AP Syllabus focus:
‘Genetically diverse populations are more likely to contain individuals that withstand environmental perturbations.’
Genetic diversity determines how many different biological “options” a population has when conditions change. When environments shift quickly, variation can separate populations that persist and reproduce from those that struggle.
What genetic diversity is (and why it matters)
Genetic diversity includes differences in DNA sequences, alleles, and genotypes among individuals in the same population. These differences can influence traits that affect survival and reproduction under new conditions (e.g., temperature tolerance, immune responses, or drought resistance).
Genetic diversity: the variety of alleles and genotypes present within a population’s gene pool.
Diversity can be described at multiple biological levels:
Allelic diversity (how many alleles exist at a locus)
Heterozygosity (how often individuals carry two different alleles at a locus)
Genome-wide variation (many loci across chromosomes contributing small effects)
Measuring diversity (conceptually)
AP Biology often emphasizes the idea that more variation increases the chance that some individuals already possess traits suited to a new challenge. In research and conservation contexts, diversity is frequently summarised with heterozygosity:
= expected heterozygosity (unitless; probability two randomly chosen alleles are different)
= frequency of the th allele (unitless)
This equation captures a key idea: when allele frequencies are more evenly distributed (not dominated by one allele), expected heterozygosity tends to be higher.
This figure plots expected heterozygosity as allele frequency changes in a two-allele system. The curve peaks when alleles are equally common (), illustrating that heterozygosity is highest when the gene pool is not dominated by a single allele. This provides an intuitive visual basis for why expected heterozygosity increases as allele frequencies become more even.Source
How diversity increases resilience to environmental change
Environmental change (an environmental perturbation) can be abiotic (temperature spikes, altered rainfall, salinity changes) or biotic (new pathogens, competitors). Genetic diversity boosts resilience because it increases the likelihood that some individuals have phenotypes that perform well under the new conditions.
Mechanism: “some individuals can already cope”
A genetically diverse population is more likely to include:
Pre-existing tolerant phenotypes (standing variation) that survive the disturbance
Genotypes with broader tolerance ranges (generalists) that maintain function across conditions
Rare alleles that become advantageous when conditions shift
If those individuals survive and reproduce more, the alleles associated with tolerance become more common in the next generation, allowing the population to persist through change.
Mechanism: disease and immune diversity
Diversity is especially important for resisting rapidly evolving biological threats.
Pathogens often spread fastest when hosts are genetically similar (similar receptors, similar immune recognition).
Diverse immune-related genes (such as MHC variants) increase the chance that some individuals can recognise and respond effectively, reducing population-wide impact.
This diagram summarizes the MHC class I antigen-presentation pathway that underlies immune recognition by CD8+ T cells. Intracellular proteins are degraded into peptides, transported into the ER (via TAP), loaded onto MHC I, and displayed at the cell surface for T-cell surveillance. Variation in MHC molecules changes which peptides can be presented, helping explain why immune-gene diversity can improve resistance to rapidly evolving pathogens. Source
Mechanism: buffering against uncertainty and trade-offs
Many traits involve trade-offs (e.g., heat tolerance vs. growth rate). When the environment is unpredictable:
A single “best” genotype may not exist across all years or microhabitats.
Diversity acts as a portfolio, spreading risk across multiple genotypes.
Fluctuating conditions can favor different alleles at different times, maintaining variation that later becomes crucial during a perturbation.
What diversity does not guarantee
Genetic diversity increases the probability that some individuals withstand perturbations, but it does not ensure persistence in every case. Outcomes depend on:
Rate and magnitude of change (too rapid/extreme can overwhelm even diverse populations)
Population size and reproductive rate (enough survivors must reproduce)
Genetic architecture of traits (some tolerances require multiple genes, limiting immediate responses)
Connectivity and habitat structure (whether tolerant individuals can access suitable microhabitats)
Key takeaways aligned to the syllabus focus
To match the syllabus statement, connect these points explicitly:
Environmental perturbations can remove many individuals quickly.
Genetically diverse populations have higher chances that at least some individuals possess protective alleles.
Those survivors can maintain reproduction, helping the population persist and recover.
FAQ
They sample many loci across the genome (e.g., SNP panels or microsatellites) and calculate statistics such as observed heterozygosity and allelic richness.
High-throughput sequencing can also reveal rare variants that strongly affect resilience.
Allele identity matters: some alleles affect tolerance directly, while others are neutral.
Also, allele frequencies matter—if one allele is near fixation, functional diversity may be low even if several alleles exist.
Effective population size ($N_e$) is the number of individuals effectively contributing genes to the next generation.
Low $N_e$ accelerates loss of diversity through chance sampling, reducing the pool of potentially tolerant genotypes during perturbations.
Approaches include:
Assisted gene flow from ecologically similar populations
Maintaining habitat corridors to support mixing
Using genetic screening to avoid introducing maladaptive alleles
The goal is to increase diversity while matching local environmental conditions.
If the necessary tolerance requires many coordinated genetic changes, existing variation may be insufficient.
Extreme events can also reduce numbers so drastically that too few survivors remain to reproduce, even if some are tolerant.
Practice Questions
Explain why a population with higher genetic diversity is more likely to persist after a sudden environmental change. (2 marks)
States that greater genetic diversity increases the likelihood that some individuals have advantageous alleles/phenotypes for the new conditions (1).
States that these individuals survive and reproduce, maintaining the population (1).
A new fungal pathogen enters two plant populations of the same species. Population A has high genetic diversity; Population B is genetically uniform. Predict and justify how the two populations are likely to differ in survival and reproduction over the next few generations. (5 marks)
Predicts Population A will have higher survival or less severe decline than Population B (1).
Justifies with presence of resistant genotypes/alleles already in Population A (standing variation) (1).
Links resistance to increased reproductive success of survivors in Population A, so resistance alleles become more common (1).
Predicts Population B will experience widespread susceptibility due to genetic uniformity (1).
Explains that limited variation in Population B reduces the chance of resistant individuals, lowering reproduction and slowing recovery (1).
