Common Ancestry - AP Biology

1. Biologists examine the amino acid sequence of the protein cytochrome c in four vertebrates. Compared with species M, species N differs by 2 amino acids, species O differs by 11, and species P differs by 14. The protein performs the same cellular role in all four species. Which inference is best supported by these data about common ancestry among the lineages?
   Answer: Species M and N share a more recent common ancestor lineage than species M shares with O or P. This question assesses the skill of inferring common ancestry from protein sequence differences in cytochrome c among vertebrates. The minimal differences between species M and N (2 amino acids) compared to M-O (11) and M-P (14) suggest M and N share a more recent common ancestor, as fewer changes have accumulated since their divergence. The conserved function of cytochrome c across species indicates that sequence similarities reflect inheritance from a shared lineage rather than convergence. This pattern allows inference of relative divergence times, with greater similarity pointing to closer relatedness. A tempting distractor is choice C, which assumes equal relatedness due to shared function, but this overlooks how sequence divergence quantifies ancestry and misapplies functional conservation to relatedness. To infer common ancestry from molecular data, compare sequence similarities quantitatively, remembering that fewer differences typically indicate more recent shared ancestry across lineages.
2. Researchers identify a shared insertion of a transposable element at the same genomic location in Species M and Species N. Species P lacks the insertion and instead has the ancestral sequence at that site. The insertion boundaries and flanking DNA sequences match in M and N, indicating the same insertion event. No other genetic markers are provided. Which inference is best supported by these data?
   Answer: Species M and N share a more recent common ancestor with each other than either shares with Species P. This question tests the skill of inferring common ancestry from shared genomic features. The identical transposable element insertion at the same site in Species M and N, absent in P, suggests M and N inherited this from a common ancestor after diverging from P's lineage. This shared-trait reasoning treats the insertion as a rare, derived event marking a specific clade, with matching boundaries confirming a single origin. Thus, the data support M and N sharing a more recent common ancestor than with P. A tempting distractor is choice C, which attributes the similarity to independent events, a misconception of convergence ignoring the improbability of identical insertions. To infer common ancestry from genomes, identify unique shared mutations like insertions as evidence of descent from a common ancestor post-event.
3. Researchers studied a pseudogene (a nonfunctional copy of a gene) in three carnivore lineages: seals, bears, and wolves. Seals and bears share the same premature stop codon at an identical nucleotide position in the pseudogene, while wolves have a different stop codon at a different position. The surrounding sequences align across all three lineages. Because the pseudogene is nonfunctional, the shared stop codon is unlikely to be maintained by selection. Which inference is best supported by these data?
   Answer: Seals and bears share a more recent common ancestor with each other than either shares with wolves. This question tests inferring common ancestry from shared mutations in nonfunctional DNA. Seals and bears sharing the same premature stop codon at an identical position in a pseudogene, while wolves have a different stop codon elsewhere, indicates seals and bears share a more recent common ancestor with each other than either shares with wolves. Since pseudogenes are nonfunctional, this shared mutation cannot be maintained by selection and must represent a single mutational event in their common ancestor. Choice E incorrectly invokes convergent evolution for a neutral trait, ignoring that selection cannot maintain specific mutations in nonfunctional sequences. When analyzing pseudogenes or other neutral sequences, shared mutations at identical positions provide strong evidence for recent common ancestry.
4. A paleontologist compared skulls of a modern horse and a fossil mammal found in older rock layers. Both skulls share the same distinctive arrangement of molars: three cusps in a specific triangular pattern and an identical groove between the second and third cusp. Several other mammals in the same deposits lack this cusp pattern. The fossil skull has smaller teeth and a shorter face, but the cusp pattern and groove occur in the same relative positions. Which inference is best supported by these observations?
   Answer: The modern horse lineage and the fossil lineage share a common ancestor with that distinctive molar pattern. This question requires inferring common ancestry from shared fossil and modern anatomical features. The identical molar cusp pattern (three cusps in a specific triangular arrangement with a groove between the second and third cusp) shared by the modern horse and fossil mammal indicates they inherited this complex trait from a common ancestor, especially since other mammals in the same deposits lack this pattern. The specific structural details make independent evolution unlikely. Choice B incorrectly assumes the fossil is a direct ancestor simply because it's older, confusing shared ancestry with direct lineage. When comparing fossil and modern forms, recognize that shared complex anatomical features indicate common ancestry rather than direct ancestor-descendant relationships.
