Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Genetic diversity = variety of alleles and genotypes within a population or species. Measure it by alleles per gene, heterozygosity (expected 2pq for two alleles) or the spread of a heritable trait.
- Diversity is gained by mutation, meiosis and fertilisation, and gene flow. It is lost by selection, chance in small populations and inbreeding.
- With little diversity, one disease or change in conditions can affect almost every individual. Examples: one potato variety and blight in 1840s Ireland; uniform crops and rusts; the few founders of today’s condors and black-footed ferrets.
- With high diversity, some individuals are more likely to already carry alleles that let them survive. Not every cell in a varied bacterial population is killed by an antibiotic.
- Adding individuals from another population (genetic rescue) restores alleles by gene flow.
Key relationships
| Situation | Effect on diversity | Consequence |
|---|---|---|
| Population crash (bottleneck) | many alleles lost | less able to respond to the next pressure |
| Inbreeding | more homozygous individuals | harmful recessive alleles expressed more often |
| Monoculture or clones | almost no variation | one pathogen strain can destroy the whole crop |
| Gene flow from another population | alleles added | resilience restored (genetic rescue) |
| Environment changes | value of each allele changes | an adaptive allele may become harmful |
| Calculation | How |
|---|---|
| Fraction surviving if only rr dies | 1 − q² |
| Fraction surviving if only the recessive homozygote survives | q² |
| Expected heterozygosity, two alleles | 2pq |
Assumptions behind the reasoning
- Hardy–Weinberg calculations assume random mating and no recent selection at that gene.
- “More diverse means more resilient” is a probability statement: it makes survival more likely, not certain.
- A small trial or survey supports a claim; it does not prove it.
Mistakes to avoid
- Saying the pressure creates the helpful allele. Selection acts on existing variation.
- Comparing only averages; the spread of a trait decides who survives an extreme.
- Calling an allele “good” without naming the environment.
- Equating a large population with a diverse one.
- Using q instead of q² for the frequency of the homozygous recessive genotype.
Quick self-check
- Why is a field of genetically identical plants at risk? (Every plant has the same susceptibility, so a pathogen that infects one can infect all.)
- If q = 0.9 and only rr individuals die, what fraction survives? (1 − 0.9² = 1 − 0.81 = 0.19)
- Give one way to raise the diversity of a small, inbred population. (Bring in individuals from another population of the species: gene flow.)
Next: practice questions.