Resistant bacteria: Bacteria can evolve rapidly because they reproduce at a fast rate.

The idea

Evolution is a change in the inherited characteristics of a population over time, driven by natural selection acting on variation produced by mutation. In bacteria, reproduction is asexual and very rapid under favourable conditions: a single cell can divide by binary fission every 20 minutes or so, so one cell can become millions in hours. Each division can produce a mutation, and because populations are huge and generation times are short, new alleles appear and are tested by selection quickly. For example, if an antibiotic is present, a mutant bacterium with a resistance allele survives, reproduces, and passes the allele to its offspring, so the resistant strain increases in frequency. This is why bacterial evolution is observed on human timescales, unlike evolution in slow-breeding organisms.

What earns the marks

• Bacteria reproduce asexually by binary fission, so a single parent cell divides into two genetically identical daughter cells.
• Under ideal conditions, bacterial populations can double in as little as 20 minutes, giving very short generation times.
• Mutations occur randomly during DNA replication; because reproduction is rapid, many mutations arise in a short time.
• Variation produced by mutation is the raw material on which natural selection acts.
• If a mutation gives resistance to an antibiotic, that bacterium survives treatment and reproduces, increasing the frequency of the resistance allele in the population.
• Rapid reproduction means many generations occur in a short time, so the proportion of resistant bacteria can rise quickly.
• Evolution is a change in the inherited characteristics of a population over time, not a change in an individual bacterium.

Common mistakes and how to put them right

• Saying bacteria 'decide' to become resistant or 'learn' to resist antibiotics.

  Put it right: Resistance arises from random mutation, and selection then increases its frequency; bacteria do not choose to mutate.

• Treating evolution as something that happens to one bacterium during its lifetime.

  Put it right: Evolution is a change in the population over generations; an individual bacterium does not evolve.

• Confusing rapid reproduction with sexual reproduction.

  Put it right: Bacteria reproduce asexually by binary fission, so offspring are clones except when mutation occurs.

• Stating that bacteria reproduce 'quickly' without giving a reason such as short generation time or binary fission.

  Put it right: Link the fast rate to binary fission and short generation times.

Exam tips

• Use the phrase 'short generation time' or 'rapid binary fission' when explaining why bacterial evolution can be observed quickly.
• Link mutation, selection and reproduction in a logical chain: mutation produces variation, selection favours resistant bacteria, reproduction increases their numbers.
• If asked to explain, avoid simply repeating the statement; add the mechanism of binary fission and the role of mutation.
• Use comparative language such as 'much faster than' when contrasting bacteria with organisms that have long generation times.