Selective pressures link environmental conditions to evolutionary change. In AP Environmental Science, focus on how human-driven and natural changes alter survival and reproduction, shifting which traits become more common in populations over generations.

Core idea: selective pressures change which traits are favoured

This diagram compares three common outcomes of natural selection: stabilizing selection (favoring intermediate phenotypes), directional selection (shifting the population toward one extreme), and diversifying/disruptive selection (favoring extremes over intermediates). It helps you see how selective pressures can change phenotype distributions across generations, which is the population-level signature of evolving trait frequencies. Source

What a selective pressure is

Selective pressures can be sudden (a pesticide application) or gradual (long-term drying of a region).

They influence populations, not individual organisms: individuals do not evolve, but their trait frequencies can shift across generations.

Fitness and why it matters

Fitness is environment-specific. A trait that increases fitness in one setting (e.g., tolerance to higher salinity) may reduce fitness elsewhere due to trade-offs (such as slower growth).

How selective pressures shape survival and reproduction over time

The mechanism linking pressure to population change

Selective pressures shape outcomes by changing:

• Survival to reproductive age (who lives long enough to breed)

• Mating success (who secures mates or fertilisations)

• Fecundity (how many offspring are produced)

• Offspring survival (whether offspring live to reproduce)

When a pressure consistently favours certain traits, individuals carrying those traits tend to leave more offspring, so those traits become more common. This is the population-level pattern behind the syllabus statement that selective pressures “change an organism’s behavior and fitness, shaping survival and reproduction over time.”

Behaviour can be part of the response

Selective pressures often act through behaviour because behaviour affects exposure to risks and access to resources. For example, altered foraging times, migration timing, or habitat choice can change:

• Predation risk

• Heat or UV exposure

• Contact with pollutants

• Access to food and nesting sites

If behavioural differences have a genetic basis (or influence reproductive success consistently), they can contribute to long-term shifts in trait frequencies.

Environmental sources of selective pressure (APES-focused)

Selective pressures commonly discussed in environmental contexts include:

• Chemical stressors: pesticides, herbicides, industrial pollutants, heavy metals

• Physical stressors: temperature extremes, drought, storms, altered fire regimes

• Habitat change: fragmentation, urbanisation, loss of breeding sites

• Resource shifts: reduced prey/plant availability, altered water supply

• Biological interactions: disease, parasites, introduced predators

In each case, the environment is “filtering” variation: organisms with traits that better tolerate or avoid the stressor tend to have higher fitness.

Key implications for environmental management

Selective pressures explain why some interventions lose effectiveness over time:

• Repeated use of the same control method can favour tolerant individuals, increasing the proportion of the population that persists under that pressure.

• Rapid environmental change can reduce average fitness if few individuals possess traits suited to the new conditions, potentially causing population decline.

• Maintaining genetic diversity helps populations respond to new pressures, because diversity increases the chance that some individuals already carry beneficial traits.