Bacteria are weird. Generally, they reproduce asexually by cloning themselves, but about one in a million will undergo quasi-sexual reproduction, taking a gene from another organism and putting it into its own genome—and they can take that gene from anything at all.
“Bacterial genetic exchange is not just rare, but it’s also promiscuous,” explained Dr. Frederick Cohan, professor of Biology. “It sounds kind of ironic, in the sense that they don’t exchange genes often but they’re not fussy about who their partners are when they do. So they can take up DNA from really anywhere—any organism in the world, a bacterium can take up a gene and put it into its own genome.”
According to Cohan, since there’s so little gene exchange that when a bacterium does improve through genetic exchange or mutation, its offspring will dominate the species until all the unadapted bacteria die out.
“If there is rare or no genetic exchange what’s going to happen is that the entire genome of [the improved] organism is going to go to 100% of the population,” said Cohan. “Every time there is an improvement in the species, there will be a purging of diversity genome-wide, within that particular population… Just one individual of the species will come to be the ancestor of the entire species.”
Cohan’s belief that these extreme cases of natural selection are the predominant mode of evolution for bacteria is controversial. His conclusion is the result of a new sophisticated terpretation of just what a species is—a field he has named “speciology”—combined with statistical modeling and the strange facts of bacterial evolution.
He makes a bold claim that scientists need to rethink what they consider a species. Cohan considers the unique ecological context of a population to be extremely important in determining whether a population is in fact a distinct species. For example, similar bacteria living on opposite sides of a canyon may be distinct species from each other. It’s this speciological line of thought that leads Cohan and his colleagues to find that what have traditionally been thought of as species of bacteria are really clusters of similar species. For instance, two bacteria that are both called E. Coli may share less than half their DNA with each other.
“The sad thing is that for over half a century bacterial systematics has been defining a species as this big,” Cohan said, holding his arms wide. “Where, in fact, those groups we’ve identified in bacterial systematics probably have hundreds of species within them.”
Cohan has always approached biology through a statistical lens. He began college with a strong interest in mathematics, and his turn to biology was steered by the opportunity to apply his love of math to science using statistical modeling. Since then, Cohan has been creating and using statistical models to learn about and make sense of the way species evolve, split, and diverge. His area is known as bioinformatics.
The use of bioinformatics is still fairly new to evolutionary biology, and it may be the key to rapid progress in the field. More precise distinctions between species make computer analyses more accurate, which in turn help Cohan determine what the defining characteristics of species are.
Cohan did his first experiments with plants and then fruit flies before moving to bacteria. In time, he has grown to appreciate his microscopic friends.
“When I first started working on bacteria I worked on them from the point of view of a model system that would help us understand general evolutionary issues that we’d rather study with elephants, but we can’t,” said Cohan. “That’s why I worked on [fruit flies]—I would have rather done these studies on rhinoceroses! But fruit flies were a good model system…As I got to know [bacteria] better, I got to realize that they are really very interesting creatures in their own right, and that everything I had really been most profoundly interested in was wide open in the study of bacteria.”
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