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On the overdispersed molecular clock.

N Takahata

    Genetics
    |May 1, 1987
    PubMed
    Summary

    Molecular evolution rates can be irregular, deviating from simple models. The fluctuating neutral space model explains this variation without requiring positive Darwinian selection, challenging common assumptions in evolutionary biology.

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    Area of Science:

    • Evolutionary Biology
    • Molecular Evolution
    • Population Genetics

    Background:

    • Molecular evolution rates at some genetic loci exhibit irregularities not explained by simple Poisson processes.
    • Existing models often assume lineage-nonspecific causes for rate variation, limiting explanations for observed diversity in substitution numbers.

    Purpose of the Study:

    • To reevaluate three scenarios where molecular evolution deviates from Poisson processes under the neutrality hypothesis.
    • To investigate the 'fluctuating neutral space' model's ability to explain variation in molecular evolution rates.

    Main Methods:

    • Reevaluation of scenarios including concomitant substitutions, fluctuating rates due to deleterious mutations and bottlenecks, and changes in selective constraints (neutral space).
    • Analysis using a time-dependent renewal process to model the fluctuating neutral space hypothesis.
    • Comparison of model predictions with observed variances in nucleotide and amino acid substitutions.

    Main Results:

    • The fluctuating neutral space model, even under stringent conditions, can generate substantial variation in substitution rates.
    • This model is described by a time-dependent renewal process and does not imply episodic molecular evolution.
    • Elevated variances in substitution numbers do not necessitate invoking positive Darwinian selection.

    Conclusions:

    • The fluctuating neutral space model provides a viable explanation for observed irregularities in molecular evolution rates.
    • Deviations from simple Poisson processes in molecular evolution do not automatically indicate positive selection.
    • Further research should consider dynamic changes in selective constraints to understand molecular evolution patterns.

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