Common Ancestry
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AP Biology › Common Ancestry
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?
The regulatory genes are identical in function, so the two lineages must be equally related to all vertebrates.
Chickens and lizards share a common amniote ancestor that had similar early developmental patterning.
Chickens evolved from lizards because chicken embryos pass through a lizardlike stage.
The shared embryonic traits show that adult chickens and adult lizards are more similar than either is to other reptiles.
The shared embryonic traits arose independently because embryos develop similarly in all environments.
Explanation
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.
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?
The modern horse is more closely related to all mammals with small teeth than to mammals with large teeth.
The modern horse lineage and the fossil lineage share a common ancestor with that distinctive molar pattern.
The molar pattern evolved independently in multiple mammals because triangular cusps are universally optimal.
The fossil lineage is more closely related to mammals lacking the cusp pattern because its face is shorter.
The fossil is a direct ancestor of the modern horse because it occurs in older rock layers.
Explanation
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.
Scientists sequenced a mitochondrial gene from four bird lineages: hawk, eagle, sparrow, and penguin. The hawk and eagle sequences differ at 6 sites, while hawk and sparrow differ at 44 sites and eagle and sparrow differ at 46 sites. Penguin differs from hawk at 62 sites and from eagle at 63 sites, and differs from sparrow at 60 sites. All sequences are the same length and align across the full gene. Which inference is best supported by these sequence comparisons?
Penguins and sparrows are the closest relatives because both differ from raptors by many sites.
Sparrows are ancestral to hawks because sparrows have more sequence differences from penguins.
Hawks and eagles share a more recent common ancestor with each other than either shares with sparrows or penguins.
Hawks and penguins independently evolved similar mitochondrial sequences due to similar body size.
All four bird lineages are equally related because they share a mitochondrial gene of the same length.
Explanation
This question tests inferring common ancestry from mitochondrial sequence differences among bird species. Hawks and eagles show only 6 sequence differences, while both differ from sparrows by 44-46 sites and from penguins by 62-63 sites, indicating hawks and eagles share a more recent common ancestor with each other than either shares with sparrows or penguins. This pattern reflects the evolutionary relationships among these bird groups, with raptors (hawks and eagles) forming a distinct clade. Choice B incorrectly groups penguins and sparrows based on their similar distances from raptors, missing that they differ from each other by 60 sites. When analyzing sequence data, focus on pairwise differences between species - smaller differences indicate more recent common ancestry.
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?
The limbs have similar functions, so the species are more closely related than other mammals.
The bone similarities show that the species recently exchanged genes through hybridization.
The four species share a common ancestor with the same basic forelimb bone pattern.
The limbs became similar because the environment caused the same bones to form in each lineage.
The four species independently evolved identical bones because they all need to move.
Explanation
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.
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?
The similar bones formed independently because similar environments produce the same bone shapes.
Mammal and reptile lineages share common ancestry in which homologous bones were modified over time.
Mammals acquired their ear bones from reptiles through horizontal gene transfer between lineages.
Mammals and reptiles are unrelated because the bones serve different functions in each lineage.
Reptiles descended from mammals because mammals have a greater number of ear bones.
Explanation
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.
A comparative study examines the pelvic bones of modern whales and terrestrial mammals. Whales have small internal pelvic bones that are not connected to hindlimbs, while a terrestrial mammal species has a pelvis attached to functional hindlimbs. Fossils of early whale ancestors show progressively reduced hindlimbs while retaining pelvic structures similar in position and composition to those of terrestrial mammals. Which inference is best supported by these observations?
Whale pelvic bones are analogous to mammal pelvises because they arose from different ancestral structures.
Terrestrial mammals evolved from whales because whale fossils show reduced hindlimbs in younger rocks.
Modern whales share common ancestry with terrestrial mammal lineages that possessed hindlimbs and a pelvis.
Whales acquired pelvic bones through hybridization with terrestrial mammals after whales became fully aquatic.
Whale pelvic bones persist because they are required for walking on land during reproduction.
Explanation
This question assesses the skill of inferring common ancestry from vestigial structures and fossil evidence in whales. The small, internal pelvic bones in modern whales, homologous to those attached to hindlimbs in terrestrial mammals, indicate inheritance from a common ancestor with functional hindlimbs that were reduced in aquatic lineages. Fossil sequences showing progressive hindlimb reduction while retaining pelvic structures support that whales share ancestry with terrestrial mammals, with the trait becoming vestigial over time. This homology, combined with positional and compositional similarities, points to a shared evolutionary origin rather than convergence. A tempting distractor is choice C, which labels the structures analogous, but this misconception dismisses the fossil and structural evidence of shared ancestry and homologous development. To infer common ancestry from vestigial traits, examine fossil transitions and compare to functional homologs in relatives, using them to trace evolutionary modifications.
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?
Species R and S evolved similar chloroplast sequences independently due to similar light environments.
Species T is a direct ancestor of R and S because its sequence identity is lower.
Species R and S share a more recent common ancestor lineage than either shares with species T.
Species T shares the most recent common ancestor with species R because both have chloroplast DNA.
All three species are equally related because chloroplasts perform photosynthesis in each lineage.
Explanation
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.
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?
Species A and B share a more recent common ancestor lineage than either shares with species C.
Species A and B evolved similar Hox clusters independently because they develop similar body plans.
Species A and C share the most recent common ancestor because both possess Hox genes.
Species C is ancestral to A and B because its Hox cluster is arranged differently.
All three species diverged at the same time because Hox genes are conserved in animals.
Explanation
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.
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?
The four species independently evolved identical limb patterns because they occupy similar habitats.
The limb similarities result from recent gene flow among the four species after they diverged.
The four species are most closely related to birds because both groups have forelimbs used for movement.
The four species inherited the forelimb bone pattern from a common tetrapod ancestor lineage.
The four species share limb bones because natural selection produces the same structures in all animals.
Explanation
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.
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?
Species M and N share a more recent common ancestor lineage than species M shares with O or P.
Species N and O are most closely related because their sequences must be identical to function.
Species P is most closely related to M because larger differences indicate earlier divergence.
Species O is the direct ancestor of species P because their sequences are most different from M.
All four species are equally related because cytochrome c has the same function in each.
Explanation
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.