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Evolution by small steps and rugged landscapes in the RNA virus phi6
1Department of Zoology, University of Maryland, College Park, Maryland 20742, USA.
Genetics
|March 2, 1999
Summary
Adaptive evolution often occurs through many small genetic changes, supporting Fisher's model. Experiments show that recovery from reduced fitness in bacteriophage phi6 typically involves small steps, especially in smaller populations, confirming advantageous mutations are usually small.
Area of Science:
- Evolutionary Biology
- Genetics
- Molecular Biology
Background:
- Fisher's geometric model posits adaptive evolution via numerous small-effect mutations.
- Empirical evidence for the model and small-step evolution remains debated.
Purpose of the Study:
- To experimentally test Fisher's geometric model of adaptive evolution.
- To investigate the role of mutation effect size in fitness recovery.
Main Methods:
- Bacteriophage phi6 populations were subjected to intensified genetic drift, reducing fitness.
- Fitness recovery was studied across various population sizes under natural selection.
- The size of adaptive steps during fitness recovery was quantified.
Main Results:
- Viral fitness declined in a single large step but recovered in multiple smaller steps.
- Smaller recovery populations resulted in smaller adaptive step sizes.
- Advantageous mutations were found to be compensatory and epistatic.
Conclusions:
- Results support Fisher's prediction that advantageous mutations are typically small.
- The study highlights the prevalence of small-effect, conditional mutations in adaptive evolution.
- The adaptive landscape for bacteriophage phi6 is likely rugged due to epistasis.