Cambridge O Level Biology · Syllabus 5090 · Inheritance
Selection Pressure
What is Selection Pressure?
Any factor that causes some individuals in a population to survive and reproduce more successfully than others, so that the frequency of particular alleles changes over generations. In natural selection the pressure comes from the environment, through factors such as predation, disease, competition for food and climate. In artificial selection the pressure is applied deliberately by people, who choose which individuals are allowed to breed. In both cases the pressure acts on inherited variation that already exists in the population; it does not create new variation.
This definition is part of the Inheritance chapter in Cambridge O Level Biology.
Questions students ask about Selection Pressure
Did the antibiotic cause the bacteria to become resistant?
No. The resistance allele arose earlier, by random mutation, in bacteria that had never encountered the drug. The antibiotic then acted as a selection pressure: it killed the bacteria that lacked the allele, leaving the resistant ones to survive and reproduce. The proportion of resistant bacteria in the population rose. The antibiotic selected variation that already existed — it did not create it.