Disruptive selection: Favors extreme phenotypes at both ends of the distribution, increasing variation.
Directional selection: Favors one extreme phenotype, shifting the population mean.
How Environmental Factors Influence Selection
Environmental factors such as temperature, food availability, predation pressure, and habitat structure can create selective pressures that shape the distribution of traits in a population.
1. Stabilising Selection
Occurs when the prevailing environment favours the average phenotype and extremes are less fit.
Example: In a stable climate, medium‑sized birds have optimal wing loading for efficient flight, while very large or very small birds expend more energy.
Result: Reduced phenotypic variance; the population mean remains relatively unchanged.
2. Disruptive Selection
Arises when the environment contains two or more distinct niches, each favouring different extreme phenotypes.
Example: A lake with both deep‑water and surface‑feeding fish; small fish exploit surface insects, large fish hunt deeper prey, while medium‑sized fish are outcompeted.
Result: Increased variance; may lead to speciation if reproductive isolation develops.
3. Directional Selection
Occurs when environmental change makes one extreme phenotype more advantageous.
Example: Rising temperatures select for beetles with lighter colouration that reflect more heat.
Result: Shift in the population mean toward the favoured extreme.
Quantifying Selection
The strength of selection can be expressed using the selection differential (\$S\$) and the selection gradient (\$\beta\$):
where \$\mu{\text{selected}}\$ is the mean trait value of the reproducing individuals and \$\mu{\text{population}}\$ is the mean trait value of the whole population.
Comparison of Selection Types
Selection Type
Environmental Context
Effect on Trait Distribution
Typical Outcome
Stabilising
Stable, homogeneous environment
Peak at intermediate values; reduced variance
Maintenance of status‑quo phenotype
Disruptive
Heterogeneous environment with multiple niches
Two peaks at extremes; increased variance
Potential divergence or speciation
Directional
Changing environment favouring one extreme
Shift of the whole distribution toward one extreme
Evolution of new adaptive phenotype
Artificial Selection and Its Parallel to Natural Selection
Artificial selection mimics natural selection but the selective pressure is imposed by humans rather than the environment. The same principles of stabilising, disruptive and directional forces apply, for example:
Stabilising: Breeding dogs for a standard size.
Disruptive: Selecting plants for both very early and very late flowering times.
Directional: Raising crop yields by selecting for larger fruit size.
Suggested diagram: A series of three bell‑shaped curves illustrating stabilising, disruptive and directional selection acting on a continuous trait.
Key Points to Remember
Environmental factors create selective pressures that can be stabilising, disruptive or directional.
Stabilising selection reduces variation; disruptive selection increases variation and can lead to speciation; directional selection shifts the mean trait value.
Artificial selection uses the same mechanisms but under human control.
Quantitative measures such as the selection differential help assess the strength of selection.