Study guide: Evolution and Population Mechanisms
Compare selection, drift, migration and speciation models.
Ideas to explain in your own words
- Population frequencies can change by selection, random drift, gene flow or new mutation. Distinguishing mechanisms requires evidence. A change that looks useful is not proof it arose because it was needed.
- Allele percentage equals 100 times focal copies divided by twice individual count. Frequency change does not by itself identify a mechanism.
- Reproductive isolation can reduce gene exchange between diverging populations. Geographic separation alone does not prove separate species. Species concepts have strengths and limitations, especially for fossils and asexual organisms.
- An explanation should connect inherited variation, reproduction and time. Individual learning or deliberate bodily change is not population evolution by natural selection. Alternatives must fit the supplied evidence.
Useful vocabulary
- selection
- trait-linked reproductive differences change frequencies
- %
- Percent means a quantity expressed per 100 of its stated reference.
- Allele percentage
- Allele percentage equals 100 times focal copies divided by twice individual count.
- geographic separation
- populations occupy disconnected places
- population explanation
- frequency changes across generations
Watch your reasoning
- Confusing selection with drift although their functions or changes differ.
- Using the distractor operation 100*a/b instead of 100*a/(2*b); Use two allele copies per stated diploid individual.
- Confusing geographic separation with reproductive isolation although their functions or changes differ.
A worked review example
Worked supplied problem: A small population’s frequencies shift after a random survivor sample, with no measured trait advantage. Compare drift and selection. Solution: Random sampling fits drift. Selection would require evidence connecting inherited traits to reproductive differences. A frequency shift alone cannot establish that advantage. This conclusion uses the stated observations and the mechanism explained above; it does not establish every unmeasured feature of a real organism, device or place. Worked distinct example: A distinct diploid model has 10 focal copies among 20 individuals: 100×10/40=25%. Method: identify the quantities in the stated rule, substitute using compatible units, and perform the operations in the correct order. Use two allele copies per stated diploid individual. Individuals and allele copies are different counts. Interpret the final unit and keep the model assumption visible; this calculation is not a measurement of a real learner or site. Worked supplied problem: Separated populations later reunite and exchange genes readily. Was geographic separation alone proof of distinct species? Solution: No. Geographic isolation can allow divergence, but continued gene exchange is evidence relevant to reproductive isolation. Species status needs an appropriate concept and more evidence. This conclusion uses the stated observations and the mechanism explained above; it does not establish every unmeasured feature of a real organism, device or place. Worked supplied problem: An individual becomes stronger through training, but offspring allele frequencies are unchanged. Explain why this is not selection-driven population evolution. Solution: Training is an individual-life response. Natural-selection explanation requires heritable variation and differential reproduction producing population change across generations; those conditions are not supplied. This conclusion uses the stated observations and the mechanism explained above; it does not establish every unmeasured feature of a real organism, device or place.
Make a practice plan
- Explain the mechanism in Separate evolutionary mechanisms using a labeled case.
- Complete Compare allele frequencies and correct the specific misconception in its feedback.
- Resolve a changed case from Interpret speciation evidence, then combine the four ideas in the transfer task.