Natural Selection and Antibiotic Resistance

Individuals within a population are not all identical. They show variation in characteristics such as colour or resistance to disease. Some differences are inherited because individuals have different genetic information. Random mutations can produce new genetic variants, although not every mutation changes an organism’s phenotype or gives it an advantage.

The environment affects which characteristics are advantageous. Organisms compete for resources, and factors such as predators or disease can reduce their chances of surviving. These factors act as selection pressures. An inherited characteristic that helps an organism in those conditions gives it a better chance of surviving and reproducing.

When these individuals reproduce, they can pass the genetic information responsible for the advantageous characteristic to their offspring. They therefore contribute more offspring to the next generation than less well-adapted individuals. Over many generations, the advantageous characteristic becomes more common: the population evolves.

This process is sometimes called ‘survival of the fittest’. Here, fitness means being well suited to the environment and successfully reproducing. A characteristic is not automatically advantageous in every environment.

Antibiotics create a selection pressure

Antibiotics are medicines that kill bacteria or prevent them from reproducing. They target bacterial structures or processes and are not effective against viruses. For example, penicillin interferes with the building of bacterial cell walls, so susceptible bacterial cells may rupture as they grow.

Bacteria within a population can differ genetically. A random mutation may give a bacterium resistance to a particular antibiotic. This means it can survive treatment that would kill susceptible bacteria or stop them reproducing.

Resistance is an advantage when that antibiotic is present. The antibiotic acts as the selection pressure; it does not deliberately produce the useful mutation.

How a resistant bacterial population emerges

Before treatment, a bacterial population may contain a small number of resistant bacteria among many susceptible bacteria. When exposed to the antibiotic, susceptible bacteria are killed or prevented from reproducing, while resistant bacteria survive.

The survivors now face less competition for nutrients. They reproduce asexually by dividing, passing their genetic information—including the variant responsible for resistance—to their offspring. As these descendants multiply, resistant bacteria make up an increasing proportion of the population.

The important change is not simply that more bacteria exist. It is that resistance becomes more common within the population. If these bacteria cause disease, an infection dominated by resistant bacteria is harder to control using that antibiotic.

Why resistance supports Darwin’s theory

Antibiotic resistance shows the sequence predicted by natural selection: genetic variation exists, a selection pressure favours some variants, survivors reproduce, and their offspring inherit the advantageous variant. The population’s inherited characteristics then change over generations.

Bacteria reproduce quickly, so many generations can occur in a short time. This allows evolutionary changes to become apparent much more rapidly than in organisms with long generation times. The emergence of antibiotic-resistant populations therefore provides evidence for Darwin’s explanation of evolution by natural selection.

• Random mutations can produce resistant bacteria.
• The antibiotic kills susceptible bacteria or prevents their reproduction; resistant bacteria survive.
• Survivors reproduce asexually and pass resistance to their offspring.
• Resistance becomes more common in the population.

Evidence for evolution

Resistance demonstrates inherited variation, natural selection and population change. Rapid bacterial reproduction makes changes observable over many generations in a short time.