Question
By the end of the 19th century in England, the dark form of the moth Biston betularia formed up to $98 \%$ of the total population in industrial areas. From 1970, the percentage of dark forms decreased significantly. What is an explanation for the decrease?
A. An increase in environmental pollution killed the dark forms more than the light forms.
B. Reduction of pollution resulted in greater camouflage for light forms of the moth.
C. Dark forms could no longer find mates.
D. Light forms had superior feeding mechanisms.
▶️Answer/Explanation
Answer: B. Reduction of pollution resulted in greater camouflage for light forms of the moth.
Explanation:
Biston betularia, or the peppered moth, is a classic example of natural selection in response to environmental changes. During the Industrial Revolution, dark-colored moths became more common in polluted areas because they were better camouflaged on soot-covered trees, avoiding predation. After 1970, environmental regulations reduced pollution, cleaning the trees and exposing dark moths to predators again, leading to a decline in their numbers.
Option Evaluation:
A. Incorrect – Pollution did not start killing dark moths directly. The issue was camouflage, not toxicity. The dark moths were more visible to predators after pollution decreased.
B. Correct – As pollution decreased, tree bark became lighter, making light-colored moths better camouflaged. This improved their survival and reproductive success, leading to an increase in their numbers and a decrease in dark moths.
C. Incorrect – There’s no evidence that mating issues caused the decline in dark forms. Mate choice was not a major factor here; predation and camouflage were the driving forces.
D. Incorrect – Feeding mechanisms did not change between the forms. Both moth types feed the same way; survival was determined by visibility to predators, not food acquisition.
Question
Three-spined stickleback fish (Gasterosteus aculeatus) vary in the number of armour plates.
The graph shows the frequency of individuals with low, partial or complete plating in a three-spined stickleback population living in Kennedy Lake, Vancouver Island, Canada.
Which type of natural selection could result in this pattern of variation in the population?
A. Disruptive
B. Directional
C. Stabilizing
D. Convergent
▶️Answer/Explanation
Answer: A. Disruptive
Explanation:
There are three main types of natural selection:
- Directional – favors one extreme trait.
- Stabilizing – favors the average trait.
- Disruptive – favors both extremes and selects against the average.
If a population shows high frequencies at both extremes (e.g., mostly low and complete plating, but few with partial plating), this indicates disruptive selection.
Option Evaluation:
A. Correct – Disruptive selection explains why both low and complete plating are common, while partial plating is rare. Different environments or predator pressures may favor either extreme.
B. Incorrect – Directional selection would result in a shift toward one extreme, not two. That’s not what the graph shows.
C. Incorrect – Stabilizing selection would increase the frequency of the average trait, which would be partial plating. But in the graph, partial plating is less frequent.
D. Incorrect – Convergent evolution is not a type of natural selection but an evolutionary pattern where unrelated species develop similar traits. It doesn’t apply to variation within a single species.
Question
The graph shows the proportion of a bacterial population of Neisseria gonorrhoeae, displaying resistance to the antibiotic tetracycline.
What can be deduced from this graph?
A. Bacteria with beneficial adaptations survive and pass on their genes.
B. Immunity to tetracycline is triggered by over-use of the antibiotic.
C. Genetic variation in this bacterial population is increasing.
D. Use of tetracycline inhibits the growth of antibiotic-resistant N. gonorrhoeae.
▶️Answer/Explanation
Answer: A. Bacteria with beneficial adaptations survive and pass on their genes.
Explanation:
Antibiotic resistance evolves through natural selection. When bacteria are exposed to antibiotics, those with random mutations that offer resistance survive, reproduce, and pass on those traits. Over time, the frequency of resistant bacteria increases in the population.
Option Evaluation:
A. Correct – This is a classic example of natural selection. The bacteria that survive tetracycline exposure do so because they have a genetic advantage, and they pass those resistant genes on.
B. Incorrect – Resistance is not triggered by antibiotic use directly. Bacteria do not develop immunity in response to antibiotics; instead, random mutations happen and those with beneficial ones survive better.
C. Incorrect – The graph shows a rise in one specific trait (resistance), which often means less overall genetic variation, not more. So variation is not necessarily increasing.
D. Incorrect – Tetracycline does not inhibit resistant bacteria. In fact, the graph shows the opposite: these bacteria continue growing despite the antibiotic, which is the definition of resistance.