When we think of evolution our minds frequently picture
a bunch of animals hanging from a crowded "family tree", or the typical sequence
of four or five
stages from ape to man packed conveniently to
fit one page. We also tend to think that this biological evolution described
in our books takes place so slowly that only after geological time spans
any change is perceptible at all. And indeed that is true... sometimes.
But it is also true that sometimes the effects
of evolution can be realized within much shorter periods of time. This is
especially obvious when we consider the evolution of species with lifetimes
much shorter than ours.
The Birch and the Moth.
A (true) tale of adaptation.
The story of peppered
moths is a great example of evolutionary changes in relatively short periods
of time.
This story tells us
about the funny changes that took place on the population of peppered moths
in England around 1850. All moths of this species captured before 1848 around
the city of Manchester looked like this:
Sometime later, in 1848, a dark moth
of the same species was captured in Manchester. Just like this one:
And that was not an
isolated case. It was just the beginning of a remarkable change in the population
taking place in just a few decades. The plot below shows the statistics of
the populations of each type of moths in 1850 and in 1900. The change is
obvious and overwhelming.
In 1850 There were 23 light moths
for each dark one whereas in 1900 the ratio is one light to 23 dark. How
can we understand this population change?.
The offspring of dark
moths is made of dark moths (except if a mutation takes place). Thus, if
natural selection was responsible for the change in population, according
to Darwin's theory, dark moths must have been better adapted to their environment.
But why?.
Environmental changes.
There are several species
of birds that feed on these moths, which rest on the trunk of abundant birch
trees during the day.
Around the middle of the
19th century, the environment of the moths suffered a radical change. Before
the industrial revolution most trees had light trunks spotted by lichens.
Towards the end of the century, the growth of industrial regions led to a
growing presence of smoke and soot that destroyed the lichens and darkened
the trees.
We can now compare both
situations and ask ourselves what moths have a better chance to survive and
procreate in each case.
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1850
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1900
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Imagine you are a hungry
bird looking for a snack. Which moths are spotted first ?
When we analyze environmental
changes, population changes are easy to explain
So... the moths adapted
to their environment by changing colors, sort of like a chameleon ?.
Not quite.
At this point it would
be good to explain that when we talk about the moths "adapting" to their
environment we are using a verbal shortcut that can lead to confusion. None
of the moths does anything to better fit to the environment.
So... how can a population
end up better adapted to the environment if each individual member doesn't
adapt to that environment ?
This apparent contradiction
disappears when we understand that evolution rests on the superabundance
of the offspring (only a small part of the moths born in a given generation
will reach maturity and will procreate) and on the variability of characters
(color in this case), derived from accidental mutations. These take place
randomly and independently of the environment and in most cases do not represent
any evolutive advantage (in our story, a change in color from light to dark
might have taken place anytime before the 19th century, but under a clean
environment dark mutant individuals would have been fast food for the birds)
Nevertheless, under certain
circumstances, a mutation can arrive to the right place at the right time
and get perpetuated like in this case of peppered moths fit to "modern times".
Interestingly enough,
this story continues.
In 1956 the British government
approved the "Clean Air Act", a law aimed at reducing air pollution. Since
then the percent of light peppered moths in industrial regions has been steadily
increasing.
Changing with times:
Other examples of (even
faster) adaptation.
Resistance to antibiotics
Antibiotics hardly ever
destroy all the bacteria they are supposed to fight. The few surviving bacteria
can reproduce and form a new colony very quickly (or at least we think that
is very quick because we have lives several orders of magnitude longer than
theirs). Thanks to their fast pace the effects of natural selection can be
made evident to us in the short range. Mutations take place, populations change
very quickly and soon are dominated by individuals not affected by the antibiotic.
We say that that particular type is (or has become) resistant to that antibiotic.
Resistance to insecticides
Just like bacteria can
turn resistant to antibiotics, insects can become resistant to insecticides.
Sometimes we try to destroy
pests with insecticides, but there are many of those little bugs and not
all of them die. If any of these weird specimens happens to be immune to
the insecticide thanks maybe to a lucky mutation, it will be the founding
father of a new breed of bugs resistant to that particu
lar chemical.
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