Environmental conditions do not stay constant. When the environment shifts, the traits that improve survival and reproduction can also shift, changing which individuals contribute more offspring and reshaping populations over generations.

Core idea: environmental change creates selection

Natural selection requires differences in reproductive success among individuals. Environmental change often triggers those differences by altering which traits work best.

Selective pressures

A changing environment introduces new challenges and opportunities (e.g., temperature extremes, novel predators, altered food supply). These conditions act as selective pressures on organisms in the population.

These paired photographs of the peppered moth show how camouflage (and therefore predation risk) depends on environmental background. When pollution darkened tree bark and reduced lichens, darker moths were less conspicuous in industrial areas, while lighter moths were better camouflaged in unpolluted areas. This is a textbook example of a changing environment shifting which phenotype has higher survival and reproductive success. Source

Selective pressures can be abiotic (nonliving) or biotic (living), and they can change in intensity, timing, or direction.

Sources of environmental change

Environmental change can be rapid (within a few generations) or gradual (over many generations). Either can alter which phenotypes are favored.

Abiotic changes

• Climate and weather shifts (heat waves, droughts, cold snaps)

• Natural disasters (fires, floods, volcanic eruptions)

• Chemistry changes (salinity, pH, oxygen availability)

• Light and seasonality changes that alter growth and breeding timing

Biotic changes

• Predator introductions or population increases

• Competition changes (new competitors or altered resource availability)

• Pathogens and parasites emerging or becoming more common

• Mutualisms breaking down (loss of pollinators or symbiotic partners)

How selective pressures “choose” among phenotypes

Selection acts on phenotypes—the observable traits affecting performance in an environment. When conditions change, the link between phenotype and reproductive success can change.

Favored vs. disadvantaged phenotypes

Selective pressures can:

• Increase the success of individuals whose phenotypes improve resource acquisition, predator avoidance, stress tolerance, or mate attraction

• Decrease the success of individuals whose phenotypes reduce performance under the new conditions (even if they were previously beneficial)

Key point: selection is not about “need.” Environmental change does not cause organisms to develop helpful traits; it changes which existing heritable variants leave more offspring.

Selection can shift in direction and strength

Environmental change can alter:

• Direction: which trait values are favored (e.g., smaller vs. larger body size)

• Strength: how big the reproductive advantage is for the favored phenotype

• Consistency: whether the same phenotype is favored every generation or alternates over time

Common patterns under changing pressures

This diagram summarizes three common selection patterns that can result when environmental conditions change. Directional selection shifts the population’s trait distribution toward one extreme, stabilizing selection narrows variation around an intermediate optimum, and disruptive selection favors both extremes, often increasing variation and potentially promoting divergence. Source

• Directional selection: one extreme phenotype is consistently favored after a shift, pushing the population toward that extreme.

• Stabilising selection: intermediate phenotypes are favoured when extreme conditions reduce performance (often in relatively steady environments, but can occur after a disruptive event removes extremes).

• Disruptive selection: both extremes are favoured when the environment becomes patchy (heterogeneous), and intermediates perform poorly.

These patterns describe outcomes of selection; the underlying cause is the environment altering survival and reproduction.

Spatial variation: different environments, different pressures

Selective pressures can vary across locations:

• Populations spread across a wide range may experience different temperature, predator communities, or resources

• Microhabitats (sun vs. shade, shallow vs. deep water) can favour different phenotypes within the same general region

If individuals mostly breed locally, spatial variation can maintain differences in which phenotypes are favoured in different places.

Temporal variation: fluctuating environments

Many environments fluctuate (seasonally or unpredictably). Fluctuation can:

• Prevent any single phenotype from being best all the time

• Favour phenotypes that perform “well enough” across conditions (broad tolerance) or that can adjust through phenotypic plasticity (environmentally influenced trait expression)

Plasticity can change which phenotypes succeed during a shift, but for evolution to occur, differences in reproductive success must be linked to heritable variation.

What students should be able to do with this idea

• Identify the environmental factor that changed (biotic or abiotic)

• Describe which phenotypes are favoured and disadvantaged under the new conditions

• Connect the selective pressure to expected changes in the population by explaining differential reproductive success (who leaves more offspring and why)