A population of insects has two alleles, and , affecting resistance to a pesticide. In Field A (strong selection), survival is: resistant = 90%, non-resistant = 10%. In Field B (weak selection), survival is: resistant = 90%, non-resistant = 85%. In which field will the resistance trait increase in frequency faster, and why?
- Field B, because higher overall survival means faster evolution.
- Field A, because the survival probability difference between types is much larger (stronger selection). (correct answer)
- Field A, because non-resistant insects will mutate into resistant insects during their lifetime.
- Both fields, at the same rate, because resistant insects survive at 90% in both places.
Explanation: This question tests your ability to use probability and differential survival/reproduction data to predict how trait and allele frequencies change in populations over time through natural selection. Probability reasoning for evolution: the RATE of evolutionary change depends on the MAGNITUDE of probability differences between variants—LARGE differences create stronger selection and faster evolution, while SMALL differences create weaker selection and slower evolution. In Field A, resistant insects have 90% survival while non-resistant have 10% survival (80 percentage point difference = STRONG selection), whereas in Field B, resistant have 90% survival while non-resistant have 85% survival (only 5 point difference = WEAK selection). Choice B correctly identifies that resistance will increase faster in Field A because the survival probability difference between types is much larger (80 points vs 5 points), creating stronger selection pressure. Choice A incorrectly focuses on overall survival rather than differential survival; C wrongly invokes mutation during lifetime when evolution works through differential survival/reproduction of existing variants; D misses that equal survival of resistant types doesn't determine rate—it's the DIFFERENCE from non-resistant that matters. Comparing selection strengths: Field A: 90% - 10% = 80 point difference = VERY STRONG selection = RAPID evolution. Field B: 90% - 85% = 5 point difference = WEAK selection = SLOW evolution. Real-world application: this explains why pesticide resistance evolves faster with higher pesticide doses (creating larger survival differences) than with lower doses!