Behavior is a major interface between organisms and their environments. Because behaviors influence survival and reproduction, they can change allele frequencies over generations, linking ecology, fitness differences, and evolution.

Core idea: behavior as a selectable trait

Natural selection can act on behavioral responses (what an organism does) when:

• Individuals vary in behavior.

• Behavioral differences cause differences in reproductive success.

• Some of the behavioral variation is heritable (genetically influenced or consistently transmitted).

Many behaviors are also plastic (change with context), but plasticity itself can be selected if it affects fitness.

Fitness and natural selection (behavior-focused)

Fitness outcomes of behavior commonly depend on:

• Survival to reproductive age (e.g., avoiding predators, avoiding lethal temperatures)

• Mating success (finding mates, courtship effectiveness, avoiding rejection)

• Fecundity (energy gained for producing offspring)

• Offspring survival (parental behaviors that improve juvenile survival)

Behaviors that raise fitness in a particular environment tend to become more common, even if they carry costs.

Quantifying fitness differences from behavior

To compare behavioral strategies, biologists often use relative fitness (fitness compared to the best-performing type in that environment).

Hardy–Weinberg calculations shown step-by-step for a population, including genotype counts, allele frequencies ($p$ and $q$), and predicted genotype frequencies ($p^2$, $2pq$, $q^2$). This kind of accounting is the mathematical baseline used to detect when evolutionary forces (including selection on behavior) are shifting allele frequencies over time. Source

This highlights selection without requiring absolute population growth data.

A behavior can be favored even if it reduces one component of fitness, as long as total reproductive success is higher than alternatives (a trade-off).

Behavioral variation and why it persists

Selection requires variation, and behavioral variation can arise from:

• Genetic differences affecting sensory systems, hormones, or neural circuitry

• Developmental environment (nutrition, stress, early-life cues)

• Learning that alters future decisions (but selection acts on the capacity/rules for learning when they are heritable)

• Frequency-dependent selection, where a behavior’s fitness depends on how common it is (rare strategies can sometimes be favored)

Graphical depiction of frequency-dependent selection in the Hawk–Dove game, where the fitness of each behavioral strategy changes as its frequency in the population changes. The intersection point represents a mixed equilibrium (both strategies have equal fitness), illustrating how selection can maintain behavioral variation rather than driving a single strategy to fixation. Source

Some behavioral traits are maintained because different strategies are favored in different conditions (environmental heterogeneity), or because trade-offs prevent a single “best” strategy across all contexts.

Environmental cues, information, and selection

Because “responses to and communication of information” affect outcomes, selection frequently targets how organisms:

• Detect cues (predator odor, food availability, day length)

• Process information (decision rules such as when to flee vs. forage)

• Respond (timing, intensity, or type of action)

Key evolutionary point: selection favors behaviors that improve decision-making accuracy when cues reliably predict fitness consequences. If cues are misleading (rapid environmental change), previously adaptive behaviors can reduce fitness.

Costs, constraints, and evolutionary change

Behaviors evolve under constraints that shape what selection can produce:

• Energy/time costs: vigilance reduces feeding time; mate searching increases exposure risk

• Physiological limits: sensory detection thresholds; neural processing capacity

• Ecological context: the same behavior can have different fitness effects in different habitats

• Genetic correlations: selection on one behavior can inadvertently change another if traits share genetic architecture

Across generations, consistent fitness differences among behavioral phenotypes shift allele frequencies, producing evolutionary change in populations, not individuals.

Illustration of allele-frequency change across generations in a small population, tracking how sampling of reproducing individuals can shift $p$ and $q$ from one generation to the next. Even though the mechanism shown is drift, the visual reinforces the same population-genetics principle your notes emphasize: evolution is measured as changing allele frequencies over generations. Source