Michael: Classifying living things is kind of like Pokemon. It appeals to our instinct to collect and identify. And the fact that we have a classification system is super useful for scientists.

I mean, it shows how every type of organism we know of is connected to all the others. We’ve mapped out all of life in one giant chart. It’s amazing, when you think about it. But deciding how to classify a new type of organism often isn’t simple — especially when it comes to assigning it a species.

In some cases, it’s obvious when two things should be separate species. A jellyfish is clearly very different from a jaguar. But the more similar organisms get, the harder it can be to sort them. There’s no one rule that will always tell you whether two organisms should be in the same species. So, different taxonomists use different rules in different situations.

The most widespread definition of a species is called the biological species concept, or BSC. It was popularized by evolutionary biologist Ernst Mayr in 1942, and he continued to refine the idea over the next several decades.

According to the BSC, organisms are part of the same species if they can produce offspring that can also breed. If they can’t produce fertile offspring, that means they belong to separate species. The BSC hinges on the idea of reproductive isolation, where something separates two species to the point that they’re just not compatible anymore. That separation could be time, or geography, or behavior, or physiology.

Alder and Willow flycatchers used to be considered part of the same species, because they meet up in the wild, they look the same, and it seems like they should be able to breed. But ornithologists eventually realized that they’re totally different species. They sing different songs to attract different mates, so an Alder flycatcher never tries to mate with a Willow flycatcher. They’re reproductively isolated.

A species is therefore basically a gene pool, one that doesn’t overlap with the gene pools of other species. That’s a good, rigorous scientific definition. It can be tested and observed in the wild. But there are lots of cases where it doesn’t work — like with microorganisms.

Bacteria, for example, don’t reproduce sexually. They can share genetic material with each other, but that’s not part of their reproductive process. So reproductive isolation is totally irrelevant, and a scientist looking to classify a new species of bacteria can’t rely on the BSC.

Instead, they might look at genetics, at the shape of the microorganisms and the way they grow, and at their evolutionary relationships to other organisms. You also can’t use the BSC to classify fossils, since you can’t exactly observe the mating behavior of a skeleton preserved in rock.

Sometimes you get unbelievably lucky and two specimens are preserved while they’re mating, but that’s way too rare to be definitive about which animals mated with which other ones. That’s why paleontologists usually use morphology, or the physical characteristics of a creature, to classify it.

Then there’s the issue of hybrids, where creatures that should be part of separate species mate. When two species hybridize, there are a number of things that can happen. Often, even if they produce offspring, the offspring are sterile.

That’s what happens when you cross a female horse and a male donkey, for example. The offspring are mules, and mules can’t breed. But sometimes two species can produce fertile offspring.

And biologists have found that animal species hybridize way more often than we used to think. If the offspring go on to mate with their parental species, that’s called introgression. It provides a means for gene flow between two species, meaning their gene pools aren’t totally isolated.

So, does that mean you have to revoke their status as separate species? Scientists haven’t settled that question yet, but some think the barriers between species are much leakier than the biological species concept allows. Sometimes this leakiness can lead to two groups being classified as subspecies within a single species.

Subspecies give you a little wiggle room. Sometimes organisms overlap geographically, and can share genes. In cases like these, subspecies means we know they can share genes, it’s just that they usually don’t.

Then there are ring species. In California, for example, there’s a species of salamander that lives in overlapping populations of subspecies on either side of the San Joaquin Valley in a big, elongated ring. Those subspecies can all interbreed — except for two.

At the southern part of the ring, two subspecies can’t interbreed. So, according to the BSC, those two subspecies should actually be separate species. But even though they can’t interbreed, genes can still flow between them through the other subspecies.

Where does that put them? You can’t answer that question with the BSC. So, having a hard and fast definition of species that you can test and demonstrate is useful — like in conservation, when you’re looking to protect a species and its habitat. But the biological species concept isn’t an absolute rule that scientists are bound to. It’s a tool, and scientists try to select the best tool for the job. That might be the BSC, and it might not. It depends on what you’re studying.

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