How Does Natural Selection Lead to Adaptation?
Learn how natural selection makes advantageous alleles more common over generations, leading to adaptation, with an antibiotic resistance example and exam tip.
Natural selection leads to adaptation when individuals with inherited characteristics that help them survive and reproduce pass on the alleles for those characteristics. Over many generations, those alleles become more common in the population. An adaptation is a feature that makes an organism better suited to its environment. An advantage depends on the environment: a characteristic that helps in one set of conditions may not help if those conditions change.
How does it happen?
Mutations occur randomly and can produce new alleles, which are alternative versions of a gene. Along with meiosis and random fertilisation, this contributes to genetic variation within a population. Individuals therefore differ in characteristics that may affect their survival. Only inherited variation can be passed to offspring; a characteristic gained solely during an individual's lifetime does not automatically change the alleles its offspring inherit.
If a characteristic gives an individual an advantage in its environment, that individual is more likely to survive and reproduce. Its offspring may inherit the advantageous allele. As this happens across generations, the allele’s frequency increases. This change in the population can make it better adapted to its environment. Allele frequency means how common an allele is in the population.
Example: antibiotic resistance
A random mutation may give a bacterium resistance to an antibiotic. When the antibiotic is used, susceptible bacteria are less likely to survive, while resistant bacteria are more likely to survive and reproduce. They pass on the resistance allele, so resistant bacteria become more common in the population. This is directional selection: selection favours one end of the range of variation.
Exam mistake to avoid: bacteria do not mutate because they need resistance. Mutations arise randomly; the antibiotic acts as a selection pressure by favouring bacteria that already have resistance. Nor does an individual evolve during its lifetime: natural selection changes allele frequencies in a population over generations.
