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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.
We're in the middle of a global biodiversity crisis. Extinction rates of nearly all groups of organisms—insects, mammals, plants, fish—are through the roof, with very scary estimates for how much could be lost in the next 20 years. Like, permanently lost.
In order to prevent the complete loss of important ecosystems on which we depend, governments and agencies from around the world are engaged in many conservation efforts. The issue is so grand in scope, a whole science has been devoted to it—conservation biology. There are some difficult and important questions and issues in conservation biology, and many of them revolve around species concepts.
It all starts with a few basic questions. How do we preserve biodiversity? What are the highest priorities? What are we even conserving?
There have been two major strategies to get at these questions: preserving places and preserving species. The place-based approach attempts to conserve land and wild places in order to protect the biodiversity that lives there. Even this place-based approach is truly species-influenced. The most obvious places to conserve are those with high biodiversity (lots of species) and/or are the most threatened (lots of species at risk). No matter which way you slice it, most conservation issues are phrased in terms of species. Rare species. Important species. Threatened species. Places with high species diversity. If you think defining a species is tricky in biology, try mixing it with politics.
How can we expect policy-makers, conservationists, and politicians to have a firm handle on species conservation when biologists aren't always sure what a species is? Choosing a species concept can have a surprising impact on conservation plans. One study published in the journal Conservation Biology looked at the number of birds that were endemic to the mountains of southern and western Mexico. Endemic means that a species is native to one location and is found nowhere else on earth. Because of their restricted distributions, endemic species are often high conservation priorities. The study identified 101 endemic bird species under the biological species concept (BSC), but a whopping 249 species if the phylogenetic species concept was used instead. That's a huge difference. Which one should be used? You tell us.
The Endangered Species Act (ESA) is the USA's primary legislation responsible for listing and managing protected species. The ESA doesn't follow an official species definition, although traditionally it has relied on the BSC when making decisions about which species to list and protect. Since we know all about the limitations of the BSC, we might guess that this could get a little tricky, especially when biologists are concerned with protecting biological entities that don't quite fall into the species category, like hybrids, subspecies, and threatened populations.
Luckily the ESA allows for the listing and protection of subspecies and subpopulations (as long as they're biologically justified), but hybrids have proven a bit trickier. In fact, even though there are cases of pretty cool threatened hybrids (the US's red wolf) that probably warrant protection, they are usually not covered because the ESA typically follows the BSC. According to the BSC, hybrids are not biologically relevant species. Bummer.
Strong species bias is another common flaw in many conservation policies in the US. The cutest, cuddliest animals should be conserved. No one is debating that. What about the ugly, semi-frightening ones? We are all a little guilty of species-ism. If one of our conservation goals is to protect the ecosystems functions important to human lives, (for example, water filtration, soil building, carbon sequestration, and pollination) it's probably just as important to protect the worms, bacteria, insects, and slime molds of the world.
Species conservation questions are often difficult to answer and can mean life or extinction for some species. Should subspecies always be eligible for protection? Is it okay to protect some populations of a threatened species and not others? How many species is enough? Is saving some rare desert beetle really worth the money and manpower? What are our ethical responsibilities as humans? Who knew species concepts could weave such a philosophical, ethical, and political web?
Artificial selection is well known in evolution classrooms as an example of the powers of selection and descent with modification. Indeed, domestication of wild species has led to loads of tasty, cuddly, and weird plants and animals. Does domestication play a role in speciation? Are domesticated plants and animals their own species?
Ever hear of Great Danes, toy poodles, broccoli, or cabbage plants? If you’ve never heard of any of these things before, we encourage you to learn about them. Go ahead. We’ll wait.
When we talked about cabbage and broccoli before, we mentioned that they are plants of the same species (Brassica oleracea). What we didn’t mention was that they also share this species umbrella with kale, Brussels sprouts, collards, and kohlrabi.
The story for the dogs is similar, except that Great Danes and toy poodles aren’t just the same species; they're the same subspecies (Canis lupus familiaris)—a subspecies of the grey wolf. That's right. Underneath all its cuddliness, your pet Malti-poo is just a wolf with thousands of years of artificial selection working to increase its cuteness and decrease its aggressiveness. As different as a Malti-poo is from a wolf, or a Brussels sprout from a kale plant, they don't qualify for separate species status.
The reason they don't qualify as species is that they can usually still interbreed with their wild ancestors. This doesn't mean that they're not well on their way to becoming their own species some day, just that the thousands of years of artificial selection haven’t done the trick yet. If they're not distinct species, what are they? This is where we enter the breeder's vocabulary of cultivars, races, breeds, and varieties. These are different ways of acknowledging and classifying the differences among organisms that belong to the same species. How convenient.
Before giving up completely on the idea of artificial speciation, there is some evidence for it in sheep and fruit flies, an odd couple to say the least. Jack Lemmon and Walter Matthau odd. Domestic sheep can no longer interbreed with their wild ancestor, Ovis orientalis. Laboratory experiments with fruit flies have shown that it's possible to induce reproductive isolation between lab populations grown in different habitat types and food substrates. From this, scientists have determined that there is evidence that it's possible, but artificial speciation hasn't happened yet. For the time being, evidence is in short supply, but even a little bit of evidence counts as evidence. Give it a few million years and see what happens.