Science stories to 
reflect on the history and future of science & society. By P. Gomez-Romero
En espa?l, por favor. En espanol
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Evolution never sleeps.

Evolution is not exclusive of the past, and not always does it need long periods of time to take place. There are many interesting examples of its working here and now.

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.

1850
1900

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.



This story is adapted from the stories on evolution at the British Museum of Natural History. You will find many other stories in the book :"El origen de las especies", British Museum (Natural History), Editorial Akal. 1992


  Questions and comments to cienciateca@mail.com |  Last modified: August 24, 1999
©Pedro Gómez-Romero, 1998,1999