Environmental change can rapidly shift which traits help organisms survive and reproduce. This page explains how genetic variation produces adaptations, how natural selection acts on that variation, and why “advantage” depends on environmental context.

Core idea: adaptations come from heritable variation

Natural selection can only act on heritable differences among individuals. When environments change, the traits that increase survival and reproduction may also change, altering which alleles become more common over generations.

Genetic variation matters because individuals with different alleles can produce different phenotypes (observable traits), and some phenotypes leave more offspring than others under specific conditions.

An adaptation is not “whatever a species has”; it is identified by its functional advantage relative to alternatives, measured by reproductive success.

Where genetic variation comes from

Key sources that generate or redistribute variation in populations include:

• Mutation: creates new alleles by changing DNA sequence (often neutral or harmful; occasionally beneficial).

• Recombination in sexual reproduction: reshuffles existing alleles (independent assortment, crossing over).

• Gene flow: movement of alleles among populations via migration and mating.

• Genetic drift: random changes in allele frequencies, strongest in small populations (can reduce variation).

Environment-specific advantage and fitness

Whether a trait is beneficial depends on the environment—biotic and abiotic factors define which phenotypes perform best. A trait can increase fitness in one context and decrease it in another.

Fitness is typically influenced by multiple components:

• Survival to reproductive age

• Mating success (finding mates, being chosen, fertilisation success)

• Fecundity (number of offspring produced)

• Offspring viability (offspring survival and reproduction)

Selective pressures created by environmental change

Environmental change can alter selective pressures—factors that cause differential survival or reproduction. Examples of changes that commonly shift selection:

• Temperature and precipitation shifts affecting enzyme performance, water balance, and growing seasons

• Resource availability changes (food, nutrients, nesting sites)

• Novel predators, pathogens, or competitors

• Habitat alteration changing camouflage value, dispersal success, or access to mates

How selection turns variation into adaptation

Selection does not “create” helpful traits; it changes allele frequencies by consistently favoring individuals whose heritable traits increase fitness under current conditions.

This plot shows Hardy–Weinberg expected genotype frequencies ($p^2$, $2pq$, $q^2$) as allele frequencies ($p$ and $q$) vary from 0 to 1. It highlights why heterozygotes are most common near $p = q = 0.5$ and why rare alleles are often found in heterozygous genotypes—key context for understanding how selection can shift allele frequencies over time. Source

Mechanistic pathway (what must be true)

For a trait to evolve as an adaptation via natural selection:

• Individuals vary in the trait (phenotypic variation).

• Some of that variation is heritable (genetic basis).

• Individuals with certain trait values have higher fitness in the environment.

• Over generations, alleles associated with higher fitness increase in frequency.

Environmental change can therefore lead to evolutionary responses when populations already contain alleles that become advantageous under new conditions (or when new alleles arise and spread).

Common selection patterns under environmental change

• Directional selection: one extreme phenotype is favored (often seen when environments shift consistently, such as warming climates).

• Stabilizing selection: intermediate phenotypes are favored (common in stable environments; can be disrupted by change).

• Disruptive selection: both extremes are favored (possible when environments become patchy, creating multiple niches).

This diagram compares how directional, stabilizing, and disruptive selection change the shape of a trait’s frequency distribution. The “before selection” curve is contrasted with an “after selection” curve to show which phenotypes leave the most offspring under each mode of selection. Source

Constraints, trade-offs, and “imperfect” adaptation

Even with strong selection, populations may not evolve the “best imaginable” phenotype because evolution is constrained by:

• Available genetic variation: selection cannot favor alleles that are absent.

• Time lag: rapid environmental change can outpace evolutionary change.

• Trade-offs: improving one function can reduce another (e.g., allocating energy to stress tolerance can reduce reproduction).

• Correlated traits: alleles often affect multiple traits; selection on one trait can drag along others.

• Historical contingency: evolution modifies existing structures; it does not start from scratch.

Distinguishing genetic adaptation from non-heritable change

Individuals can show phenotypic flexibility in response to the environment (developmental or physiological shifts), but these changes are not necessarily genetic adaptations unless they are heritable and increase fitness.

Key distinctions:

• Acclimation/acclimatization: within-lifetime phenotype change; can help survival but does not itself change allele frequencies.

• Genetic adaptation: population-level change across generations driven by differential reproduction of heritable variants.

Evidence used to infer adaptation to environmental change

Biologists infer adaptation by linking trait variation to fitness differences in specific environments and demonstrating a heritable basis. Common lines of evidence include:

• Trait–fitness correlations measured in natural or controlled settings

• Comparing populations across different environments (local adaptation patterns)

• Tracking allele frequency changes across generations during environmental shifts

• Demonstrating heritability using breeding designs or genomic association approaches