What the exam asks
- Explain how competition for limited resources leads to differential survival and reproduction.
- Describe or evaluate evolutionary fitness strictly as reproductive success (number of fertile offspring produced) using data tables or graphs.
- Predict how changes in biotic or abiotic selective pressures alter the direction and rate of evolution.
- Connect molecular variation (e.g., enzyme isoforms, membrane lipids, chlorophyll types) to organismal survival in varying environments.
- Analyze data from artificial selection experiments to explain how human-directed breeding alters phenotypic and genetic variation in populations.
Core ideas
Darwin’s mechanism of natural selection
Natural selection is a non-random mechanism of evolution that acts on existing phenotypic variation in a population:
- Overproduction and competition: Organisms produce more offspring than limited environmental resources (food, space, mates) can support, creating competition.
- Heritable variation: Individuals in a population possess different phenotypes due to underlying genetic variation (arising from random mutations, meiosis, and sexual reproduction).
- Differential survival and reproduction: Individuals with traits better suited to the current environment are more likely to survive, reproduce, and pass alleles for those advantageous traits to their offspring.
- Adaptation: Over generations, advantageous heritable traits become more common in the population, while unfavorable traits become less common.
Evolutionary fitness and selective pressures
In AP Biology, evolutionary fitness is measured strictly by reproductive success—an individual’s genetic contribution to the next generation (producing viable, fertile offspring). Living a long time or being physically strong does not confer high fitness unless it translates into more surviving offspring.
Selective pressures are any biotic (predators, pathogens, competition) or abiotic (temperature, drought, salinity, toxins) factors that influence survivorship and reproduction:
- Stability vs. fluctuation: A phenotype with high fitness in a wet year may have low fitness in a drought year. As biotic and abiotic environments fluctuate, both the direction (which traits are favored) and the rate (how fast allele frequencies shift) of evolution change.
- Genetic diversity as insurance: More genetically diverse populations are more resilient to environmental perturbations because they are more likely to contain individuals capable of withstanding new selective pressures.
| Mode of Selection | Selective Pressure | Effect on Phenotypic Variation |
|---|---|---|
| Directional | Favors one extreme phenotype (e.g., pesticide resistance). | Shifts population mean toward the favored extreme. |
| Stabilizing | Favors intermediate phenotypes; selects against both extremes (e.g., human birth weight). | Decreases phenotypic variation around the mean. |
| Disruptive | Favors both extreme phenotypes over the intermediate (e.g., beak sizes for strictly large or small seeds). | Increases variation; leads to a bimodal distribution. |