5. A botanist compares chloroplast DNA sequences from three plant species. Two flowering plant species (R and S) share 99.2% sequence identity across a chloroplast region, while a moss species (T) shares 93.5% identity with R and 93.4% with S. The region is present in all three species. Which inference is best supported by these data about common ancestry?
   Answer: Species R and S share a more recent common ancestor lineage than either shares with species T. This question assesses the skill of inferring common ancestry from chloroplast DNA sequence similarities in plants. The high sequence identity (99.2%) between flowering plants R and S, compared to their lower identity (~93%) with moss T, indicates that R and S share a more recent common ancestor than either does with T. This pattern suggests that mutations accumulated more in the lineages leading to T after divergence, while R and S retained greater similarity due to closer relatedness. The presence of the region in all three supports inheritance from a shared photosynthetic ancestor, with degrees of similarity revealing branching order. A tempting distractor is choice E, which attributes similarities to independent evolution from environments, but this convergent misconception overlooks the improbability of such high identity arising separately. To infer common ancestry from organelle DNA, calculate sequence identities and interpret higher values as signs of more recent divergence, aiding in reconstructing plant phylogenies.
6. In a comparative anatomy lab, students observe that the forelimbs of humans, cats, whales, and bats each contain a humerus, radius, ulna, carpals, metacarpals, and phalanges arranged in the same relative order, although the bones differ in length and thickness. Embryological observations show that these limb bones develop from similar tissue buds in early embryos of all four species. Fossils of early tetrapods show a similar limb-bone pattern. Which inference is best supported by these observations about evolutionary relationships?
   Answer: The four species inherited the forelimb bone pattern from a common tetrapod ancestor lineage. This question assesses the skill of inferring common ancestry from anatomical, embryological, and fossil evidence. The shared forelimb bone pattern among humans, cats, whales, and bats suggests these species inherited the trait from a common tetrapod ancestor, as the identical arrangement indicates homology rather than independent evolution. Embryological similarities in tissue buds further support that the limbs develop from conserved developmental pathways inherited from a shared lineage. Fossil evidence of early tetrapods with the same pattern reinforces that this trait was present in their common ancestor and modified over time in descendant lineages. A tempting distractor is choice B, which attributes the similarities to convergent evolution due to similar habitats, but this ignores the homologous bone arrangement and embryological data that point to inheritance rather than independent origins. To infer common ancestry effectively, look for homologous structures supported by multiple lines of evidence like fossils and development, as greater similarity in complex traits often indicates shared evolutionary history.
7. Researchers sequence a 900-base-pair region of a mitochondrial gene from three species: Species X and Y differ at 9 sites, Species X and Z differ at 90 sites, and Species Y and Z differ at 92 sites. The gene region is confirmed to be orthologous in all three species. No other traits are considered. Which inference is best supported by these sequence comparisons about evolutionary relationships?
   Answer: Species X and Y share a more recent common ancestor than either shares with Species Z. This question tests the skill of inferring common ancestry from genetic sequence similarities. The fewer differences between Species X and Y (9 sites) compared to X-Z (90) and Y-Z (92) indicate that X and Y diverged more recently, sharing a common ancestor after their lineages split from Z. This shared-trait reasoning relies on the principle that orthologous genes accumulate mutations over time, so closer relatives have more similar sequences due to less divergence time. Thus, the data support X and Y having a more recent common ancestor, as their sequences are most alike. A tempting distractor is choice E, which misinterprets greater differences as evidence of closer relation, a misconception of equating dissimilarity with relatedness instead of similarity. To infer common ancestry from sequences, calculate pairwise differences and interpret smaller values as indicating more recent shared ancestry.
8. Researchers compared an intron sequence from the same nuclear gene in three primates. Humans and chimpanzees share an identical 120-base-pair insertion at the same position in the intron, while gorillas lack the insertion and instead have a different 14-base-pair deletion at that location. The surrounding intron sequence aligns among all three lineages. Because intron changes are typically selectively neutral, shared insertions at identical positions are unlikely to occur independently. Which inference is best supported by these data?
   Answer: Humans and chimpanzees share a more recent common ancestor with each other than either shares with gorillas. This question tests inferring common ancestry from shared genetic insertions in neutral DNA regions. The identical 120-base-pair insertion at the same position in both human and chimpanzee introns, absent in gorillas, indicates humans and chimpanzees share a more recent common ancestor with each other than either shares with gorillas. Since introns are selectively neutral, this shared insertion is extremely unlikely to have arisen independently and instead represents a single insertion event in their common ancestor. Choice C incorrectly interprets the insertion as an ancestral trait that humans lost, misunderstanding that both species share the derived state. When analyzing neutral genetic markers like intron insertions, shared derived traits at identical positions provide strong evidence for recent common ancestry.
