Natural selection

Individuals in a population are not genetically identical. They carry different versions of genes, called alleles, which can produce differences in inherited characteristics. In a particular environment, some of these characteristics may help individuals survive and reproduce more successfully than others.

More offspring are produced than the environment can support. Individuals therefore face a struggle for survival, which includes competition for limited resources such as food, water or space. Individuals with advantageous inherited characteristics are generally more likely to survive and reproduce, although this is not guaranteed for every individual.

When better-adapted individuals reproduce, they can pass the alleles associated with those characteristics to their offspring. If the characteristics continue to be advantageous, this pattern can be repeated over many generations. The advantageous alleles can then become more common in the population. This change is called natural selection.

Natural selection changes the frequency of alleles in a population over generations; it does not mean that an individual develops a useful inherited characteristic simply because it needs one. Whether a characteristic is advantageous also depends on the environment.

Selective breeding

Humans can increase a desirable inherited feature in a population of plants or animals by choosing which individuals reproduce. This is called selective breeding, or artificial selection. The feature must be inherited so that it can be passed from parents to offspring.

Selective breeding across generations

For example, a grower might want crop plants with larger fruits. The process is repeated over several generations:

1. Identify the feature to improve: fruit size.
2. Choose parents that show the feature: plants with large fruits.
3. Cross the parents to produce offspring.
4. Select offspring that produce large fruits and cross them as parents of the next generation.
5. Repeat the selection and crossing over many generations.

Offspring do not all have exactly the same features, so the grower selects those that best show the desired feature each time. Over generations, large fruit size can become more common in the crop population.

The same approach can be used to increase desirable inherited features in domesticated animals: humans choose parents that show a desired feature, then repeatedly select and breed offspring that also show it.

Evolution of antibiotic resistance in bacteria

Bacteria in a population are not all genetically identical. Some may carry an allele—a version of a gene—that makes them resistant to a particular antibiotic. This means they are more likely to survive exposure to it than bacteria without the resistance allele.

When the antibiotic is used, susceptible bacteria are more likely to die. Resistant bacteria are more likely to survive and reproduce, passing the resistance allele to their offspring. As these offspring reproduce, the allele is passed on again.

Over many generations, resistant bacteria can make up an increasing proportion of the population. This change can produce an antibiotic-resistant strain. The antibiotic does not make individual bacteria develop resistance because they need it; it favours bacteria that already carry a resistance allele.