Genetic variation shapes how populations grow, shrink, and persist across generations. In AP Biology, connect allele and genotype diversity to survival, reproduction, and changing population size through time.

Core idea: linking genes to population-level change

What counts as genetic variation?

Genetic variation refers to differences in DNA among individuals in the same population, producing differences in alleles and genotypes that can affect traits.

Because individuals differ genetically, they also often differ in fitness-related traits (e.g., disease resistance, metabolic efficiency, fertility). These differences alter birth rates, death rates, and the number of breeding individuals, which together drive population dynamics (changes in population size and composition over time).

Why population dynamics depend on variation

Genetic variation influences population dynamics through three closely related pathways:

• Trait variation affecting survival: some genotypes survive better under the population’s current conditions, raising the proportion of survivors and stabilising or increasing population size.

• Trait variation affecting reproduction: differences in mating success, fecundity, or gamete viability change the number of offspring entering the next generation.

• Maintenance of breeding potential: variation supports healthier patterns of mating and reduces the chance that many individuals share the same harmful alleles.

Measuring genetic variation in a population

Allele frequencies and heterozygosity

A practical way to describe variation is the fraction of individuals that are heterozygous at a locus (carry two different alleles). Higher heterozygosity usually indicates a broader set of genotypes contributing to survival and reproduction.

Expected heterozygosity for a two-allele locus is often summarised as:

Expected heterozygosity ($H=2pq$) is shown as a function of allele frequency $p$. The curve peaks when $p=q=0.5$, illustrating that genetic variation is maximized when alleles are at similar frequencies and minimized when one allele is near fixation. Source

Heterozygosity is informative because many fitness-relevant traits are influenced by alleles that can be beneficial, neutral, or harmful depending on how they are combined in genotypes.

Effective population size and “who is really breeding”

Not all individuals contribute equally to the next generation. When only a subset reproduces, the population’s genetic variation can behave like that of a smaller population.

Population dynamics depend strongly on Ne because:

This graph shows how effective population size ($N_e$) changes as the number of breeding females ($N_f$) varies while total breeders remain constant. $N_e$ is highest when the sex ratio is balanced and declines as one sex becomes rare, demonstrating why populations can “act small” genetically even if the census size is large. Source

• fewer breeders can cause rapid shifts in which alleles are represented among newborns

• unequal reproductive success concentrates reproduction into fewer lineages, changing genotype frequencies generation to generation

• loss of variation can reduce average reproductive output if harmful alleles become more commonly paired

Mechanisms by which variation alters population growth

Inbreeding and inbreeding depression

When mates are genetically similar, offspring are more likely to be homozygous for deleterious recessive alleles, which can reduce survival or fertility.

The figure compares fitness distributions for outbred and inbred groups and illustrates a drop in mean fitness (inbreeding depression). It also highlights that environmental conditions can modulate the size of this fitness reduction, which matters because survival and fecundity shifts directly change population growth. Source

Effects on population dynamics commonly include:

• lower juvenile survival, reducing recruitment into the breeding population

• reduced fecundity (fewer offspring per adult)

• increased variance in reproductive success (some lineages fail entirely)

Genetic load and demographic consequences

Genetic load (the presence of harmful alleles lowering average fitness) can change population growth by subtly increasing mortality or reducing reproductive rates across many individuals. Even small per-individual effects can scale to noticeable changes in population size across generations, especially when many loci contribute.

Variation in key life-history traits

Genetic differences influencing traits such as development time, age at first reproduction, or offspring number can change:

• generation time (how quickly the population turns over)

• age structure (proportion of juveniles vs adults)

• net reproductive rate (average contribution of individuals to the next generation)

These shifts matter because population growth is determined not just by how many individuals exist, but by how many are reproducing and how effectively their offspring survive to reproduce.