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Question

Difficulty: HardMain Idea and Primary Purpose

Passage:
For much of the late twentieth century, evolutionary phylogenetics operated under the foundational assumption that cellular life descended along a bifurcating "tree of life," where genetic information was transmitted almost exclusively vertically from ancestral to daughter organisms. This paradigm, largely solidified by Carl Woese’s landmark ribosomal RNA sequencing, positioned ribosomal RNA as an immutable molecular clock capable of resolving the deepest evolutionary lineages. However, the advent of whole-genome sequencing in the late 1990s exposed a profound complication: widespread horizontal gene transfer (HGT)—the lateral exchange of genetic material across disparate prokaryotic domains. Genome-wide comparative analyses revealed that while genes encoding core informational machinery (such as translation apparatuses) resist lateral swapping and preserve vertical signals, metabolic and operational genes have frequently crossed taxonomic boundaries throughout evolutionary history. This genetic mosaicism led some radical critics to claim that early tree-based phylogenetics was entirely illusory. Yet contemporary evolutionary biologists largely reject both extreme positions—the rigid vertical tree and total phylogenetic chaos. Instead, they advocate for a synthesis that conceptualizes early evolution as a reticulate network: a web-like framework where vertical trunks are discernible for informational core systems, while lateral branches capture the dynamic functional adaptations driven by HGT.

Which of the following best states the primary purpose of the passage?

  1. To describe how whole-genome sequencing challenged a long-standing evolutionary paradigm and to outline the emerging synthesis regarding early microbial evolution.Answer
  2. B
    To argue that Carl Woese’s reliance on ribosomal RNA sequencing fundamentally invalidated modern attempts to reconstruct early prokaryotic lineages.
  3. C
    To detail the specific biochemical mechanisms that prevent core translation apparatuses from undergoing horizontal gene transfer.
  4. D
    To assert that traditional phylogenetic trees must be entirely abandoned in favor of non-historical functional classifications of microorganisms.
  5. E
    To highlight a historical conflict between proponents of metabolic gene mapping and defenders of ribosomal RNA sequencing.

Answer

The primary purpose of the passage is to describe how whole-genome sequencing challenged a long-standing evolutionary paradigm and to outline the emerging synthesis regarding early microbial evolution.
The passage follows a classic academic structure: establishing a long-held scientific assumption (the bifurcating tree of life), introducing counter-evidence (horizontal gene transfer via whole-genome sequencing), and concluding with the current scientific synthesis (a reticulate network model). The option stating that the passage describes how genome sequencing challenged a paradigm and outlines the emerging synthesis perfectly captures this overall purpose.

Step-by-Step Solution

1
Analyze the passage structure and rhetorical progression.
The text introduces the traditional vertical 'tree of life' model, presents horizontal gene transfer revealed by genome sequencing as a major complication, outlines extreme interpretations, and resolves with a middle-ground 'reticulate network' synthesis.
Determining the overall structure isolates the primary goal of the passage from subsidiary details.
2
Identify the author's ultimate objective.
The author aims to contextualize an evolutionary paradigm shift, showing how new genomic data challenged an old framework and led to a nuanced synthesized model.
Primary purpose questions require an evaluation of what the passage as a whole sets out to achieve.
3
Evaluate and eliminate distractors based on scope, tone, and rhetorical distortions.
Options that overstate the invalidation of phylogenetics, focus narrowly on ribosomal mechanisms, extrapolate total rejection of tree models, or mischaracterize the text as a personal conflict are eliminated.
Standard GRE distractor traps rely on extreme language, detail focus, and misrepresenting the author's nuanced thesis.

Key Concept

Synthesizing passage architecture and authorial intent to determine primary purpose
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Topics

Main Idea and Primary Purpose

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