The evolution of genetic drift over 50,000 generations

Area of Science:

• Evolutionary biology
• Population genetics
• Microbial evolution

Background:

• Genetic drift, driven by random variation in reproductive success, is a key factor shaping genetic diversity and evolution.
• The strength of genetic drift is determined by the variance in descendant number, which influences evolutionary outcomes like mutation establishment.
• It was unclear if the variance in descendant number itself evolves over long timescales.

Purpose of the Study:

• To investigate whether the variance in descendant number, a key parameter of genetic drift, evolves over long timescales.
• To disentangle the contributions of census population size and variance in descendant number to effective population size.
• To analyze the divergent evolution of genetic drift in two replicate populations of E. coli.

Main Methods:

• Utilized model-based Bayesian inference combined with joint tracking of neutral lineage frequency fluctuations and census population sizes.
• Analyzed data from 33 clones across 50,000 generations in the E. coli Long-Term Evolution Experiment (two replicate populations: Ara-2 and Ara+2).
• Quantified changes in census size and variance in descendant number to infer effective population size and drift strength.

Main Results:

• The strength of genetic drift evolved markedly and divergently between the two replicate E. coli populations.
• Both census size and variance in descendant number changed substantially over time, with variance in descendant number driving most changes in effective population size.
• After ~2,000 generations, the variance in descendant number diverged, with Ara-2 exhibiting 1.5-5x stronger drift than expected, indicating evolved increased stochasticity.

Conclusions:

• The key parameter governing genetic drift (variance in descendant number) can itself evolve.
• Divergent evolution of genetic drift has direct consequences for the rate and probability of adaptation.
• Beneficial mutations are approximately twice as likely to establish in the Ara+2 population compared to Ara-2 due to differences in drift strength.