AP Biology: Natural Selection — Practice Questions & Answers
How evolutionary mechanisms reshape populations: natural and artificial selection acting on heritable variation, differential reproductive success (fitness), the Hardy-Weinberg model for measuring genetic change, the evidence and history of evolution, and how new species arise or go extinct.
531 practice questions available for this unit — here are 10 with full answers and explanations.
In evolutionary biology, the term fitness most accurately refers to which of the following?
The physical strength and endurance of an individual
The relative reproductive success of an individual compared to others in the population
The total lifespan of an individual organism
The number of mutations an individual carries
Tap an answer to check it.
WhyFitness is defined as an organism's reproductive success, that is, its relative contribution of offspring (and thus alleles) to the next generation. It is not about physical strength or lifespan per se, except as those traits affect reproduction.
2Multiple choice · Easy
Which of the following is a necessary condition for natural selection to occur in a population?
All individuals in the population are genetically identical
There is heritable variation that affects survival and reproduction
The environment remains perfectly constant over time
Mutations stop occurring within the population
Tap an answer to check it.
WhyNatural selection requires heritable variation in traits that affect reproductive success. Without heritable variation, differential survival cannot change allele frequencies across generations. Identical individuals or non-heritable traits would not permit selection.
3Multiple choice · Medium
A population of birds shows a range of beak sizes. After a drought leaves only large, hard seeds available, the average beak size of the population increases over several generations while small beaks become rare. This pattern best illustrates which type of natural selection?
Stabilizing selection
Disruptive selection
Directional selection
Artificial selection
Tap an answer to check it.
WhyDirectional selection favors one extreme of a trait distribution, shifting the population mean in that direction. Here, large beaks are favored, so the distribution shifts toward larger beaks. Stabilizing selection favors the mean; disruptive selection favors both extremes.
4Multiple choice · Medium
In a population of snails, individuals with intermediate shell color survive best because both very light and very dark shells are more easily spotted by predators. Over time the variance in shell color decreases. This is an example of which type of selection?
Directional selection
Stabilizing selection
Disruptive selection
Sexual selection
Tap an answer to check it.
WhyStabilizing selection favors intermediate phenotypes and selects against both extremes, reducing variation in the trait. The result is a narrower distribution centered on the mean.
5Multiple choice · Hard
In an environment containing two distinct seed sizes (very small and very large) but few intermediate seeds, a finch population evolves so that both small-billed and large-billed birds become common while intermediate-billed birds decline. Which statement best describes the long-term evolutionary potential of this pattern?
It is stabilizing selection and will reduce overall genetic variation
It is disruptive selection and could promote sympatric speciation if the two forms become reproductively isolated
It is directional selection that will eliminate one extreme entirely
It is genetic drift and is unrelated to the seed environment
Tap an answer to check it.
WhyThis is disruptive selection, which favors both extremes against the intermediate. By increasing the frequency of two divergent phenotypes, disruptive selection can promote a bimodal distribution and, if combined with reproductive isolation between the two forms, can ultimately contribute to sympatric speciation.
6Multiple choice · Easy
Which of the following is the best example of artificial selection?
Bacteria developing antibiotic resistance after exposure to an antibiotic
Humans breeding dogs over generations to produce specific breeds with desired traits
Moths becoming darker as tree bark darkens from pollution
Finches evolving different beak shapes on different islands
Tap an answer to check it.
WhyArtificial selection occurs when humans intentionally breed organisms for desired heritable traits, such as developing many dog breeds from wolf ancestors. The other options describe natural processes acting without deliberate human breeding choices.
7Multiple choice · Hard
A breeder repeatedly selects the highest-yielding corn plants for several generations, but eventually yield stops increasing despite continued selection. Which explanation best accounts for this plateau?
The corn plants began producing their own new beneficial mutations to resist selection
The genetic variation for high yield has been largely exhausted, leaving little for selection to act on
Artificial selection cannot change allele frequencies after the first generation
The environment forced the plants to revert to their ancestral phenotype
Tap an answer to check it.
WhyContinued directional artificial selection can deplete the additive genetic variation for the trait, fixing favorable alleles. Once little heritable variation remains, selection has nothing to act on and the response to selection stalls. This illustrates that selection cannot create new variation, only act on existing variation (until new mutations arise).
8Multiple choice · Medium
Which of the following describes a population in the context of population genetics?
All organisms of different species living together in an ecosystem
A group of interbreeding individuals of the same species sharing a common gene pool
A single individual and all of its offspring
The total number of genes in one organism's genome
Tap an answer to check it.
WhyIn population genetics, a population is a group of individuals of the same species living in the same area that can interbreed and share a common gene pool. The gene pool is the collection of all alleles in that population.
9Multiple choice · Hard
In a population at Hardy-Weinberg equilibrium for a gene with two alleles, the recessive phenotype is found in 16% of individuals. What is the frequency of the dominant allele?
0.16
0.40
0.60
0.84
Tap an answer to check it.
WhyThe recessive phenotype frequency equals q^2 = 0.16, so q = 0.4. Since p + q = 1, p = 1 - 0.4 = 0.6. The dominant allele frequency is 0.6.
10Multiple choice · Hard
In a population of 1000 individuals at Hardy-Weinberg equilibrium, the dominant allele A has a frequency of 0.7 and the recessive allele a has a frequency of 0.3. Approximately how many individuals are expected to be heterozygous (Aa)?
90
210
420
490
Tap an answer to check it.
WhyHeterozygote frequency = 2pq = 2(0.7)(0.3) = 0.42. With 1000 individuals, 0.42 x 1000 = 420 heterozygous individuals.
Ready to test yourself?
Practise Natural Selection with instant marking and per-unit score tracking.
Evolution: A change in the heritable genetic makeup (allele and genotype frequencies) of a population across successive generations.
Natural selection: The process by which individuals with heritable traits that improve survival and reproduction in a given environment leave more offspring, gradually shifting the population's allele frequencies over time.
Microevolution: Small-scale changes in allele frequencies within a single population from one generation to the next.
Macroevolution: Large-scale evolutionary patterns above the species level, such as the origin of new species, major lineages, and broad biodiversity changes over long timescales.
Adaptation: An inherited trait shaped by natural selection because it raises an organism's reproductive success in its particular environment.
Fitness: An individual's relative contribution of offspring to the next generation compared with other individuals in the population.
Relative fitness: The reproductive output of one genotype expressed as a fraction of the output of the most successful genotype in the population.
Heritability: The degree to which variation in a trait is passed from parents to offspring through genes rather than caused by the environment.