It's a comment that has stuck with me. We perceive species as being these little cubbyholes with impenetrable sides, and once you've filed something there, it stays put. Of course a polar bear and a grizzly bear are different species. How could they be otherwise?
But when you start pushing at the definition a little, you find that it gives way almost immediately. Ask some non-scientist how they know polar bears and grizzly bears are different species, and you'll likely get an answer like, "Because they look completely different." And, to be fair, that's more or less how the father of taxonomy, Carl Linnaeus, did it.
Problems creep in almost immediately, though. The "of course different species" polar and grizzly bears look far more alike than do, say, a chihuahua and a St. Bernard. (Imagine trying to convince an alien biologist that those two are members of the same species.) So very quickly, scientists were forced into refining the definition so as to capture the separateness of two different species in such a way that the term could be applied consistently.
What they ultimately landed on was the canonical definition used in just about every biology textbook in the world: "Members of the same species are capable of potentially interbreeding and producing viable and fertile offspring." (The "fertile" part had to be added because of the famous example of a horse and a donkey being able to produce a viable hybrid -- but that hybrid, the mule, is almost always infertile.)
The problem was, even that wasn't enough to clarify things. Polar bears and grizzly bears, for example, can and do hybridize in the wild, and the offspring (the rather unfortunately-named "pizzly bear") are almost always fertile. This isn't an aberration. These kinds of situations are common in the wild. In fact, in my part of the world, there are two birds that look dramatically different -- the blue-winged warbler and the golden-winged warbler -- but they will happily crossbreed. When the hybrids were first observed by scientists, they were different enough from both parents that it was thought they were a third separate species, which was called Brewster's warbler. It was only after long observation that biologists figured out what was going on -- especially given that "Brewster's warblers" are potentially interfertile with either parental species.
In fact, the more you press the definition, the more it falls apart, the more exceptions you find. Today's taxonomists are usually wary about labeling something a "species" -- or when they do, they're aware that it's potentially an artificial distinction that has no particular technical relevance. They are much more comfortable talking about genetic overlap and most recent common ancestry, which at least are measurable.
The reason all this comes up is because of a startling discovery brought to my attention by a friend and long-time loyal reader of Skeptophilia. Researchers in Hungary have produced a hybrid between an American paddlefish and a Russian sturgeon -- two species no one could confuse with each other -- and they appear to be fertile, and normal in every other way.
The more you look at these "sturddlefish," the more shocking they get. Sturgeon and paddlefish are not only separate species, they're in separate families -- two layers of classification above species. "I’m still confused," said Prosanta Chakrabarty, ichthyologist at Louisiana State University. "My jaw is still on the floor. It’s like if they had a cow and a giraffe make a baby."
He quickly amended that statement -- giraffes and cows have a recent common ancestor only a few million years ago, whereas paddlefish and sturgeons have been separate lineages for 184 million years. To get anything comparable, Chakrabarty said, you'd have to have something like a human coming out of a platypus egg.
The scientists believe that the reason this happened is because of the relatively slow rate of evolution of both lineages (especially the sturgeons). Sturgeons now look pretty similar to sturgeons two hundred million years ago, while almost all of the mammalian biodiversity you see around you -- divergence between, say, a raccoon and a squirrel -- happened since the Cretaceous Extinction, 66 million years ago. But even so, it's pretty remarkable. To my eye, paddlefish and sturgeon look way more different than lots of pairs of species that can't interbreed, so once again, we're confronted with the fact that the concept of species isn't what we thought it was -- if it has any biological relevance at all.
Atlantic Sturgeon (Acipenser oxyrhynchus)
American Paddlefish (Polyodon spathula) [Both this and the above image are in the Public Domain]
This brings us back to the unsettling (but exciting) fact that whenever we think we have everything figured out, nature reaches out and astonishes us. It's why I'll never tire of biology -- to paraphrase Socrates, the more we know, the more we realize how little we know.
But one thing I know for sure is that the biologists really need to come up with better names than "sturddlefish" and "pizzly bear."
This week's Skeptophilia book recommendation of the week is by the brilliant Dutch animal behaviorist Frans de Waal, whose work with capuchin monkeys and chimps has elucidated not only their behavior, but the origins of a lot of our own. (For a taste of his work, watch the brilliant TED talk he did called "Moral Behavior in Animals.")
In his book Mama's Last Hug: Animal Emotions and What They Tell Us About Ourselves, de Waal looks at this topic in more detail, telling riveting stories about the emotions animals experience, and showing that their inner world is more like ours than we usually realize. Our feelings of love, hate, jealousy, empathy, disgust, fear, and joy are not unique to humans, but have their roots in our distant ancestry -- and are shared by many, if not most, mammalian species.
If you're interested in animal behavior, Mama's Last Hug is a must-read. In it, you'll find out that non-human animals have a rich emotional life, and one that resembles our own to a startling degree. In looking at other animals, we are holding up a mirror to ourselves.
[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]