The Question is from neet-zoology genetics-evolution , and it was created by the author T SAILAJA

Question

Which observation MOST directly challenges the idea of a universally applicable “molecular clock” for dating evolutionary divergence?

T SAILAJA · Accepted Answer

## CORE INFORMATION
✓ **Answer**: C
## CONCEPT FRAMEWORK
 **Primary Concept**: Molecular Clock
 **Related Topics**: Evolutionary Biology, Phylogenetics, Mutation Rates
 **Prerequisites**: Understanding of the molecular clock hypothesis and its assumptions.
## SOLUTION PATHWAY
**Given Information**:
The question asks which observation most directly challenges the molecular clock hypothesis.
**Method & Steps**:
The molecular clock hypothesis assumes a constant rate of molecular evolution (e.g., mutations) across different lineages and genes.  Let's analyze each option:
1. **Option A:** Fossil evidence contradicting molecular clock estimates indicates a problem with the accuracy of either the fossil dating or the molecular clock, but doesn't directly challenge the *concept* of a constant rate.  Inaccuracy is different from the principle being flawed.
2. **Option B:** Environmental factors influencing mutation rates directly contradict the assumption of a constant rate. However, option C is more direct.
3. **Option C:** This directly refutes the core assumption of a universal molecular clock. If different genes evolve at different rates, then there isn't a single, universally applicable clock.
4. **Option D:** Horizontal gene transfer complicates phylogenetic analysis and can disrupt the accuracy of molecular clock estimations, but it doesn't directly challenge the fundamental assumption of a constant rate within a single lineage.
Therefore, option C is the most direct challenge.

**Result**: Option C
**Verification**: Option C directly addresses the core assumption of the molecular clock – a constant rate of evolution across all genes.
 ## OPTION ANALYSIS
A :x: Fossil evidence discrepancies could be due to errors in either the fossil dating or the molecular clock calibration, not necessarily a flaw in the concept itself.
B :x:  While variable mutation rates are a problem for the molecular clock, option C is a more direct challenge to the fundamental assumption of a universal rate.
C ✓: This observation directly contradicts the central tenet of a universal molecular clock – a constant rate of evolution across all genes within a lineage.
D :x: Horizontal gene transfer is a complicating factor, but it doesn't directly challenge the core assumption of a constant rate within a single, non-transferred gene.
 ## LEARNING AIDS
 **Common Mistakes**: Choosing options that show problems with the molecular clock's accuracy instead of its fundamental assumptions.
 **Quick Tips**: Focus on the core assumption of the molecular clock: a constant rate of evolution across all genes and lineages. The option that most directly challenges this assumption is the correct answer.

T SAILAJA · Answer

Fossil evidence sometimes contradicts molecular clock estimates.

T SAILAJA · Answer

Mutation rates can be influenced by environmental factors.

T SAILAJA · Answer

The correct Option is "Different genes evolve at different rates, even within the same lineage."

T SAILAJA · Answer

Horizontal gene transfer can introduce new genes into a lineage.

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