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Go to CuriosityStream.com/microcosmos to start streaming thousands of documentaries and nonfiction TV shows. If you know about the species Lacrymaria olor, then you know what you're getting when you see it under a microscope.

There's the oval body, the long neck, the grasping and reaching out for food. It has a distinct shape, a distinct way of life. The combination of its own genetics and its surrounding environment.

It's also distinct that even when you look at different Lacrymaria olor and see that, sure, these individuals don't necessarily look exactly alike, you can still spot the underlying framework of the species that makes them resemble each other. You see the morphology and the behavior all united under the name Lacrymaria olor. It's like looking at a house cat.

You know they have their own colors and patterns and preferences, but at the same time, it makes sense why we might consider them all to be the same species. Felis catus. Then you look at the species Felix manul, better known as Pallas's cat, and the idea that these two are relatives of each other also feels right.

There is a sense of same but different. And that can make the idea of a species seem simple enough. There are things that are like each other, and there are things that are less like each other.

So the idea of a species is to define individuals so that we can define groups so that we can define relationships. Except it is never so simple when it comes to trying to categorize life. Because at the end of the day, species are only a little bit about the organisms we're looking at.

They're really more about us and how we see the world. Life is complicated and evolving, which means it will always be pushing against the bounds of our categorization. But that doesn't mean we can't try.

The human attempt to define a species is an exercise in futility that is nonetheless vital to our understanding of biology. It provides us with the language to describe the world around us so that we can make comparisons and find patterns. So what even is a species?

Well, it depends on who you ask. There have been several species concepts over the past few centuries. One of the definitions you may be familiar with is attributed to the zoologist Ernst Mayr who in 1942 defines species as: “groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.” This describes what is known as the Biological Species Concept.

Two species might look the same, but if they can't reproduce together, then they're different. One of the major challenges with this definition is obvious from our journey through the microcosmos. Not all organisms interbreed.

Some reproduce asexually. So how does one draw biological borders around species that just make copies of themselves? There are several other species concepts out there, and we won't be able to explore all of them in this episode.

But one of the other salient definitions that helps avoid the challenges of the Biological Species Concept is similar to how we started this episode: Just look at the organisms. Look at their morphology, describe their shape and color in organelles and whatever strange features appear and use that to define an individual species. It seems as simple as saying that if it's almond shaped in body and long and neck then it's Lacrymaria olor.

Why make it any more complicated than that? But deciding on what traits define a morphological species is so dependent on who is observing and describing those traits. And it creates so many more questions for how we use morphology to describe species and their taxonomy.

What if the neck is a little shorter? The body a little stouter? Does that make a different species?

And just how much can we infer about the relationships between two species based on their appearance? If they look similar does that mean they're close relatives, or is that some kind of phylogenetic optical illusion? There's no single answer to any of those questions.

It all just depends on the organisms and what it is you want to understand about them. Let's look at Lacrymaria sapropelica, a fellow member of the Lacrymaria genus. Its body is stout compared to the Lacrymaria olor, like setting down a teapot next to a champagne glass.

Most importantly, its neck doesn't seem to extend much beyond its body. Instead of the constant stretching movement of the Lacrymaria olor’s neck, the Lacrymaria sapropelica’s neck neck is stubby and immobile. In fact, it looks a little bit like this other species called Phialina caudatum, which belongs to the genus Phialina, a genus that closely resembles Lacrymaria except for the obvious issue of the neck.

So if Lacrymaria sapropelica doesn't have an extending neck, why is it classified as Lacrymaria and not Phialina? Well, mostly because no one's taken the time to officially change it. Naming and classifying species doesn't just happen automatically.

A person has to come along and do it. Putting in the effort to study the creatures and write up their reasons for changing its identity. And, you know, sometimes that person just hasn't come along.

That's the thing about people. We've come up with these incredible systems for describing nature. But at the end of the day, those systems are still subject to our very human whims.

And just to emphasize how messy the process of organizing organisms is, we should add that Phialina caudatum actually went by another name before Lacrymaria caudatum. But unlike Lacrymaria sapropelica, someone did come along and do the work of revising its place. We don't know what motivated them to take on the species.

Maybe they were constantly irritated when they read the words “Lacrymaria caudatum.” Maybe they just happened to be studying the species already. Or they just wanted to publish a paper Or maybe it's a combination of all three. Human motivations and desires are just as difficult to categorize as microbes.

All this messiness and subjectivity seems built into morphology. But we live in the age of genetics now where sequencing an organism is cheap and the data objective, right? Yes and no.

There are ways that molecular phylogeny the use and comparison of genetic sequences to understand evolutionary relationships has allowed us to see past the surface similarities between organisms. Amoebas, for example, were thought to be closely related because of their use of pseudopodia or false feet, but their genetic underpinnings revealed that amoebas are actually the culmination of many different species and groups and phyla. They just happened to resemble each other in this one major way.

And those results, the specificity they've allowed us in redefining these species, have given us new understanding about their evolution and behaviors. But data, as concrete and undeniable as it feels, is still in the eye of the beholder. Like, let's say we found a new species with a short, not extending neck and say we decided to name it.

Lacrymaria microcosmosi. we're excited to find it, and we're even more excited to study it and report all the genetic information we can find describing the organism in terms of its genetic code that we registered to a depository for other scientists to look at. But then let's say that shortly after our discovery other masters of microscopes turn up their own Lacrymaria microcosmosi discoveries. The morphologies match what we've described.

But when we compare our genetic sequences to what they've found, there are differences. The question is how big do those differences have to be to make the organism itself different? Do they define a new species, an entirely new genus?

At what point do we decide something is too different to be the same or even just similar? That's not necessarily a problem that DNA in computers will be able to solve for us. There's still going to be a person who has to decide, a person who has to make a subjective call based on the data presented and the questions asked.

And their definition will probably have exceptions to leaving blanks, in our understanding. But blanks are exciting. They leave us room for new ways of seeing things, of asking things, because our definitions are an attempt to bring the expansiveness of biology into something we can understand, something we can speak and read.

But life expands beyond those constraints in ways we know, in ways we have yet to find out. Thank you for coming on this journey with us. As we explore the unseen world that surrounds us.

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