Darwin, natural selection and antibiotic resistance

The work of Darwin and WallaceSpec 4.1BTriple only

The theory of evolution by natural selection was developed by two scientists working separately: Charles Darwin and Alfred Wallace.

Charles Darwin

• Darwin collected observations and specimens on a long sea voyage, including studying plants and animals in different places.
• He noticed that individuals of the same species show variation, and that organisms in different places are suited to their different environments.
• He spent many years collecting evidence and developing his ideas before publishing them in his book On the Origin of Species in 1859.

Alfred Wallace

• Wallace independently developed a very similar theory of evolution by natural selection.
• His ideas were based on his own observations and collections of plants and animals in different parts of the world.
• His work encouraged Darwin to publish, and the two men's ideas were presented together.

Impact on modern biology

• Natural selection gave a unifying explanation for the huge diversity of living things and how species are related.
• When combined with later knowledge of genes, it explains how variation arises (through mutation) and how characteristics are inherited.
• It helps us understand the emergence of resistant organisms, such as antibiotic-resistant bacteria.
• It supports the idea that living things can be classified according to how they are related.

Evolution by natural selectionSpec 4.2

Evolution is a change in the inherited characteristics of a population over many generations. Darwin explained how it happens using natural selection.

1. Individuals in a population show variation. Some of this is genetic, caused by differences in alleles, which arise by mutation.
2. More offspring are born than can survive, so individuals compete for resources such as food, space and mates, and some are eaten or die of disease.
3. Individuals with characteristics that suit their environment are more likely to survive and reproduce.
4. They pass on the alleles for those characteristics to their offspring.
5. Over many generations, the proportion of the population with the advantageous alleles increases.

Individuals with a characteristic that gives an advantage are sometimes described as the 'fittest'. This means best suited to their environment, not strongest.

Natural selection

The process by which organisms better adapted to their environment are more likely to survive, reproduce and pass on their alleles.

Evolution

A change in the inherited characteristics of a population over many generations.

Resistant organisms and antibiotic resistanceSpec 4.3

The emergence of resistant organisms is an example of natural selection that can be seen happening within a human lifetime. Bacteria reproduce very quickly, so changes in a population happen fast.

1. In a population of bacteria, a mutation produces an allele that makes one bacterium resistant to an antibiotic. This is variation.
2. The antibiotic is used. It kills the non-resistant bacteria, but the resistant bacterium survives. The antibiotic is the selection pressure.
3. The resistant bacterium reproduces with less competition, and passes the resistance allele on to its offspring.
4. Over time the population is mostly resistant bacteria, so the antibiotic no longer works against the infection.

This supports Darwin's theory because all the steps of natural selection can be observed: variation, a selection pressure, survival and reproduction of the better adapted, and inheritance of the useful allele.