Natural selection explains how populations, not individuals, evolve. When heritable variation affects reproductive output, certain traits become more common across generations, producing adaptation and measurable genetic change within the population.

Core idea: natural selection links variation to evolutionary change

Natural selection is a mechanism that can change a population’s genetic makeup over time because individuals differ and those differences can affect how many offspring they contribute to the next generation.

Natural selection is therefore a major driver of evolution at the population level, because it produces predictable, non-random shifts in inherited characteristics across time.

Hardy–Weinberg genotype frequencies ($p^2$, $2pq$, and $q^2$) plotted against allele frequency ($p$, with $q=1-p$). The curves show how genotype composition shifts as allele frequencies change, linking the population-level definition of evolution to measurable genetic outcomes. Source

What natural selection acts on (and what evolves)

Selection acts on phenotypes in individuals

Selection “sees” the phenotype—observable traits such as morphology, physiology, and behavior—because phenotypes influence survival to reproduction and/or number of offspring produced.

• Individuals do not evolve during their lifetime; they may survive or reproduce at different rates.

• Differences among individuals in reproduction lead to differences in which alleles are passed on.

Evolution is measured in populations across generations

Because offspring inherit alleles from parents, consistent differences in reproductive contribution can alter the genetic composition of the next generation.

• If a trait is heritable and associated with higher reproductive output, alleles contributing to that trait tend to increase in frequency.

• If a trait is heritable and associated with lower reproductive output, its contributing alleles tend to decrease in frequency.

Requirements for natural selection to cause evolution

For natural selection to drive evolutionary change, three conditions must be met simultaneously.

1) Variation exists in the population

Individuals within a population must differ in one or more traits. Variation can involve:

• Structural traits (e.g., body covering, coloration)

• Physiological traits (e.g., tolerance to dehydration)

• Behavioral traits (e.g., foraging timing)

If there is no variation, selection cannot favor one trait over another.

2) Variation is heritable

The trait differences must be linked to genetic differences that can be passed from parents to offspring.

• Heritability does not require a trait to be controlled by a single gene; many traits are polygenic.

• Traits that are purely environmental (not genetically influenced) may affect an individual’s success but do not directly cause allele frequency change.

3) Variation leads to differential reproductive contribution

Individuals with certain phenotypes must, on average, leave different numbers of viable offspring than others in the same population.

• The key outcome is differential reproduction, because it directly determines which alleles enter the next generation.

• Survival matters mainly insofar as it affects the probability of reproducing.

How natural selection produces evolutionary change over time

Natural selection drives evolution through a repeated, generational feedback loop:

• Existing heritable variation produces different phenotypes in a population.

• Phenotypic differences cause differences in reproductive output among individuals.

• Offspring inherit alleles disproportionately from individuals with higher reproductive output.

• The next generation has shifted allele frequencies, changing trait distributions in the population.

• Over many generations, the population can become better matched to its environment (an adaptation), as alleles associated with higher reproductive output become more common.

This mechanism is consistent with the syllabus emphasis that natural selection is a major mechanism driving evolutionary change in populations over time: selection is a population-level process with measurable genetic consequences.

Important clarifications for AP Biology

Natural selection is not random, but it depends on random inputs

• The sorting of existing variation is non-random: individuals with certain heritable traits contribute more offspring.

• The presence of variation in any given individual is influenced by processes such as recombination and occasional new alleles, which arise without respect to “need.”

Selection does not create “perfect” organisms

Natural selection can only favor traits that:

• Already exist in some heritable form in the population

• Increase reproductive contribution under current conditions

• Are not constrained by trade-offs (a trait can improve one function while reducing another)

Natural selection acts on individuals, but adaptation describes populations

• Individuals are selected for or against based on phenotype.

• Populations become adapted when allele frequencies change so that advantageous phenotypes become more common across generations.

Natural selection can maintain multiple alleles

Even when one phenotype has higher reproductive output in some contexts, variation may persist if:

• Different phenotypes have advantages in different microhabitats or times

• The advantage depends on how common a phenotype is (frequency-related effects)

• Heterozygotes show higher reproductive output than either homozygote (in relevant genetic systems)

Graph modeling how the frequency of the sickle-cell allele can rise across generations under strong malarial selection when heterozygotes have the highest fitness. This is a classic balancing-selection scenario: higher heterozygote reproductive success can maintain multiple alleles in the population rather than driving one allele to fixation. Source