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19. Phylogeny, Speciation, and Extinction — Practice Questions
This chapter explores how scientists infer evolutionary relationships using phylogenies, the processes leading to speciation, and how extinction and continuing evolution shape life on Earth.
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(Multiple Choice — Click to Reveal Answer)
1. What is the primary purpose of a phylogenetic tree?
(A) To track population size
(B) To show feeding relationships
(C) To represent evolutionary relationships
(D) To classify organisms based on color
Answer
(C) — Phylogenetic trees show hypothesized evolutionary relationships among species, based on common ancestry.
2. In a phylogenetic tree, what does a node represent?
(A) An extinct organism
(B) A speciation event and common ancestor
(C) A fossil record
(D) A branch of modern species
Answer
(B) — Nodes indicate a common ancestor from which two or more descendant groups diverged.
3. Which of the following is considered the most reliable evidence for constructing phylogenies?
(A) Fossil position
(B) Biogeography
(C) Morphological traits
(D) Molecular data
Answer
(D) — Molecular data such as DNA sequences provide more consistent evidence and are less prone to convergent evolution.
4. What is a shared derived characteristic?
(A) A trait present only in outgroups
(B) A trait found in all species
(C) A newly evolved trait unique to a clade
(D) A trait caused by the environment
Answer
(C) — Shared derived characteristics are traits that evolved in the most recent common ancestor of a clade and are unique to that group.
5. In a cladogram, which group is least closely related to the others?
(A) The outgroup
(B) The root
(C) A species with the most traits
(D) A species at the top of the diagram
Answer
(A) — The outgroup serves as a point of comparison and is least closely related to the ingroup.
6. Which of the following best defines a clade?
(A) A group of species that share a common habitat
(B) A group of organisms with similar diets
(C) An ancestor and all its descendants
(D) A family of closely related species
Answer
(C) — A clade includes a common ancestor and all species that descended from it, forming a complete branch on the tree of life.
7. Which method uses shared derived characteristics to build phylogenetic trees?
(A) Radiometric dating
(B) Cladistics
(C) Comparative embryology
(D) Molecular clock analysis
Answer
(B) — Cladistics groups organisms based on shared derived characteristics to infer evolutionary relationships.
8. What is the result of a speciation event on a phylogenetic tree?
(A) Collapse of a branch
(B) Creation of a node and new branches
(C) Deletion of ancestral data
(D) Reversal to a common trait
Answer
(B) — A speciation event results in a node on the tree, where one lineage splits into two or more new lineages.
9. Which type of speciation occurs without geographic isolation?
(A) Allopatric speciation
(B) Sympatric speciation
(C) Adaptive radiation
(D) Temporal isolation
Answer
(B) — Sympatric speciation occurs when new species arise from a single population in the same geographic area, often due to behavioral or genetic changes.
10. Which condition most directly leads to reproductive isolation?
(A) Natural selection
(B) Genetic drift
(C) Geographic barriers
(D) Random mating
Answer
(C) — Physical barriers like mountains or rivers can prevent gene flow, leading to reproductive isolation and eventually speciation.
11. What does "extinction" mean in a biological context?
(A) A species becomes rare
(B) A species migrates
(C) A species ceases to exist
(D) A species becomes dormant
Answer
(C) — Extinction occurs when the last individual of a species dies, and it no longer exists anywhere.
12. Which of the following is an example of a prezygotic barrier to reproduction?
(A) Sterile offspring
(B) Hybrid inviability
(C) Mating occurs at different times
(D) Offspring with low fitness
Answer
(C) — Temporal isolation is a prezygotic barrier because it prevents mating based on timing differences.
13. What is the primary difference between homology and analogy?
(A) Homology involves convergent evolution, analogy does not
(B) Homology refers to shared ancestry, analogy refers to similar function
(C) Analogy results from mutation, homology does not
(D) Homology is less useful in phylogenetics
Answer
(B) — Homologous traits arise from shared ancestry; analogous traits arise independently due to similar selection pressures.
14. What evidence suggests two species share a recent common ancestor?
(A) They have identical ecological roles
(B) They are found in the same environment
(C) They have many shared derived traits
(D) They are the same size
Answer
(C) — Shared derived traits suggest the species diverged recently from a common ancestor.
15. Which of the following would most likely lead to extinction?
(A) High genetic variation
(B) Sudden environmental change
(C) Stable population growth
(D) Broad ecological niche
Answer
(B) — Rapid or severe environmental changes can outpace a species’ ability to adapt, increasing the risk of extinction.
