Emily: In your high school Biology class, You most likely learned that the definition of a species is: "A group of individuals that can breed with one another, give birth to fertile offspring, and which cannot breed with other groups." It sounds pretty straight forward, right? Well, in the world of taxonomy, which is the branch of science responsible for classifying organisms, this is known as the Biological Species Concept. This is only one of as many as a few dozen such concepts used by taxonomists when describing a new species today.

So, why are there so many interpretations? For starters, Charles Darwin, himself, didn't give a concrete definition of a species in: "On the Origin of Species" in 1859, and the word species is in the title of the book. He wrote: "No one definition [of species] has as yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species." Darwin basically said, "Meh, you'll probably know it when you see it."

If you put that aside, and fast-forward nearly 160 years later, then, why can't scientists today agree on one universal definition of a species? The short answer is: "The criteria a scientist uses to describe an organism to the species level is dependent on the information and data available for that organism.

A species concept is a hypothesis. It's a way to outline a particular set of criteria that can be tested and applied across many different types of life, but, there still is no one-size-fits-all technique. For example, the biological species concept isn't helpful for organisms that went extinct thousands or millions of years ago. And I say it's not helpful because it can't really be tested. Unless two animals died during the course of mating, And the conditions were perfect for them to become fossilized in the process, we can't know if, say, two dinosaurs would have been able to breed.

It has been known to happen: check out these amorous and unfortunate Eocene turtles. But, because finding two fossil vertebrates "caught in the act" is rare and therefore unreliable, many paleontologists describe new species based off of a morphological species concept.

Essentially, they compare and describe the sizes and shapes of their new fossil bones to similar and already described fossil species. Extinct lifeforms aside, it can be really difficult to observe whether or not two organisms in the wild are successfully interbreeding-- so, researchers have found other ways to come to this conclusion.

Take the Eastern and Western Meadowlarks. Without knowing they were different, you'd simply think that this was a gorgeous bird With a massive range, covering all of the United States. It was actually Meriwether Lewis of the Lewis and Clark Expedition who observed that these similar looking birds have completely different songs. You can assume that if these two different birds have two different songs, Then they're searching for two different mates.

Based off of this information, John James Audubon gave the Western Meadowlark its own species name. Further research has confirmed that these two species cannot really successfully interbreed, making this a good example of the biological species concept.

Today, there are plenty of other organisms that can't be defined by the biological species concept. Those that reproduce via parthenogenesis -- or asexually -- give birth to fertile offspring without needing to sexually reproduce. Asexual reproduction is incredibly common in organisms like starfish, and honeybees, but, also in the New Mexico Whip-Tail Lizard, and in sharks. Recently, a Field Museum scientist was part of a research team that discovered parthenogenesis in a type of endangered Sawfish.

In cases of organisms that asexually reproduce, taxonomists can define these different species by examining their genes. And this is known as a Genotypic Species Concept. Speaking of genes, hasn't gene sequencing solved this species-definition problem yet? Well, not exactly.

While knowing an organism's genetic code is certainly helpful, it still doesn't offer up a black-and-white way of determining how different two organisms' codes need to be before you can clearly say that they're two different species. Take Polar bears and Grizzly bears: They look different, eat different things, and live in different climates. Genetic information reveals that Polar bears split from Grizzlies less than 500 thousand years ago But, the two remain very similar genetically-- So much so, that when their populations overlap, The two have been known to interbreed and produce fertile offspring.

But, over the last half of a millennium, adaptations have allowed Polar bears to inhabit a unique ecosystem and thrive on a heavy diet of blubber, things that Grizzly bears can't do. Polar bears are even classified as a marine species, in part because their primary habitat is sea ice and they rely on marine prey for their diet. So, this is an example of an Ecological Species Concept. Genetic similarities between polar bears and grizzly bears alone don't take all of those other important differences into account.

You'll often hear taxonomists refer to trees of life: these are formal charts that really do look and act like the trunks and branches of trees, and are used to outline evolutionary relationships between organisms. In fact, all life on Earth is part of a great, big tree of life.

And, if you're curious, you can even find a version of this entire thing online. If that's too overwhelming and, understandably it probably is, then, think about your own family tree: You can look at your parents, and your grandparents, and your great-grandparents And so on, and so forth, back 200 or 2,000, or 200 million years. Imagine that same type of tree for all mammals, or all vertebrates, or all animals, all the way back to the beginning of all life on Earth.

That giant tree of life still exists, but on a much different scale than your own family tree. Instead of a parent-child gene flow, like between you and your mother and father, it's an ancestor-to-descendent flow. Now, try to imagine the exact point in time where one branch of the tree of life splits from another. Therein lies the difficulty of determining when one species becomes two.

But, adding an evolutionary context around an organism helps us to understand how it came to be today, In relation to other lifeforms These are examples of Evolutionary and Phylogenetic Species Concepts. Just as there isn't one single definition of a species, Taxonomists typically won't rely on only one species concept when describing a new species.

Instead, it's likely they're using all sorts of information and data to make the case for their new classification and a majority of those concepts involve the evolutionary tree of life. Learning more about the biological, morphological, or ecological characteristics of certain organisms helps us to place them accordingly on this evolutionary tree.

The life on our planet is so diverse and complicated, that it's understandable the naming conventions we humans use to organize all of it sometimes falls short. But, it's still important to continue studying and classifying the life we do discover, because having a better understanding of these complexities helps us to find solutions to everything from environmental problems to world-wide health concerns, food production and other sustainability issues.

After all, we can't study it or protect it if we don't know what it is. While a species cares not about the name we give it, that name is the first step in recognizing and appreciating the uniqueness of that organism, which opens the door for more questions about what it is and how it came to be. And that never-ending pursuit of curiosity, and trying to find our place in this exceedingly complicated world is a very human thing worth celebrating.