PCB 4674 | PCB4674 Final Exam: Evolution -
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Questions and Correct Answers with Rationale
1. In a small island population of lizards, a rare allele increases in frequency over several
generations despite no apparent fitness benefit. Which process most likely accounts for this?
A. Natural selection
B. Gene flow
C. Genetic drift
D. Mutation pressure
Correct Answer: C
Expert Explanation: Genetic drift refers to random fluctuations in allele frequencies
within a population. These effects are particularly strong in small populations where
chance events can cause alleles to disappear or reach fixation. Unlike natural selection, drift
does not depend on the adaptive value of a trait. Gene flow would require immigration,
which is often limited in island systems. Therefore, the increase of a neutral allele is a
classic hallmark of stochastic evolutionary forces.
2. Which of the following best defines a monophyletic group or clade in phylogenetic
systematics?
A. A group consisting of an ancestor and some of its descendants
B. A group based on shared analogous traits
C. A group consisting of an ancestor and all of its descendants
D. A group of organisms that look similar due to convergence
Correct Answer: C
Expert Explanation: A monophyletic group is the fundamental unit used to describe
evolutionary lineages. It must include the most recent common ancestor and every single
descendant from that point. Groups that exclude some descendants are termed
paraphyletic, while those based on convergent traits are polyphyletic. Cladistic analysis
relies on identifying these groups to reconstruct the true tree of life. Understanding these
distinctions is critical for accurately mapping historical relationships among taxa.
3. A population is in Hardy-Weinberg equilibrium with the frequency of the recessive allele
(q) being 0.3. What is the expected frequency of the heterozygotes?
A. 0.09
B. 0.49
,C. 0.42
D. 0.21
Correct Answer: C
Expert Explanation: In Hardy-Weinberg equilibrium, the frequencies of alleles p and q
sum to one. If q is 0.3, then p must be 0.7 to satisfy the equation p + q = 1. The frequency of
heterozygotes is calculated using the term 2pq in the expansion p^2 + 2pq + q^2.
Multiplying 2 by 0.7 and 0.3 results in a frequency of 0.42. This mathematical model serves
as a null hypothesis to detect evolutionary change in populations.
4. If the dN/dS ratio of a protein-coding gene is significantly less than 1 (dN/dS < 1), what
does this indicate about the type of selection acting on it?
A. Positive selection
B. Purifying (negative) selection
C. Directional selection
D. Balancing selection
Correct Answer: B
Expert Explanation: The ratio dN/dS compares the rate of non-synonymous substitutions
to synonymous substitutions. A ratio less than one indicates that amino acid changes are
being filtered out by selection. This suggests that the current protein sequence is already
optimized and changes are likely deleterious. Purifying selection is the most common form
of selection observed in highly functional genes. Conversely, a ratio greater than one would
imply selection for novelty or change.
5. The Biological Species Concept (BSC) primarily uses which criterion to define species
boundaries?
A. Morphological similarity
B. Niche occupancy
C. Reproductive isolation
D. DNA sequence divergence
Correct Answer: C
Expert Explanation: The Biological Species Concept focuses on the ability of individuals to
interbreed and produce fertile offspring. It defines species as groups of actually or
potentially interbreeding natural populations. This criterion highlights the importance of
gene flow in maintaining species cohesion. However, the BSC has limitations when applied
to asexual organisms or extinct species found in fossils. Despite these issues, it remains a
central concept in evolutionary biology for sexual taxa.
, 6. Which scenario best illustrates allopatric speciation via vicariance?
A. A small group of birds blown to a distant island
B. A host-switch by insects to a new plant species in the same forest
C. A new mountain range rising and splitting a population of beetles
D. Plants doubling their chromosome number and becoming self-fertile
Correct Answer: C
Expert Explanation: Vicariance occurs when a physical barrier arises within the existing
range of a species. This barrier prevents gene flow and allows separated populations to
diverge genetically over time. Examples include the formation of mountains, canyons, or
the shifting of tectonic plates. Unlike dispersal, where individuals move to a new location,
vicariance involves the habitat itself splitting. This process is a major driver of global
biodiversity patterns across many different taxa.
7. In phylogenetic trees, traits that are shared because they were inherited from a common
ancestor are called:
A. Analogous traits
B. Homoplasies
C. Convergent traits
D. Homologous traits
Correct Answer: D
Expert Explanation: Homology is defined as similarity resulting from shared ancestry
rather than similar environmental pressures. For example, the forelimb bones of mammals
are homologous because they derive from the same ancestral structure. This is distinct
from analogy, where structures look similar because they perform similar functions.
Analogy results from convergent evolution, where unrelated lineages solve problems in
similar ways. Distinguishing homology from homoplasy is the central challenge of modern
phylogenetic reconstruction.
8. What is the primary consequence of a severe population bottleneck?
A. Increased genetic diversity due to rapid mutation
B. A significant loss of genetic variation and increased inbreeding
C. An increase in the effective population size
D. Enhanced adaptation to multiple environments
Correct Answer: B
Expert Explanation: A population bottleneck occurs when a large population is drastically
reduced in size for at least one generation. During this period, rare alleles are often lost
Florida State University Updated and Latest
Questions and Correct Answers with Rationale
1. In a small island population of lizards, a rare allele increases in frequency over several
generations despite no apparent fitness benefit. Which process most likely accounts for this?