16. Which of the following is an example of postzygotic isolation?
(A) Behavioral differences during mating
(B) Offspring cannot reproduce
(C) Different mating times
(D) Incompatible genitalia
Answer
(B) — Postzygotic isolation occurs after fertilization, such as when hybrids are sterile.
17. What is adaptive radiation?
(A) Slow accumulation of mutations
(B) Species converging into a single niche
(C) Rapid diversification of species from a common ancestor
(D) One species going extinct repeatedly
Answer
(C) — Adaptive radiation involves many new species evolving rapidly to exploit different niches from a single ancestral species.
18. Why are molecular clocks useful in phylogenetics?
(A) They show exact time of extinction
(B) They help estimate divergence times between species
(C) They predict future mutations
(D) They prove fossil ages
Answer
(B) — Molecular clocks use mutation rates to estimate how long ago species diverged from a common ancestor.
19. What is a polyphyletic group?
(A) A group that includes the ancestor and all its descendants
(B) A group with one common ancestor
(C) A group missing common ancestry
(D) A group with unrelated species lumped together
Answer
(D) — A polyphyletic group consists of species that do not share an immediate common ancestor and are incorrectly grouped based on similarities.
20. Which type of speciation is driven by geographic separation?
(A) Parapatric
(B) Sympatric
(C) Allopatric
(D) Disruptive
Answer
(C) — Allopatric speciation occurs when a physical barrier divides a population, preventing gene flow.
21. Which factor contributes most to extinction vulnerability?
(A) Broad diet
(B) Small population size
(C) High genetic diversity
(D) Large geographic range
Answer
(B) — Small populations are more susceptible to random events, inbreeding, and reduced adaptability, increasing extinction risk.
22. Which branch point on a phylogenetic tree is interpreted as a speciation event?
(A) Leaf
(B) Root
(C) Node
(D) Outgroup
Answer
(C) — Nodes represent points where one lineage splits into two, indicating a speciation event.
23. What type of evolution results in similar traits in unrelated species?
(A) Divergent evolution
(B) Artificial selection
(C) Convergent evolution
(D) Stabilizing selection
Answer
(C) — Convergent evolution leads to analogous traits due to similar environmental pressures, not shared ancestry.
24. Which trait is least useful in building phylogenetic trees?
(A) DNA sequence data
(B) Convergent morphological traits
(C) Shared derived characteristics
(D) Homologous bone structure
Answer
(B) — Convergent traits evolve independently and may mislead phylogenetic analyses.
25. Which of the following is an example of behavioral isolation?
(A) Two species have different mating calls
(B) Hybrids die early in development
(C) Two species have nonviable offspring
(D) Gametes cannot fuse
Answer
(A) — Behavioral isolation prevents mating because individuals don’t recognize each other as potential mates.
26. Two species of frogs have overlapping ranges but breed at different times of year. What type of reproductive barrier is this?
(A) Behavioral isolation
(B) Temporal isolation
(C) Mechanical isolation
(D) Postzygotic isolation
Answer
(B) — Temporal isolation occurs when species reproduce at different times, preventing interbreeding.
27. What would most likely cause a phylogenetic tree to be revised?
(A) Discovery of similar coloration among species
(B) New fossil evidence contradicting relationships
(C) Increase in extinction rates
(D) Shared habitat preferences
Answer
(B) — New fossil or genetic data can change our understanding of evolutionary relationships and revise the tree.
28. Which process can result in rapid speciation, especially in plants?
(A) Mutation accumulation
(B) Sympatric speciation via polyploidy
(C) Gene flow
(D) Genetic drift in large populations
Answer
(B) — Polyploidy can lead to instant reproductive isolation and new species, particularly common in plants.
29. Which statement about extinction is most accurate?
(A) It always results from natural selection
(B) It occurs only due to human impact
(C) It is a natural part of evolution and biodiversity
(D) It prevents adaptive radiation
Answer
(C) — Extinction is a normal evolutionary process that can open niches and drive diversification.
30. Why is an outgroup included in a phylogenetic analysis?
(A) To eliminate homologous traits
(B) To confirm convergence
(C) To root the tree and identify shared derived traits
(D) To predict extinction likelihood
Answer
(C) — An outgroup provides a point of comparison to infer the direction of evolutionary change and root the tree.
