What the exam asks
- Explain ongoing evolution: Use data on antibiotic, pesticide, herbicide, or chemotherapy resistance, or pathogen mutations, to explain how allele frequencies shift over generations.
- Evaluate genetic diversity and extinction risk: Explain why genetically uniform populations (such as bottlenecked wild species or agricultural monocultures) are vulnerable to novel environmental pressures.
- Contextualize fitness: Explain why an allele that is adaptive in one environment can be deleterious or neutral when the environment changes.
- Design or extend investigations: Propose a follow-up experiment, identify controls, or state a testable hypothesis to measure ongoing evolutionary changes in a population.
Core ideas
Evidence of continuing evolution
All extant species continue to evolve. Because evolution is defined as a change in allele frequencies in a population over generations, any ongoing shift in genotype or phenotype distributions demonstrates continuing evolution.
| Line of Evidence | Mechanism & Exam Context |
|---|---|
| Chemical Resistance | Application of antibiotics (bacteria), pesticides (insects), herbicides (weeds), or chemotherapy drugs (tumor cells) acts as a strong selective pressure. Individuals with pre-existing random mutations that confer resistance survive and reproduce at higher rates, increasing the frequency of the resistance allele. |
| Emerging Pathogens | Viruses and bacteria have short generation times and high mutation rates (especially RNA viruses lacking proofreading enzymes). Antigenic changes allow pathogens to evade host immune systems or jump to new host species. |
| Genomic & Fossil Change | Direct DNA sequencing across generations shows ongoing mutation, chromosomal rearrangements, and allele shifts. The fossil record shows continuous morphological shifts across geological strata up to the present. |
Genetic diversity and population resilience
A population’s ability to respond to environmental change depends on its standing genetic variation—the pool of alleles already present before the environment shifts.
- High genetic diversity: Increases the probability that at least some individuals possess alleles encoding phenotypes that can survive a new selective pressure (such as a novel pathogen, drought, or toxin). The population adapts as those individuals reproduce.
- Low genetic diversity: In small, inbred, or bottlenecked populations (e.g., Tasmanian devils, cheetahs, or cloned crop monocultures), individuals share nearly identical genomes. If a lethal pathogen or sudden climate shift occurs and no individual carries a protective allele, the entire population faces decline or extinction.