Questions: Evolution Through Natural Selection

Question 1

A population of bacteria is treated with an antibiotic. Within weeks, most survivors are resistant. What best explains this rapid shift?

• A. The antibiotic caused the bacteria to mutate and develop resistance as an adaptive response to the threat
• B. The most fit bacteria transferred their resistance directly to neighboring cells, spreading the trait horizontally
• C. Bacteria with pre-existing resistance mutations survived and reproduced more, increasing the frequency of resistance alleles in each successive generation
• D. The antibiotic selected for bacteria with better immune systems, which then passed on their stronger immunity

Answer: (C)

Natural selection works on pre-existing heritable variation — it does not create new mutations in response to selective pressure. Bacteria resistant to the antibiotic already existed in small numbers before treatment. When the antibiotic was introduced, susceptible bacteria died while resistant ones survived and reproduced, passing resistance to offspring. Over generations, resistant alleles became common. Option A describes Lamarckian inheritance (organisms adapting in response to need), which is incorrect. Horizontal gene transfer (option B) is a real phenomenon but not 'natural selection.'

Question 2

Stabilizing selection is acting on birth weight in a human population — both very small and very large babies survive less well than babies of intermediate weight. What genetic pattern do you expect over many generations?

• A. The population shifts toward smaller babies as selection consistently removes the largest individuals
• B. Allele frequencies remain unchanged because stabilizing selection conserves the status quo without affecting genetics
• C. Variation in birth weight increases as selection favors a wider range of values
• D. Alleles producing intermediate birth weights increase in frequency; alleles contributing to extreme values decrease

Answer: (D)

Stabilizing selection favors intermediate phenotype values and acts against both extremes, so alleles that push birth weight toward the extremes are selected against in each generation. Over time, those alleles decrease in frequency and phenotypic variation narrows. Option B is a common misconception: stabilizing selection absolutely changes allele frequencies — it just doesn't shift the mean. Option A describes directional selection, not stabilizing.

Question 3

Natural selection acts directly on alleles — it evaluates an organism's genetic sequence and selects which alleles persist in the next generation.

• T. True
• F. False

Answer: False

Natural selection acts on phenotypes — the observable traits expressed by organisms — not directly on DNA sequences. An allele that improves survival or reproduction increases in frequency because the organisms carrying it leave more offspring; selection 'sees' the trait (camouflage color, beak shape, disease resistance), not the underlying allele. The allele frequencies then change as a downstream consequence. This distinction matters: recessive alleles in heterozygotes often escape selection because they're not expressed in the phenotype.

Question 4

For natural selection to produce evolutionary change across generations, variation in a population must be heritable.

• T. True
• F. False

Answer: True

Heritability is one of Darwin's three required conditions for natural selection to work. If variation is not heritable — if offspring don't tend to resemble parents in the relevant trait — then even dramatic differences in reproductive success cannot change allele frequencies across generations. Non-heritable phenotypic variation (caused by diet, injury, or developmental environment rather than genetics) disappears in each generation and leaves no evolutionary legacy. This is why Darwin's argument requires both variation AND inheritance.

Question 5

Explain why individual organisms do not evolve, but populations do. How does this distinction clarify what natural selection actually does?

Model answer: Evolution is defined as a change in allele frequencies across generations in a population. Individual organisms have fixed genomes that do not change during their lifetimes (with minor exceptions like somatic mutation). What changes is which alleles are represented — and how frequently — in the breeding population across successive generations. Natural selection determines which individuals survive and reproduce, thereby influencing which alleles get passed to the next generation. The individual is the unit of selection; the population is the unit of evolution.

This distinction corrects the common misconception that organisms 'adapt' during their lifetimes in response to the environment (Lamarckian thinking). A giraffe doesn't grow a longer neck because it reaches for leaves; giraffes with genetically longer necks happened to survive better and left more offspring, so long-neck alleles became more frequent. The individual giraffe is the same from birth to death. The population, measured across generations, is what changes.