A. Natural selection
B. Gene flow
C. Genetic drift
D. Mutation pressure
Correct Answer: C
Expert Explanation: Genetic drift refers to random fluctuations in allele frequencies
within a population. These effects are particularly strong in small populations where
chance events can cause alleles to disappear or reach fixation. Unlike natural selection, drift
does not depend on the adaptive value of a trait. Gene flow would require immigration,
which is often limited in island systems. Therefore, the increase of a neutral allele is a
classic hallmark of stochastic evolutionary forces.
2. Which of the following best defines a monophyletic group or clade in phylogenetic
systematics?
A. A group consisting of an ancestor and some of its descendants
B. A group based on shared analogous traits
C. A group consisting of an ancestor and all of its descendants
D. A group of organisms that look similar due to convergence
Correct Answer: C
Expert Explanation: A monophyletic group is the fundamental unit used to describe
evolutionary lineages. It must include the most recent common ancestor and every single
descendant from that point. Groups that exclude some descendants are termed
paraphyletic, while those based on convergent traits are polyphyletic. Cladistic analysis
relies on identifying these groups to reconstruct the true tree of life. Understanding these
distinctions is critical for accurately mapping historical relationships among taxa.
3. A population is in Hardy-Weinberg equilibrium with the frequency of the recessive allele
(q) being 0.3. What is the expected frequency of the heterozygotes?
A. 0.09
B. 0.49
,C. 0.42
D. 0.21
Correct Answer: C
Expert Explanation: In Hardy-Weinberg equilibrium, the frequencies of alleles p and q
sum to one. If q is 0.3, then p must be 0.7 to satisfy the equation p + q = 1. The frequency of
heterozygotes is calculated using the term 2pq in the expansion p^2 + 2pq + q^2.
Multiplying 2 by 0.7 and 0.3 results in a frequency of 0.42. This mathematical model serves
as a null hypothesis to detect evolutionary change in populations.
4. If the dN/dS ratio of a protein-coding gene is significantly less than 1 (dN/dS < 1), what
does this indicate about the type of selection acting on it?
A. Positive selection
B. Purifying (negative) selection
C. Directional selection
D. Balancing selection
Correct Answer: B
Expert Explanation: The ratio dN/dS compares the rate of non-synonymous substitutions
to synonymous substitutions. A ratio less than one indicates that amino acid changes are
being filtered out by selection. This suggests that the current protein sequence is already
optimized and changes are likely deleterious. Purifying selection is the most common form
of selection observed in highly functional genes. Conversely, a ratio greater than one would
imply selection for novelty or change.
5. The Biological Species Concept (BSC) primarily uses which criterion to define species
boundaries?
A. Morphological similarity
B. Niche occupancy
C. Reproductive isolation
D. DNA sequence divergence
Correct Answer: C
Expert Explanation: The Biological Species Concept focuses on the ability of individuals to
interbreed and produce fertile offspring. It defines species as groups of actually or
potentially interbreeding natural populations. This criterion highlights the importance of
gene flow in maintaining species cohesion. However, the BSC has limitations when applied
to asexual organisms or extinct species found in fossils. Despite these issues, it remains a
central concept in evolutionary biology for sexual taxa.
, 6. Which scenario best illustrates allopatric speciation via vicariance?
A. A small group of birds blown to a distant island
B. A host-switch by insects to a new plant species in the same forest
C. A new mountain range rising and splitting a population of beetles
D. Plants doubling their chromosome number and becoming self-fertile
Correct Answer: C
Expert Explanation: Vicariance occurs when a physical barrier arises within the existing
range of a species. This barrier prevents gene flow and allows separated populations to
diverge genetically over time. Examples include the formation of mountains, canyons, or
the shifting of tectonic plates. Unlike dispersal, where individuals move to a new location,
vicariance involves the habitat itself splitting. This process is a major driver of global
biodiversity patterns across many different taxa.
7. In phylogenetic trees, traits that are shared because they were inherited from a common
ancestor are called:
A. Analogous traits
B. Homoplasies
C. Convergent traits
D. Homologous traits
Correct Answer: D
Expert Explanation: Homology is defined as similarity resulting from shared ancestry
rather than similar environmental pressures. For example, the forelimb bones of mammals
are homologous because they derive from the same ancestral structure. This is distinct
from analogy, where structures look similar because they perform similar functions.
Analogy results from convergent evolution, where unrelated lineages solve problems in
similar ways. Distinguishing homology from homoplasy is the central challenge of modern
phylogenetic reconstruction.
8. What is the primary consequence of a severe population bottleneck?
A. Increased genetic diversity due to rapid mutation
B. A significant loss of genetic variation and increased inbreeding
C. An increase in the effective population size
D. Enhanced adaptation to multiple environments
Correct Answer: B
Expert Explanation: A population bottleneck occurs when a large population is drastically
reduced in size for at least one generation. During this period, rare alleles are often lost
