AP Syllabus focus:
‘Species and populations with little genetic diversity are at greater risk of decline or extinction.’
Low genetic diversity limits how populations respond to threats and can directly reduce survival and reproduction. In AP Biology, focus on how genetic mechanisms and chance events combine to increase decline and extinction risk.
Core idea: why low genetic diversity is dangerous
Genetic diversity provides the range of inherited differences that selection can act on and that can buffer populations against change.
Genetic diversity: The variety of alleles and genotypes in a population, often reflected in measures such as heterozygosity and allelic richness.
When diversity is low, a population tends to be more genetically uniform, so a single stressor (disease, drought, new predator) can affect many individuals similarly, increasing the chance of rapid decline.
Pathways from low diversity to decline and extinction
Reduced capacity to adapt to environmental change
If a new selective pressure arises, adaptation requires heritable variation in traits linked to survival or reproduction.
With fewer alleles, there are fewer potentially advantageous phenotypes available.
Populations may fail to “track” changing conditions (e.g., temperature shifts, altered rainfall, new parasites).
Even if some individuals survive, reproduction may not restore diversity quickly enough to prevent continued decline.
Inbreeding and inbreeding depression
Small or isolated populations often experience inbreeding (mating among relatives), which increases homozygosity.
This figure depicts identity-by-descent (IBD) segments and shows why small populations tend to produce offspring with long homozygous tracts after a bottleneck. As related individuals share more of the same ancestral chromosome segments, recessive deleterious alleles are more likely to become homozygous and expressed, contributing to inbreeding depression. Source
Inbreeding depression: A reduction in fitness caused by increased homozygosity, which exposes harmful recessive alleles and can reduce performance of heterozygote-advantage traits.
Fitness impacts that elevate extinction risk include:
Lower fertility or sperm quality
Higher juvenile mortality
Weaker immune responses (greater disease susceptibility)
Reduced growth, stress tolerance, or competitive ability
These effects can create a feedback loop: reduced fitness shrinks population size further, which increases inbreeding even more.
Genetic drift and loss of beneficial alleles
In small populations, genetic drift can dominate allele frequency change, causing random loss of alleles, including beneficial ones.
The expected loss of heterozygosity due to drift can be approximated using effective population size:
= Heterozygosity at time (dimensionless)
= Initial heterozygosity (dimensionless)
= Effective population size (breeding individuals; individuals)
= Number of generations (generations)
As decreases, heterozygosity is lost faster, making the population progressively less resilient and more vulnerable to additional stresses.
Accumulation of deleterious alleles (mutation load)
Even in healthy populations, new deleterious mutations arise. Large populations can more effectively reduce their frequency through selection, but small, low-diversity populations may:
Retain harmful alleles by chance (drift)
Express harmful recessives more often due to inbreeding
Experience gradual declines in average survival and reproduction
Interaction with non-genetic threats (the “extinction vortex”)
Low genetic diversity rarely acts alone. It interacts with:
Demographic stochasticity: random variation in births and deaths that can crash small populations
Environmental stochasticity: unpredictable events (storms, fires, heat waves) that disproportionately affect small, uniform populations
Disease outbreaks: low variation in immune-related genes can allow rapid spread
Together, these forces can drive an extinction vortex, where shrinking population size and worsening genetic health reinforce one another.
The extinction vortex diagram illustrates a self-reinforcing spiral in which small population size amplifies genetic and ecological problems (e.g., genetic drift, inbreeding depression, and stochastic events). As these pressures intensify, survival and reproduction decline, further reducing population size and accelerating the approach to extinction. Source
Recognising low-diversity extinction risk in real populations
Signs that low diversity may be contributing to decline include:
Persistent low reproductive success despite adequate habitat and resources
High incidence of congenital defects or poor juvenile survival
Strong population fragmentation with little interchange among subgroups
Rapid declines following novel pathogens or abrupt environmental shifts
Genetic data (e.g., heterozygosity, allelic richness) are used alongside demographic trends to infer whether genetic factors are likely elevating extinction risk.
FAQ
They combine genetic metrics (e.g., heterozygosity) with demographic and ecological evidence.
Common clues include poor reproduction across good years, high defect rates, and declines that persist after habitat improvement.
Genetic rescue introduces migrants to increase diversity and reduce inbreeding depression.
Risks include outbreeding depression if populations are too divergent, and introduction of pathogens or maladaptive alleles.
Heterozygosity reflects current relatedness and inbreeding, while allelic richness captures rare alleles important for future adaptation.
Many assessments use both because they can change at different rates.
Usually not. New diversity depends on mutation, which is slow relative to conservation timescales.
Without gene flow, drift can continue removing variation even if census numbers rebound.
No. Some persist if environments are stable and harmful alleles have been reduced, but risk remains higher.
A new disease or rapid climate shift can expose the vulnerability of genetic uniformity.
Practice Questions
Explain why a population with low genetic diversity is more likely to go extinct when the environment changes. (1–3 marks)
States that low genetic diversity means fewer alleles/less heritable variation available (1)
Links this to reduced ability to adapt via natural selection to new conditions (1)
Connects reduced adaptation to increased mortality or reduced reproductive success leading to extinction (1)
A small, isolated mammal population shows increasing relatedness among individuals and reduced juvenile survival over several generations. Describe how low genetic diversity can contribute to population decline and extinction risk in this scenario. (4–6 marks)
Identifies inbreeding/ mating between relatives in small populations (1)
Explains increased homozygosity and expression of deleterious recessive alleles (1)
Links this to inbreeding depression causing reduced juvenile survival and/or fertility (1)
Describes genetic drift removing alleles and further reducing genetic diversity in small populations (1)
Explains reduced ability to respond to disease/environmental change due to fewer alleles (1)
Connects declining fitness and size to a positive feedback/extinction vortex increasing extinction risk (1)
