Predation, disease and competition for the means of survival result in…

The idea

A selection pressure is any factor that makes some phenotypes more likely than others to survive and reproduce. Predation removes the individuals easiest to catch, so alleles for better camouflage, speed or vigilance become more common in the prey population. Disease removes the individuals whose immune systems cannot cope, so alleles giving resistance are favoured. For example, in regions where malaria is common, the sickle-cell allele persists because heterozygotes have some resistance to the parasite, not because of a general immune-system improvement. Competition, whether between members of the same species or between species, means that when food, water, light, mineral ions, territory or mates are in short supply the individuals best able to obtain them survive and breed. None of these pressures changes an organism during its life; they change which organisms leave offspring.

What earns the marks

• Naming the selection pressure acting on the population, such as predation, a named pathogen or competition for a named resource.
• Variation already existing in the population, caused by mutation.
• Individuals with the advantageous phenotype being more likely to survive.
• Those survivors reproducing and passing on the advantageous allele.
• The frequency of the advantageous allele increasing over generations.

Common mistakes and how to put them right

• Writing that organisms adapt, get stronger or become resistant during their lifetime, instead of the population changing over generations.

• Saying the fittest survive without saying which characteristic makes them fitter in that particular environment.

• Referring to immunity when the answer requires a resistance allele.

• Leaving out reproduction, so the advantage never reaches the next generation.

• Naming competition without naming the resource being competed for.

Exam tips

• Give the selection pressure by name, then the advantageous phenotype, then survival, then reproduction, then allele frequency.
• Start with variation that already exists, because natural selection explanations normally begin there.
• Use more likely to rather than will, because selection is about probability, not certainty.
• Name the specific resource or pathogen so the selection pressure is clearly identified.