Questions: Natural Selection

Question 1

A population of bacteria is exposed to an antibiotic. Most bacteria die, but a few with a pre-existing resistance mutation survive and reproduce. Which condition of natural selection explains why resistance was already present before antibiotic exposure?

• A. Differential reproduction
• B. Limited resources
• C. Heritable variation
• D. Phenotypic plasticity

Answer: (C)

The resistance mutation existed as heritable variation in the population before any selection pressure was applied. This is a critical point: natural selection does not create new traits on demand — it filters pre-existing variation. The bacteria didn't 'develop' resistance in response to the antibiotic; individuals carrying a pre-existing resistance allele simply survived to reproduce.

Question 2

If a giraffe stretches its neck throughout its life to reach higher leaves, its offspring will tend to be born with slightly longer necks.

• T. True
• F. False

Answer: False

This describes Lamarckian inheritance of acquired characteristics, which is incorrect. Natural selection operates on heritable genetic variation, not traits acquired during an organism's lifetime. A giraffe's neck length is determined by alleles inherited from its parents, not by its own neck-stretching behavior. Only heritable variation — encoded in DNA and passed to offspring — can be acted on by natural selection.

Question 3

Natural selection acts on phenotypes, but evolution is measured at the level of allele frequencies. Explain why this distinction matters.

Model answer: Selection 'sees' and filters phenotypes (observable traits), but only the underlying alleles are inherited. A beneficial phenotype can only spread if it has a genetic basis. Tracking allele frequencies is how we measure whether evolution has actually occurred.

Two individuals might share an identical phenotype but have different genotypes (e.g., one is heterozygous, one is homozygous). Selection on phenotype affects which individuals reproduce, but what gets passed to the next generation are alleles. Evolution — change over time in a population — is therefore defined as change in allele frequencies across generations, not as change in any individual organism.