For most species we see, evolution is a slow process, requiring generations to show its effects. But the species we can't see—bacteria and other microbes—can go through dozens of generations in a single day. For them, evolution can be a rapid process, as antibiotic resistance has made us painfully aware.

That's why researchers often use bacteria to study evolutionary processes. In perhaps the most famous experiment, a single lab has now sent E. coli through tens of thousands of generations of competing with each other for limited resources and has tracked the resulting changes on the DNA level.

But evolution isn't always a constant competition of all against all, as takes place in these experiments. Instead, you get migrations and exploitation of new habitats, allowing rare founders to spawn entire populations. Now, a research team has figured out a nice way to study founder dynamics in a bacterial culture and has consequently allowed the branching of evolutionary lineages to be watched like a movie.

The work relied on what its creators are calling MEGA, the microbial evolution and growth arena. MEGA is essentially a giant rectangular Petri dish, 120cm in length, and is set up a bit like a US football field. Its two ends have nothing but bacteria food, but conditions change along a series of stripes leading to the center of the pitch. As you get closer to the center, the concentration of an antibiotic rises so that by the time you reach the midfield of MEGA, there's 3,000 times the amount of antibiotic that E. coli can tolerate.

The bottom of the plate contains a high concentration of gelling agent, creating a semi-solid area where the bacteria will stay in place. But above that is a thin layer with much less gelling agent, creating a semi-liquid that the bacteria can swim through and migrate to new habitats.

Bacteria were seeded at the ends and rapidly spread through the antibiotic-free zone, stalling at the edge of the region where the drug's concentration was three times what the bacteria could tolerate. By three days later, however, the first mutants had evolved a tolerance for this level of the drug and fanned out into the new zone from a single starting point. The process repeated itself at each of the boundaries with higher drug levels until the bacteria finally spread into the zone with the highest levels of antibiotic.

Because of the way the experiment was set up, it was like watching an evolutionary tree expand in time lapse. Each new drug-tolerant lineage could be watched as it entered a new zone, fanned out, and then started competing with other lineages. It made for a compelling movie.

As with other experiments, the authors sequenced the genomes of various lineages to determine how the bacteria were changing over time. One trait that evolved several times is a high rate of mutations, caused by mutations in the error-correction region of the enzyme that copies DNA. In essence, the bacteria had evolved to generate more mutations, which are the feed stock of further evolution. Oddly, however, these "mutator" strains didn't seem to adapt any more quickly than some of their peers.

Many of the mutations that allowed the bacteria to tolerate antibiotics came at a cost: slower growth. Still, slower growth was advantageous because it allowed these bacteria to expand into what was an uninhabited habitat. In many cases, further mutations elsewhere later restored normal rates of growth.

In many cases, by the time these useful mutations had occurred, it was too late; rapidly migrating bacteria had now left their healthier peers in the dust, consigned to a crowded, competitive region of the MEGA plate. If the authors started the experiment over again, seeding it with these well-adapted strains, they outcompeted their peers. But as the researchers describe it, "the fitness of the population is not driven by the fittest mutants but rather by those that are both sufficiently fit and arise sufficiently close to the advancing front."

This is likely to be a useful evolutionary model, given that expanding populations often occur in nature, including in our own species' past as we expanded out of Africa. In many ways, MEGA is a complement to the multi-year evolution experiment; both types of competition occur in the wild, and they can each help us understand the dynamics of them and test some of our theoretical models.

But in terms of an evolution movie, MEGA clearly has the edge.

Science, 2016. DOI: 10.1126/science.aag0822 (About DOIs).