Anole lizard species that occur together (sympatrically) on certain Caribbean islands occupy different habitats: some live only in the grass,...
GMAT Reading Comprehension : (RC) Questions
Anole lizard species that occur together (sympatrically) on certain Caribbean islands occupy different habitats: some live only in the grass, some only on tree trunks, and some only on twigs. These species also differ morphologically: grass dwellers are slender with long tails, tree dwellers are stocky with long legs, twig dwellers are slender but stubby-legged. What is striking about these lizards is not that coexisting species differ in morphology and habitat use (such differences are common among closely related sympatric species), but that the same three types of habitat specialists occur on each of four islands: Puerto Rico, Cuba, Hispaniola, and Jamaica. Moreover, the Puerto Rican twig species closely resembles the twig species of Cuba, Hispaniola, and Jamaica in morphology, habitat use, and behavior. Likewise, the specialists for other habitats are similar across the islands.
The presence of similar species on different islands could be variously explained. An ancestral species might have adapted to exploit a particular ecological niche on one island and then traveled over water to colonize other islands. Or this ancestral species might have evolved at a time when the islands were connected, which some of these islands may once have been. After the islands separated, the isolated lizard populations would have become distinct species while also retaining their ancestors' niche adaptations. Both of these scenarios imply that specialization to each niche occurred only once. Alternatively, each specialist could have arisen independently on each of the islands.
If each type of specialist evolved just once, then similar specialists on different islands would be closely related. Conversely, if the specialists evolved independently on each island, then a specialist on one island would be more closely related to other types of anoles on the same island—regardless of their ecological niches—than it would be to a similar specialist on a different island.
Biologists can infer how species are related evolutionarily by comparing DNA sequences for the same genes in different species. Species with similar DNA sequences for these genes are generally more closely related to each other than to species with less-similar DNA sequences. DNA evidence concerning the anoles led researchers to conclude that habitat specialists on one island are not closely related to the same habitat specialists elsewhere, indicating that specialists evolved independently on each island.
The primary purpose of the passage is to
1. Passage Analysis:
Progressive Passage Analysis
| Text from Passage | Analysis |
|---|---|
| Anole lizard species that occur together (sympatrically) on certain Caribbean islands occupy different habitats: some live only in the grass, some only on tree trunks, and some only on twigs. | What it says: Different types of lizards live in different places on Caribbean islands - some in grass, some on tree trunks, some on twigs. What it does: Introduces the main subject and establishes a basic pattern of habitat separation Source/Type: Scientific fact/observation Connection to Previous Sentences: This is our starting point - no previous information to connect to Visualization: Caribbean Island: • Grass areas → Lizard Type A • Tree trunks → Lizard Type B • Twigs → Lizard Type C What We Know So Far: Lizards divide up living spaces on islands What We Don't Know Yet: Why this matters, how many islands, what the lizards look like Reading Strategy Insight: The colon and examples make this immediately concrete and understandable |
| These species also differ morphologically: grass dwellers are slender with long tails, tree dwellers are stocky with long legs, twig dwellers are slender but stubby-legged. | What it says: The lizards that live in different places also look different - each type has a body shape that fits where they live What it does: Adds physical descriptions to match the habitat categories from sentence 1 Source/Type: Scientific observation Connection to Previous Sentences: This directly builds on sentence 1 by describing the same three lizard types, but adding their physical appearance Visualization: • Grass lizards → slender + long tails (good for moving through grass) • Tree trunk lizards → stocky + long legs (good for climbing) • Twig lizards → slender + stubby legs (good for balancing on thin branches) What We Know So Far: 3 lizard types live in 3 different places and have matching body shapes What We Don't Know Yet: Why this pattern is significant Reading Strategy Insight: The parallel structure (grass dwellers/tree dwellers/twig dwellers) makes this easy to follow - same categories, just adding detail |
2. Passage Summary:
Author's Purpose:
To explain how scientists solved a puzzle about why the same types of lizards appear on different Caribbean islands by testing competing theories through DNA analysis.
Summary of Passage Structure:
In this passage, the author walks us through a scientific investigation that moves from observation to theory to resolution:
- First, the author describes an interesting pattern - the same three types of habitat-specialized lizards (grass, tree trunk, and twig dwellers) appear on four different Caribbean islands, and each type looks remarkably similar across islands.
- Next, the author presents three competing theories to explain this pattern: lizards spread from one island to others, lizards evolved when islands were connected, or lizards evolved independently on each island.
- Then, the author explains how these theories make different predictions about which lizards should be closely related, and introduces DNA analysis as the method to test these predictions.
- Finally, the author reveals that DNA evidence supported the independent evolution theory - lizards on the same island are more closely related to each other than to similar specialists on other islands.
Main Point:
DNA evidence shows that the same types of lizard specialists evolved independently on each Caribbean island, rather than spreading from island to island or evolving once when the islands were connected.
Why It's Wrong:
• The passage does describe lizard features, but only as setup for the main scientific investigation
• The "unusual features" are mentioned in the first paragraph but aren't the focus - they're the puzzle that needs explaining
• The bulk of the passage (sentences 6-16) focuses on theories and DNA evidence, not on describing lizard characteristics
Common Student Mistakes:
- Did you focus too much on the opening descriptions of lizard habitats and morphology?
→ Remember that opening details often serve as setup for the main purpose - Are you treating the lizard descriptions as the main point rather than the puzzle to be solved?
→ Look at where the passage spends most of its time - on competing theories and DNA evidence
Why It's Wrong:
• The passage focuses on habitat specialization and morphology, not behavior patterns
• While behavior is mentioned briefly, it's not the central focus of the scientific investigation
• The theories and DNA evidence address evolutionary relationships, not behavioral explanations
Common Student Mistakes:
- Did you confuse habitat use with behavior?
→ The passage is about where lizards live and their body shapes, not how they behave - Are you focusing on a minor detail mentioned only briefly?
→ Scan the passage again - behavior is mentioned once while habitat specialization dominates
Why It's Wrong:
• The passage doesn't contrast two types of evidence - it presents morphological observations first, then DNA evidence as the resolution
• There's no "particular hypothesis" being supported by contrasting evidence types
• The DNA evidence doesn't support a hypothesis - it resolves the competition between multiple hypotheses
Common Student Mistakes:
- Are you seeing morphological similarity and DNA evidence as contrasting types of evidence?
→ Actually, morphological evidence created the puzzle, and DNA evidence solved it - they work together, not in contrast - Did you miss that DNA evidence actually rejected some hypotheses rather than supporting one particular hypothesis?
→ The DNA evidence eliminated theories 1 and 2, supporting theory 3 by process of elimination
Why It's Right:
• The passage follows the complete arc of a scientific investigation from puzzle to resolution
• The author systematically presents the question, competing theories, methodology, and conclusion
• The entire structure builds toward showing how DNA evidence resolved the theoretical debate
Key Evidence: "DNA evidence concerning the anoles led researchers to conclude that habitat specialists on one island are not closely related to the same habitat specialists elsewhere, indicating that specialists evolved independently on each island."
Why It's Wrong:
• While the passage does examine different explanations, this isn't the complete picture of the author's purpose
• The passage goes beyond just examining explanations - it shows how the question was resolved
• This choice misses the crucial resolution component that takes up the final portion of the passage
Common Student Mistakes:
- Are you stopping your analysis before the passage conclusion?
→ The passage doesn't just examine theories - it reveals which one was correct - Did you focus only on the middle portion about competing theories?
→ Include the DNA methodology and resolution in your analysis of the author's complete purpose