Natural selection

Today I’m talking to doctor James Smith, who will discussing the subject of natural

selection. Dr. Smith, can you please explain what it’s all about?

Natural selection is a central concept of evolution, sometimes called the survival of the

fittest. It was an idea of Charles Darwin and Alfred Russel Wallace. (1) Darwin chose the

name as an analogy with artificial selection (selective breeding).

Natural selection is the process where organisms with favorable traits are more likely to

reproduce. In doing so, they pass on these traits to the next generation. Over time this

process allows organisms to adapt to their environment. This is because the frequency

of genes for favorable traits increases in the population.

(2) Members of a species are not all alike, partly because of differences in heredity

(genetics). This is true even with children of the same parents. Some of these

differences might make one organism better at surviving and reproducing than others in

a particular habitat. When this organism reproduces, it passes along the genes, which

gave it the advantage, to its children. Some adaptations are extremely long-lasting,

useful in many habitats. The evolution of wings in birds is an example. Others are good

only as long as the environment stays the same. If the environment changes enough,

then another organism might do better.

OK, then. Can you give us an overview of what you’ll talk about today?

Contents

� 1 The process

� 2 Examples

o 2.1 Antibiotic resistance

o 2.2 Camouflage

o 2.3 Mimicry

Right, so I guess you’ll start by talking about the process.

The process

Natural selection explains why living organisms change over time to have the anatomy,

the functions and behaviour that they have. It works like this:

1. All living things have such fertility that their population size could (3)increase

rapidly forever.

2. We see that the size of populations does not increase to this extent. Mostly,

numbers remain (4)about the same.

3. The food and other resources are limited. Therefore, there is (5)competition for

food and resources.

4. No two individuals are alike. Therefore, they will not have the same chances to

(6)live and reproduce.

5. Much of this variation can be inherited. The parents pass such traits to the

(7)children through their genes.

6. The next generation can only come from those that survive and reproduce. After

many generations of this, the population will have more helpful genetic

differences, and fewer harmful ones. Natural selection is really a process of

(8)elimination. The elimination is being caused by the relative fit between the

individuals, and the environment they live in.

Ok, so that clarifies how it happens. Do you think you could give us some examples

now?

Examples

There are now quite a number of examples of natural selection in natural populations.

Antibiotic resistance

Resistance to antibiotics is increased by the survival of individuals which are immune to

the effects of the antibiotic. Their offspring inherit the resistance, creating a new

population of resistant bacteria.

A well-known example of natural selection in action is the development of antibiotic

resistance in microorganisms. (9) Since the discovery of penicillin in 1928 by Alexander

Fleming, antibiotics have been used to fight bacterial diseases. Natural populations of

bacteria contain, among their vast numbers of individual members, considerable

variation in their genetic material, as the result of mutations. When exposed to

antibiotics, most bacteria die quickly, but some may have mutations that make them

slightly less susceptible. If the exposure to antibiotics is short, these individuals will

survive the treatment. This selective elimination of maladapted individuals from a

population is natural selection.

Given enough time, and repeated exposure to the antibiotic, a population of antibiotic-

resistant bacteria will emerge. This leads to what is known as an evolutionary arms race,

or co-evolution, in which bacteria continue to develop strains that are less susceptible

to antibiotics, while medical researchers continue to develop new antibiotics that can

kill them. Response strategies typically include the use of different, stronger antibiotics;

however, new strains of MRSA have recently emerged that are resistant even to these

drugs. A similar situation occurs with pesticide resistance in plants and insects, and with

malarial resistance to quinine.

Camouflage

Some day-flying moths were light in colour, and some of the moths were dark. At first,

the light colored moths survived better because they were camouflaged against the

white colour of the nearby trees. This made it hard for birds to see them. (10) When

factories were built, it caused pollution that made all the trees look black. Now the light

colored moths were obvious against the dark bark. The dark colored moths now had the

advantage, after the environment changed. The genes controlling dark colour spread

through the population of moths.

Mimicry

Another example: Some harmless insects mimic other insects which are dangerous, or

which taste foul. (11) Mimicry evolves because the better mimics survive better. They

live to produce more offspring than the less good mimics. The genes of the better

mimics become more common in the species. Over time, mimic species get closer to

their models. This is the process of evolution by natural selection.

Great stuff... Thanks for sharing your knowledge with us today.