish species that live in completely dark caves have
vestigial, non-functional eyes. When their sighted
ancestors ended up living in caves, there was no longer
any natural selection that maintained the function of the
fishes’ eyes. So, fish with better sight no longer outcompeted fish with worse sight. Today, these fish still
have eyes—but they are not functional and are not an
adaptation; they are just the by-products of the fishes’
evolutionary history.
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 adaptive 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 a “just so”
story, 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 gomphothere1. 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.
An artist’s
representation of a
gomphothere (extinct).
A modern-day calabash
fruit.
4. 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.
Neutral Theory: The relative importance
of drift and selection
It might seem like everywhere we look, we see evidence of natural selection:
organisms seem to be pretty well adapted to their environments. But the
neutral theory of molecular evolution suggests that most of the genetic
variation in populations is the result of mutation and genetic drift and not
selection.
Basically, the theory suggests that if a population carries several different
versions of a gene, odds are that each of those versions is equally good at
performing its job—in other words, that variation is neutral: whether you
carry gene version A or gene version B does not affect your fitness.
The neutral theory is easily misinterpreted. It does NOT suggest:




That organisms are not adapted to their environments
That all morphological variation is neutral
That ALL genetic variation is neutral
That natural selection is unimportant in shaping genomes
The main point of the neutral theory is simply that when we see several
versions of a gene in a population, it is likely that their frequencies are
simply drifting around. The data supporting and refuting the neutral theory
are complicated. Figuring out how widely the neutral theory applies is still
the topic of much research.
Exaptations
An “exaptation” is just one example of a characteristic that evolved, but that
isn’t considered an adaptation. Stephen Gould
and Elizabeth Vrba1 proposed vocabulary to let
biologists talk about features that are and are
not adaptations:

Adaptation—a feature produced by
natural selection for its current function
(such as echolocation in bats, right).

Exaptation—a feature that performs a
function but that was not produced by natural selection for its current
use. Perhaps the feature was produced by natural selection for a
function other than the one it currently performs and was then coopted for its current function. For example, feathers might have
originally arisen in the context of selection for insulation, and only
later were they co-opted for flight. In this case, the general form of
feathers is an adaptation for insulation and an exaptation for flight.
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.
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.
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.