9. A developmental biologist compared early embryo stages of a chicken and a lizard. In both lineages, embryos form a transient pharyngeal-arch pattern and a segmented tail bud, and the same set of regulatory genes is expressed in similar spatial regions during these stages. Later development diverges, producing distinct adult morphologies. The shared embryonic structures and gene-expression pattern appear before major lineage-specific traits form. Which inference is best supported by these observations?
   Answer: Chickens and lizards share a common amniote ancestor that had similar early developmental patterning. This question requires inferring common ancestry from shared embryonic features and gene expression patterns. The presence of pharyngeal arches, segmented tail buds, and similar regulatory gene expression in both chicken and lizard embryos indicates these features were inherited from their common amniote ancestor, even though adult forms differ significantly. These complex developmental similarities are best explained by common descent rather than convergence. Choice B incorrectly assumes one lineage evolved from the other based on embryonic similarities, confusing shared ancestry with direct descent. When comparing developmental patterns, recognize that shared embryonic features and gene expression indicate inheritance from a common ancestor possessing those developmental programs.
10. In a phylogenetic study, researchers compared a 900–base-pair region of a developmental gene in five vertebrates. The frog and salamander sequences differ at 18 sites, the lizard and mouse differ at 21 sites, and the frog and lizard differ at 79 sites. The salamander and lizard differ at 82 sites, and the mouse differs from the frog at 85 sites and from the salamander at 87 sites. All sequences align without gaps, indicating the same gene region is present in each lineage. Which inference is best supported by the sequence differences?
    Answer: Frogs and salamanders share a more recent common ancestor than either shares with lizards or mice. This question tests your ability to infer common ancestry from sequence differences between species. The key is recognizing that fewer sequence differences indicate more recent common ancestry - frog and salamander differ at only 18 sites, while frog-lizard (79 sites) and frog-mouse (85 sites) show much greater divergence. This pattern indicates frogs and salamanders share a more recent common ancestor than either shares with lizards or mice, supporting their grouping as amphibians. Choice C incorrectly assumes convergent evolution without evidence, ignoring that sequence similarity is better explained by shared ancestry. When comparing sequences, always use the principle that fewer differences indicate more recent divergence from a common ancestor.
11. Biologists examined forelimb skeletons from a bat, whale, cat, and lizard. Each forelimb contains a humerus, radius, ulna, carpals, metacarpals, and phalanges arranged in the same relative order, though their sizes and shapes differ among lineages. The forelimbs perform different functions (flight, swimming, running, and walking). Which inference is best supported by the shared bone arrangement across these vertebrate lineages?
    Answer: The lineages share a common tetrapod ancestor that had the same basic forelimb bone pattern. This question requires inferring common ancestry from shared anatomical structures across diverse vertebrates. The identical bone arrangement (humerus, radius, ulna, carpals, metacarpals, phalanges) in the same relative order across bat, whale, cat, and lizard forelimbs indicates inheritance from a common tetrapod ancestor, despite their different functions today. This homologous structure pattern is strong evidence for common descent rather than independent evolution. Choice B incorrectly assumes convergent evolution would produce identical bone arrangements, when convergence typically produces similar functions through different structures. When examining shared anatomical features, recognize that complex structural similarities in the same relative positions indicate common ancestry rather than independent origins.
12. In two insect lineages, researchers found the same transposable element inserted at the same genomic location within a noncoding region. A third insect lineage lacks the element and shows an uninterrupted sequence at that site. The flanking DNA sequences align among all three lineages, indicating the same locus was compared. Insertions of transposable elements at identical positions are rare events. Which inference is best supported by this evidence?
    Answer: The two lineages sharing the insertion have a more recent common ancestor than either does with the third lineage. This question tests inferring common ancestry from rare genomic events like transposable element insertions. Two insect lineages sharing the same transposable element at an identical genomic location, while a third lineage lacks it, indicates the two lineages with the insertion share a more recent common ancestor than either does with the third lineage. Identical insertions are extremely rare events unlikely to occur independently, making shared ancestry the best explanation. Choice D incorrectly invokes convergent evolution for a molecular marker that has no functional significance, ignoring the extreme improbability of independent identical insertions. When analyzing rare genomic events like transposon insertions, recognize that sharing such events at identical positions provides strong evidence for recent common ancestry.
