As if species definitions weren't bad enough with plants and animals—things we can see—they all but fall apart when dealing with prokaryotes (bacteria and archaea). The trouble is these microorganisms reproduce asexually, change rapidly, and often exchange genes with "distantly" related species. What to do? Current bacterial species concepts rely heavily on DNA and molecular techniques to identify groups that share an evolutionary history, and as new methods become available the species concepts are being updated and refined.
Identifying prokaryotes may sound like the world's most boring job, but the who's who of the microbial world is strangely important. You likely self-identify yourself as a member of the species Homo sapiens, but did you know that there are more bacterial cells in your body than human cells? And you call yourself a human?
There are actually lots of ways in which different prokaryotic species affect our human lives:
Antibiotic resistance: The ability of potentially scary microbes to develop immunity to antibiotics is a hot news topic these days. Speciation plays a big role—the fast generation times and ability for bacteria to share genes for immunity via horizontal gene transfer make them prime candidates for fast-speciation, or at least evolution, related to drug resistance.
Human health: Microbes are responsible for all sorts of human ailments: food poisoning, strep throat, skin infections, stomach ulcers, you get the idea. Not all of them are bad, in fact most of them are quite helpful. Unfortunately, it can be difficult to sort out the helpful from the harmful. Many bacteria species have both pathogenic (illness-causing) strains and non-pathogenic (non-illness-causing) strains. Before we go all crazy trying to wipe out whole species of what we call "harmful" bacteria, a closer inspections is always warranted.
Bioremediation & Biometabolism: Bacteria have incredible metabolic diversity, meaning they can eat a whole bunch of things that nothing else can eat. In turn, they produce a bunch of byproducts that nothing else can make. Scientists are now using bacterial species—and constantly looking for new ones—for big jobs in bioremediation and biometabolism. Bioremediation involves using microbes to clean up pollution, chemical spills, and other things we don't know how to get rid of. Biometabolism involves using microbes to digest different materials to produce byproducts that we can use for fuel. Talk about alternative energy. Sometimes you have to give it to the little guys.