Overproduction, competition and selection pressuresSpec D4.1.3, D4.1.4, D4.1.5
In short
Natural selection is promoted by overproduction of offspring and competition for limited resources. Populations produce more offspring than the carrying capacity allows, so individuals compete for food, water and space. Abiotic factors such as extreme temperatures also act as selection pressures. Individuals with higher fitness survive and reproduce more, passing on their alleles.
Overproduction and competition
Most species produce far more offspring than can survive. The carrying capacity of an environment is the maximum population size it can support sustainably. Above it, there are not enough resources, so individuals of the same species compete: intraspecific competition.
- Animals: food (for example seeds for seed-eating birds), water, nesting or breeding sites, territory and shelter.
- Plants: light, water, mineral ions such as nitrate and phosphate, and space for roots.
Abiotic factors as selection pressures
A selection pressure is any factor that affects the chance of survival or reproduction differently depending on an individual's traits. Abiotic factors such as very high or very low temperatures, drought or flooding are density-independent: they affect survival whatever the population size. In a severe frost, for example, individuals with better insulation or more cold-tolerant tissues are more likely to survive.
Differences in adaptation, survival and reproduction
Individuals differ in how well adapted they are. Better-adapted individuals win more resources, survive longer and leave more offspring. This difference between individuals is the basis of natural selection.
- Carrying capacity
- The maximum population size of a species that an environment can support sustainably.
- Density-independent factor
- A factor, usually abiotic, that limits a population regardless of its density, such as a heatwave or a hard frost.
- Fitness
- The survival value and reproductive potential of a genotype: how many offspring it contributes to the next generation compared with other genotypes.
Survival alone earns no mark. Write 'survive and reproduce, passing on their alleles'.
Linking question: how do intraspecific interactions differ from interspecific interactions? Intraspecific competition is usually most intense because individuals of one species have identical resource needs.
Written and checked against the IB Biology HL specification · Updated October 2026