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Zorluk: ZorNecessary Assumption

Biologist: Migratory garden warblers possess cryptochrome 4 (Cry4) proteins in their retinal cells, which undergo magnetically sensitive chemical reactions only when exposed to short-wavelength blue light. In an experiment, captive warblers displayed normal migratory orientation when tested under dim light containing blue wavelengths. However, when tested under monochromatic red light—which does not excite Cry4—the birds completely lost their directional orientation, even though their overall activity levels remained normal. This demonstrates that the excitation of retinal Cry4 proteins is indispensable for garden warblers to orient themselves during nocturnal migration.

Which one of the following is an assumption on which the biologist's argument depends?

  1. A
    Cry4 excitation is the only physiological process in garden warblers that is triggered exclusively by short-wavelength blue light.
  2. B
    Any migratory bird whose retinal Cry4 proteins are excited by blue light will orient itself accurately toward its migratory destination.
  3. The disorientation observed under monochromatic red light was not the result of red light disrupting an orientation mechanism that operates independently of Cry4.Cevap
  4. D
    Garden warblers rely on magnetic compass orientation more than on celestial or landscape cues during nocturnal migration.
  5. E
    Other species of nocturnal migratory birds also depend on retinal Cry4 excitation to achieve directional orientation.

Cevap

The argument depends on the assumption that the disorientation observed under monochromatic red light was not caused by an impairment of an orientation mechanism operating independently of Cry4 excitation.
The argument infers that because orientation failed in conditions where Cry4 was inactive (monochromatic red light), Cry4 activation must be indispensable for orientation. For this inference to hold, the author must presuppose that monochromatic red light did not cause disorientation through some extraneous mechanism independent of Cry4. If red light actually impaired a completely separate orientation pathway, the failure to orient under red light would provide no evidence that Cry4 activation is necessary.

Adım Adım Çözüm

1
Analyze the argument structure and isolate the premises and conclusion.
Premise 1: Cry4 undergoes magnetic reactions only under blue light. Premise 2: Warblers oriented under blue light but lost orientation under red light (which lacks blue wavelengths), while maintaining normal activity. Conclusion: Retinal Cry4 excitation is necessary for garden warblers to orient during nocturnal migration.
Identifying the explicit logical steps exposes the gap between the experimental observation and the causal necessity claimed in the conclusion.
2
Identify potential alternative explanations (Defender assumption gap).
The author assumes that the loss of orientation under red light was specifically due to the lack of Cry4 activation, rather than red light actively interfering with an independent sensory mechanism required for orientation.
If an experimental variable introduces an extraneous confounding effect, the observed outcome cannot be attributed exclusively to the intended independent variable.
3
Apply the Negation Test to the candidate assumption.
Negation: 'The disorientation observed under monochromatic red light WAS the result of red light disrupting an orientation mechanism that operates independently of Cry4.' If this negated statement is true, the experimental evidence fails to prove that Cry4 excitation is indispensable, completely undermining the conclusion.
A required unstated assumption, when negated, must logically destroy the author's conclusion.

Anahtar Kavram

Necessary Assumption Identification via Defender Negation
Tahmini Süre:1m 30s
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Necessary Assumption

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