31. What does the principle of parsimony suggest when building a phylogenetic tree?
(A) Choose the tree that includes the most analogies
(B) The tree with the fewest evolutionary changes is preferred
(C) Only DNA data should be used
(D) Oldest fossils should define branching points
Answer
(B) — Parsimony favors the simplest explanation with the least number of evolutionary steps.
32. What is true about hybrid zones that are stable over time?
(A) They always lead to extinction
(B) They result in increased reproductive isolation
(C) They allow gene flow between species
(D) They collapse rapidly
Answer
(C) — Stable hybrid zones allow some gene flow between species without leading to complete speciation or fusion.
33. Which condition would most likely favor sympatric speciation in animals?
(A) A physical barrier divides the population
(B) New mutation allows reproductive isolation within the same habitat
(C) Two populations migrate together
(D) Seasonal migration patterns emerge
Answer
(B) — Sympatric speciation often requires mutations or behaviors that isolate individuals reproductively in the same location.
34. Why might species with low genetic variation face higher extinction risk?
(A) They evolve more rapidly
(B) They adapt better to new environments
(C) They are more vulnerable to environmental change
(D) They attract more predators
Answer
(C) — Low genetic diversity limits a species’ ability to adapt, increasing vulnerability to diseases and environmental shifts.
35. What does it mean when two species are sister taxa in a phylogenetic tree?
(A) They diverged from the same immediate ancestor
(B) They have identical DNA
(C) They are the most distant relatives
(D) They are polyphyletic
Answer
(A) — Sister taxa share a most recent common ancestor and are each other’s closest relatives on the tree.
36. Explain how a molecular clock can be used to estimate the time since two species diverged.
Answer
Molecular clocks estimate divergence times based on the rate of genetic mutations. By comparing DNA differences and knowing the mutation rate, scientists can approximate how long ago two species shared a common ancestor.
37. Describe one way in which convergent evolution can complicate phylogenetic analysis.
Answer
Convergent evolution leads to similar traits in unrelated species, which can be mistaken for homology, potentially misleading phylogenetic trees if not correctly identified as analogy.
38. What evidence suggests that two species are more closely related to each other than to a third species?
Answer
They share more derived traits and may have fewer genetic differences compared to the third species, indicating a more recent common ancestor.
39. How can reproductive isolation lead to speciation?
Answer
Reproductive isolation prevents gene flow between populations. Over time, genetic divergence accumulates, leading to the formation of new species.
40. Give an example of a postzygotic reproductive barrier and briefly explain it.
Answer
Hybrid sterility is a postzygotic barrier. For example, a mule (offspring of a horse and donkey) is sterile and cannot reproduce, preventing gene flow between species.
41. What is the difference between a monophyletic group and a polyphyletic group?
Answer
A monophyletic group includes a common ancestor and all its descendants, while a polyphyletic group includes species without a shared recent ancestor, often grouped by superficial traits.
42. Why is adaptive radiation common after mass extinction events?
Answer
Mass extinctions leave many ecological niches unoccupied. Surviving species diversify rapidly to exploit these open niches, leading to adaptive radiation.
43. Describe one factor that contributes to the stability of a hybrid zone.
Answer
When hybrids are viable and fertile but not as fit as parent species, limited gene flow continues while species remain distinct, maintaining a stable hybrid zone.
44. How does geographic isolation promote allopatric speciation?
Answer
Geographic isolation blocks gene flow between populations. Independent mutations and selection pressures in each area lead to reproductive divergence and eventually speciation.
45. What role does extinction play in shaping biodiversity?
Answer
Extinction removes species, reducing biodiversity, but it can also create opportunities for other species to evolve and diversify, especially after mass extinctions.
46. How can DNA sequence comparisons improve phylogenetic trees?
Answer
DNA sequences provide detailed information on genetic similarities and differences, allowing more accurate reconstructions of evolutionary relationships than morphology alone.
47. Why are behavioral differences considered prezygotic reproductive barriers?
Answer
Behavioral differences, such as mating calls or courtship rituals, prevent mating between species even if they live in the same area, stopping fertilization from occurring.
48. What is the significance of a node on a phylogenetic tree?
Answer
A node represents the most recent common ancestor of the descendant species that branch from it, marking a speciation event.
49. Explain how parsimony is used in choosing the best phylogenetic tree.
Answer
Parsimony favors the tree that requires the fewest evolutionary changes, assuming that simpler explanations are more likely to be correct.
50. Describe one limitation of using only fossil records to construct phylogenies.
Answer
Fossil records are often incomplete or biased toward hard-bodied organisms, which may lead to inaccurate assumptions about evolutionary relationships without molecular data.