13. Researchers examine the arrangement of bones in the middle ear of mammals and the jaw of certain reptiles. Mammals have three middle ear bones (malleus, incus, stapes). In reptiles, two jaw bones (articular and quadrate) occupy positions and have shapes that correspond to the malleus and incus in mammals. Developmental studies show that mammalian malleus and incus arise from embryonic tissues that also form the reptilian articular and quadrate. Which inference is best supported by these observations?
    Answer: Mammal and reptile lineages share common ancestry in which homologous bones were modified over time. This question assesses the skill of inferring common ancestry from homologous bone arrangements in mammal ears and reptile jaws. The correspondence between mammalian middle ear bones (malleus, incus) and reptilian jaw bones (articular, quadrate), supported by developmental studies showing shared embryonic origins, indicates inheritance and modification from a common ancestor. This homology suggests that mammal and reptile lineages diverged from a shared tetrapod ancestor, with bones repurposed for hearing in mammals over evolutionary time. The positional and shape similarities further reinforce that these are not independent structures but evolved from the same ancestral bones. A tempting distractor is choice D, which claims independent formation due to environments, but this ignores developmental and positional evidence, embodying a convergence misconception. To infer common ancestry from homologous structures, trace developmental and fossil origins, using them to map trait modifications across related lineages.
14. A botanist studies flowers of two plant species that both attract hummingbirds. Despite similar red coloration and tubular shape, dissections show that the arrangement and number of stamens, the ovary position, and the pattern of vascular bundles in the petals are highly similar between the two species and match those of several other species in the same genus. No pollinator data are used beyond hummingbird visitation. Which inference is best supported by the internal structural similarities?
    Answer: The two species share a recent common ancestor within their genus, indicated by shared floral structural patterns. This question tests the skill of inferring common ancestry from shared morphological traits. The matching internal flower structures, such as stamen arrangement, ovary position, and vascular patterns, between the two species suggest inheritance from a common ancestor within their genus. This shared-trait reasoning distinguishes these homologous features from potentially convergent external traits like color and shape driven by pollination. Therefore, the similarities support a recent common ancestor, as they align with patterns in related species. A tempting distractor is choice E, which attributes similarities solely to adaptation for pollinators, a misconception of overemphasizing convergence while ignoring structural homology. To infer common ancestry in plants, dissect and compare internal anatomies to identify inherited traits beyond superficial adaptations.
15. A paleontologist finds a series of fossils from sediment layers of increasing age. Younger fossils show a mammal with a single jawbone and three small middle-ear bones. Older fossils show similar skulls but with two small bones attached to the jaw joint that gradually appear smaller and more ear-associated in intermediate layers. The fossils occur in a continuous stratigraphic sequence at one site. Which inference is best supported by this pattern?
    Answer: The sequence supports a lineage in which jaw bones in ancestral forms correspond to middle-ear bones in later descendants. This question tests the skill of inferring common ancestry from fossil patterns. The transitional fossils showing jaw bones gradually becoming middle-ear bones suggest a lineage where ancestral reptiles gave rise to mammals, with bones repurposed over time. This shared-trait reasoning is based on the homology between jaw elements in older fossils and ear bones in younger ones, indicating inheritance and modification from a common ancestor. The continuous stratigraphic sequence supports an evolutionary progression within a single lineage, linking the forms through common descent. A tempting distractor is choice C, which incorrectly implies traits evolve in response to needs, a misconception of Lamarckian inheritance rather than natural selection on variations. To infer common ancestry from fossils, examine transitional series for homologous structures that show gradual changes across layers.
16. Biologists compare a gene involved in oxygen transport across vertebrates. In humans and chimpanzees, the coding sequence differs by 1 nucleotide; in humans and gorillas it differs by 6 nucleotides; in humans and orangutans it differs by 18 nucleotides. The gene is confirmed to be the same locus in all species. Which inference is best supported by these data?
    Answer: Humans and chimpanzees share a more recent common ancestor than humans and gorillas do. This question tests the skill of inferring common ancestry from genetic differences. The minimal nucleotide differences between humans and chimpanzees (1) compared to humans-gorillas (6) and humans-orangutans (18) indicate humans and chimpanzees share a more recent common ancestor. This shared-trait reasoning uses the molecular clock concept, where fewer mutations accumulate between closely related species due to shorter divergence time. Thus, the data support closer relatedness between humans and chimpanzees, with gorillas branching off earlier. A tempting distractor is choice A, which misreads greater differences as indicating recency, a misconception that reverses the logic of similarity equating to closer ancestry. To infer common ancestry from genes, compare sequence divergences and associate smaller differences with more recent shared ancestors.
