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EPISTASIS AND THE INCREASE IN ADDITIVE GENETIC VARIANCE: IMPLICATIONS FOR PHASE 1 OF WRIGHT'S SHIFTING-BALANCE
1Department of Zoology, University of Vermont, Marsh Life Science Building, Burlington, Vermont, 05405-0086.
Summary
Wright's shifting-balance theory explains how gene interactions and genetic drift can form adaptive gene complexes. Increased epistasis raises individual genetic variance, promoting allele reordering and complex formation.
Area of Science:
- Evolutionary Biology
- Quantitative Genetics
- Population Genetics
Background:
- Wright's shifting-balance theory posits that genetic drift and selection in interacting gene systems create adaptive gene complexes.
- Existing genetic drift theory often assumes additive gene effects, potentially overlooking the impact of gene interactions.
- Wright emphasized the universality of gene interaction effects, suggesting additive models may be insufficient for evolutionary differentiation.
Purpose of the Study:
- To investigate the role of gene interactions, specifically epistasis, in Wright's shifting-balance theory.
- To model the variance in local breeding values (LBVs) and the covariance of LBVs between individuals within and across demes.
- To determine how epistasis influences the independent variation of individual LBVs and the potential for allele reordering.
Main Methods:
- Developed a model to analyze the variance in local breeding values (LBVs) of an individual.
- Examined the covariance in LBVs for pairs of individuals mated within the same deme versus different demes.
- Quantified the impact of increasing epistasis on the variance and covariance of LBVs.
Main Results:
- In purely additive models, the covariance of LBVs equals the variance in LBVs.
- Increasing epistasis elevates the variance in an individual's LBV while decreasing the covariance between paired individuals' LBVs.
- A larger divergence between variance and covariance indicates greater independent variation in LBVs, signifying potential allele reordering.
Conclusions:
- The study confirms a key aspect of Wright's shifting-balance theory: gene interactions facilitate allele reordering by genetic drift.
- This reordering, when combined with selection, is crucial for the formation of adaptive gene complexes.
- Non-additive gene effects (epistasis) are essential for understanding evolutionary processes beyond simple additive models.
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