learning objectives 5 Flashcards
What is required for evolution by natural selection to occur?
Evolution by natural selection occurs when there is variation in a heritable characteristic that affects survival and/or reproduction. This process is experimentally testable.
What happens to the population histogram after natural selection if the trait is heritable?
The population histogram will change, showing a shift in the distribution of the trait. If the trait is heritable, the population in subsequent generations will likely reflect this shift.
What is directional selection, and what is its effect on the population mean?
Directional selection occurs when an extreme phenotype is the most fit. The effect is a change in the population mean toward the more fit phenotype.
Example: Altitude tolerance in Tibetan women.
What is stabilizing selection, and how does it affect population variation?
Stabilizing selection occurs when an intermediate phenotype is the most fit. The population mean stays the same, but the population variation decreases as less fit phenotypes are selected against.
Example: Human birth weight, where babies weighing 7-8 pounds have higher survival rates.
What is disruptive selection, and what happens to population variation as a result?
Disruptive selection occurs when two extreme phenotypes are more fit than the intermediate phenotype. The population mean stays the same, but the variation increases as the intermediate phenotype is selected against.
Example: The beak size of the black-bellied seedcracker finch in West Africa, where birds have beaks either <12mm or >15mm.
How can you infer which mode of selection is at work from a population histogram?
By analyzing the change in phenotypes over time. Directional selection will shift the mean, stabilizing selection will narrow the variation, and disruptive selection will increase the variation with a bimodal distribution.
