It is generally accepted that biological evolution, that is, the process of accumulating changes in a population of living beings over time, is too slow a process for a person to appreciate throughout his life.
Of course, we can analyze the evidence that the evolutionary process leaves in living beings, both in the fossil record and in phylogenetic terms, and thanks to these observations and the tools that genetics provides us, we can reconstruct the family trees of any group. But, in general, evolutionary changes in populations require tens, hundreds, or even thousands of generations to become observable ; changes that happen in a shorter time would go unnoticed.
However, the key term to be able to observe the evolutionary process is not in how slow or fast it is, but in how it is measured. And it is not measured in years, but in generations. And while human generations last decades, there are organisms whose generations last years, months, days or even minutes . And, therefore, the evolutionary changes in their populations can occur in time ranges short enough for researchers to observe them in real time.
Lenski’s bacteria
In 1988, a team led by Michigan State University professor of microbiology and molecular genetics Richard E. Lenski began an experiment that is still ongoing today . The researchers obtained a strain of Escherichia coli bacteria and seeded twelve populations in their respective culture media . Every day —which is estimated that the population undergoes about 6.64 generations—, 1% of the population is extracted and transferred to a new culture medium, and every 500 generations (75 days) a sample of the lineage is cryopreserved, freezing it in glycerol, to have historical samples with which to compare ancestors with descendants.
Since the beginning, tens of thousands of generations of bacteria have already been recorded in Lenski’s experiment—50,000 were recorded in a scientific publication in the April 2021 Journal of Bacteriology . During this time, multiple significant evolutionary changes have been observed.
Among others, a metabolic innovation stands out: the ability to use citrate as a carbon source . This evolutionary novelty took place in a lineage —which receives the name of Ara-3— that, for more than 30,000 generations, had this resource at its disposal, without taking advantage of it. The ability was observed in a population around generation 31,500. The long delay in the appearance of the evolutionary advantage, and the fact that it only appeared in one lineage, told the researchers that this was an extraordinarily rare mutation.
Other evolutionary changes that have been observed in the extensive experiment of Lenski and his bacteria include an increase in the size of some lineages, or the acquisition of resistance to antibiotics.
the evolution video
In 2016, and knowing that the evolutionary process that leads to the acquisition of antibiotic resistance is one of the easiest to observe, a team of researchers from Harvard University decided to try something even more impressive than the Lenski experiment. . They tried to record the evolutionary process on video .
The approach was the following. In a huge 120 × 60 cm plate they put a solid agar culture medium impregnated with black ink and divided it into nine transverse bands . The two extreme bands were left unchanged. The next two impregnated them with a concentration of the antibiotic trimethoprim three times higher than the minimum inhibitory concentration —that is, three times the dose from which the bacteria can no longer grow—, or what is equivalent, the dose that It is considered lethal to bacteria. The following bands, towards the center, were impregnated with a concentration of 10 times the lethal dose, 100 and 1000. A thin layer of light agar was placed on this culture medium to allow the growth and movement of the bacteria.
In each outer band, where there was no antibiotic, they seeded six populations of bacteria sensitive to the antibiotic, turned on the video camera, and let the bacteria grow with the intention of capturing on video the evolutionary process of acquisition of resistance to trimethoprim of the bacteria. .
And they got it.
The bacteria colonized the last band in 264 hours . Real – time video revealed how different bacterial colonies were growing and distributed across the plate and they were able to see where evolutionary innovations were taking place ; In addition, they were able to take samples at different points and analyze them, thus developing the evolutionary tree of bacteria. The video, in its accelerated version, can be seen on YouTube.
Why are they still bacteria?
Some people deny that the evolutionary process is real, even though it is a widely observed phenomenon, as the experiments shown illustrate. Despite this, these people reject the tests claiming that the bacteria are still bacteria , and have not become something else.
That argument fails at its core, resting on a false idea of what evolution is and how it works. In the evolutionary process there are gradual and cumulative changes in populations, depending on environmental pressure , but the phylogenetic group to which the population belongs does not change . In other words, bacteria, regardless of the number of evolutionary changes they undergo, will continue to be bacteria, in the same way that human beings continue to be primates, we continue to be mammals, and we continue to be animals, regardless of the changes suffered during our evolutionary process.
Every organism that descends from an ancestor belongs to the same group as its ancestor; this is called monophyletism , and it is something that evolution always conserves. And although new species may arise that form new groups, they will continue to belong, all of them, to the same ancestral group.
For the same reason, birds are still dinosaurs . Because its ancestor, that feathered animal from the Jurassic, with teeth and a long tail, was a dinosaur. And it doesn’t matter how many new species of birds emerge, how many new groups form, how many lineages evolve, and how many forms they take. They will remain birds, and they will always remain dinosaurs.
And for that reason, Lenski’s bacteria, however much they evolve, will remain bacteria. Because that’s how the evolutionary process works .
References:
Baym, M. et al. 2016. Spatiotemporal microbial evolution on antibiotic landscapes.
Science, 353(6304), 1147-1151. DOI: 10.1126/science.aag0822 Blount, Z. D. et al. 2008. Historical contingency and the evolution of a key innovation in an experimental population of Escherichia coli. Proceedings of the National Academy of Sciences, 105(23), 7899-7906. DOI: 10.1073/pnas.0803151105
Grant, N. A. et al. 2021. Changes in Cell Size and Shape during 50,000 Generations of Experimental Evolution with Escherichia coli. Journal of Bacteriology, 203(10). DOI: 10.1128/JB.00469-20
Harvard Medical School. 2016, septiembre 9. The Evolution of Bacteria on a “Mega-Plate” Petri Dish (Kishony Lab).
Jordan, J. A. et al. 2022. Idiosyncratic Fitness Costs of Ampicillin-Resistant Mutants Derived from a Long-Term Experiment with Escherichia coli. Antibiotics, 11(3), 347. DOI: 10.3390/antibiotics11030347
