E. coli long-term evolution experiment

Lenski's 12 long-term lines of E. coli on 25 June 2008

E. coli Long-Term Evolution Experiment (LTEE) is an ongoing study in experimental evolution led by Richard Lenski that has been tracking genetic changes in 12 initially identical populations of asexual Escherichia coli bacteria since 24 February 1988. The experiment is conducted at Michigan State University. As of 2023, the LTEE has surpassed 75,000 bacterial generations, providing invaluable insights into the process of evolution, mutation, and natural selection.

Background

The LTEE was designed to examine the dynamics of evolution in a simple and controlled environment. Escherichia coli was chosen for its short generation time, ease of laboratory culture, and well-understood genetics. The experiment started with 12 populations of genetically identical E. coli B strain REL606. These populations have been grown in Davis minimal broth supplemented with glucose as a limiting nutrient, which imposes a bottleneck and selection pressure on the populations.

Procedure

Each day, 1% of each bacterial culture is transferred to a new flask containing fresh medium, allowing for approximately 6.64 generations per day. This daily transfer simulates a consistent environment while also imposing a regular bottleneck effect, which is a critical aspect of the experiment's design. The remaining culture is frozen at -80°C, creating a "fossil record" that can be revived for comparative studies.

Key Findings

Over the course of the LTEE, several significant findings have emerged:

1. Adaptation: All populations have shown improvements in fitness relative to the ancestral strain, though the rate and trajectory of adaptation have varied among populations.

2. Divergence: Despite starting from identical conditions, the populations have diverged genetically and phenotypically, illustrating the stochastic nature of evolution.

3. Historical Contingency: A striking example of historical contingency was observed in one population that evolved the ability to metabolize citrate under aerobic conditions (a trait not found in the ancestral strain). This event underscored the role of "potentiating mutations" and the non-linear path of evolutionary changes.

4. Genetic Changes: Sequencing of the bacterial genomes over time has revealed insights into the molecular basis of evolution, including the roles of point mutations, deletions, duplications, and horizontal gene transfer.

Implications

The LTEE has profound implications for our understanding of evolutionary biology. It provides empirical evidence for theoretical concepts such as clonal interference, the benefit of sexual reproduction (via comparison with asexual reproduction), and the predictability of evolution. Furthermore, it offers a unique resource for studying the genetic mechanisms underlying adaptation and speciation.

Criticism and Limitations

While the LTEE has been widely praised, it has also faced criticism, primarily related to its relevance to natural populations. Critics argue that the highly controlled laboratory conditions may not accurately reflect the complexity and variability of natural environments. However, supporters of the LTEE counter that it provides a clear and manipulable system to test evolutionary theories that are difficult to study in the wild.

Conclusion

The E. coli Long-Term Evolution Experiment stands as a landmark study in evolutionary biology. It continues to yield insights into the fundamental processes that drive genetic variation and adaptation. As the experiment progresses, it promises to further our understanding of evolutionary dynamics in a changing world.