17. In a comparative anatomy lab, students examine the forelimbs of a bat, whale, cat, and human. Each limb contains a humerus, radius, ulna, carpals, metacarpals, and phalanges arranged in the same relative order, but the bones differ in length and thickness among species. The students note that these limbs are used for flight, swimming, running, and grasping, respectively. No information is provided about embryology or DNA. Which inference is best supported by the bone arrangement described?
    Answer: The four species share a common ancestor with the same basic forelimb bone pattern. This question tests the skill of inferring common ancestry from shared anatomical traits. The similar arrangement of bones in the forelimbs of bats, whales, cats, and humans suggests these structures are homologous, inherited from a common ancestor that possessed the same basic bone pattern. Despite differences in bone proportions and limb functions, the consistent relative order of humerus, radius, ulna, carpals, metacarpals, and phalanges indicates divergence from a shared ancestral form rather than independent origins. This shared-trait reasoning supports that the four species evolved from a common ancestor with this forelimb blueprint, which was later modified for different uses. A tempting distractor is choice B, which incorrectly attributes the similarities to convergent evolution, a misconception that confuses homology with analogy based on function alone. To infer common ancestry in similar scenarios, compare structural patterns while considering if they reflect inheritance rather than environmental convergence.
18. A botanist compared flower structure in five plant lineages. Two lineages (X and Y) both have petals fused into a single tubular corolla and share the same unusual arrangement of vascular bundles supplying the fused petals. The other three lineages have separate petals and lack that vascular-bundle arrangement. Genetic analysis of a chloroplast gene shows X and Y differ by 3 substitutions, while each differs from the other three lineages by more than 40 substitutions. Which inference is best supported by the combined evidence?
    Answer: Lineages X and Y share a more recent common ancestor with each other than either shares with the other lineages. This question requires inferring common ancestry from multiple lines of evidence - morphology and molecular data. Lineages X and Y share both fused petals with an unusual vascular bundle arrangement and differ by only 3 chloroplast substitutions, while differing from other lineages by over 40 substitutions, strongly supporting that X and Y share a more recent common ancestor. The combination of shared complex morphology and low genetic divergence makes independent evolution extremely unlikely. Choice B incorrectly assumes convergent evolution despite the genetic evidence, failing to integrate both data types. When multiple independent lines of evidence (morphological and molecular) point to the same relationship, this provides particularly strong support for common ancestry.
19. Scientists compare the arrangement of Hox genes in two animal species. Species A and species B each have a cluster of Hox genes in the same order along the chromosome, and several corresponding genes share highly similar DNA sequences. A third species, C, has a Hox cluster with a different gene order and lower sequence similarity to A and B. Which inference is best supported by these observations about ancestry?
    Answer: Species A and B share a more recent common ancestor lineage than either shares with species C. This question assesses the skill of inferring common ancestry from the arrangement and sequence similarity of Hox genes in animals. The identical gene order and high sequence similarity between species A and B indicate they inherited their Hox clusters from a more recent common ancestor than either shares with species C, which has a different order and lower similarity. This conservation of complex gene arrangements suggests shared developmental pathways passed down from a common lineage, with divergence leading to the differences seen in C. The pattern reflects evolutionary history, where closer relatives retain more similar genetic architectures. A tempting distractor is choice D, which posits independent evolution of similar clusters, but this underestimates the improbability of converging on identical complex arrangements and ignores inheritance as the simpler explanation. To infer common ancestry from genetic data, compare both sequence identity and structural organization, using higher similarity to identify more recent shared lineages.
20. A comparative anatomist examined the middle-ear region of a mouse, dog, and lizard. Both mammals have three small middle-ear bones (malleus, incus, stapes) with the malleus and incus articulating in the same distinctive joint shape, while the lizard has a single middle-ear bone and lacks that joint. The mammal and lizard skulls otherwise share many vertebrate features. Which inference is best supported by the middle-ear evidence?
    Answer: Mice and dogs share a more recent common ancestor with each other than either shares with lizards. This question requires inferring common ancestry from shared derived anatomical features. Mice and dogs both possess three middle-ear bones with a distinctive malleus-incus joint, while lizards have only one middle-ear bone and lack this joint, indicating mice and dogs (both mammals) share a more recent common ancestor with each other than either shares with lizards. The three-bone middle ear is a derived mammalian trait inherited from their common ancestor. Choice D incorrectly assumes convergent evolution of this complex anatomical system, ignoring that such detailed structural similarities are better explained by common descent. When comparing anatomical features, recognize that shared derived traits (like the mammalian middle ear) indicate more recent common ancestry than shared primitive traits.

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