What leeches and ligers can teach you about evolution

If you aren't totally clear on what constitutes a species, or how scientists draw the line between one species and another, don't feel bad.

Quite frankly, the scientists are a little shaky on this stuff, as well.

That's because species aren't easily defined, and there's a lot of debate over whether an individual animal, plant, fungus, or bacterium belongs in one species group or another. In fact, if you want to know what a species is, it's best to not bother trying to grope for a strict definition, taxonomists told me. Instead, every species is really a hypothesis. "It's a testable conjecture," said Mark Siddall, curator of the phylums Annelida and Protozoa at the American Museum of Natural History. "It's a hypothesis about common ancestry, and the recency of that common ancestry."

But that hasn't always been the case.

A lot of the language we use to talk about taxonomy today was handed down from the work of 18th-century European scientists. These men, including Carl Linneaus (who is called the father of taxonomy), were working off of a very different understanding of the world. To them, taxonomy was mostly about organizing the natural world that had been given to humanity, in its current form, by God.

From their perspective, the deity created things separately, and those things had remained separate. So all you had to do was look around and spot the obvious difference between one group of things and another. Leeches were very clearly different from lions. Plants with three leaves and yellow flowers could be separated from plants with four leaves and red flowers. It was a human responsibility, as God's bookkeepers, to assign names to those distinct groups.

The trouble is, that view has some pretty obvious flaws, right off the bat. Yes, there are clear delineations between a leech and a lion. But what about between leeches?

This is a leech.

And so is this.

Leeches come in a rainbow of different colors, shapes, and sizes. They live in different places. They eat different things. (In fact, there are a surprising number of leeches that do not want to suck your blood.) And how do you draw the line between a leech and a worm? It's not always an open-and-shut case.

Yup. Still totally a leech.

Today, scientists recognize roughly 700 different species of leeches, Siddall said. We also know that leeches, as a whole, are themselves a sub-class. Those 700 leech species are all types of segmented worm.

All of this flows out of our understanding of evolution. When we say those 700 species are all types of leech, we're saying that we think they share a common ancestor. When we say that leeches are a type of worm, what we're really saying is that leeches and worms share a common ancestor — and that that ancestor is not as recent as the one shared by all the different leeches.

Those are hypotheses, and they could be wrong. Because evolution is an ongoing process, the relationships those hypotheses describe could also change.

"In some ways we're still at a very early stage in taxonomy, despite doing this for 250 years," Ellinor Michel said. She's a researcher with London's Natural History Museum and an expert on mollusks. Everybody agrees that nature is clustered in units, she said, but that's about where the agreement ends. "Some people think that if you knuckle down, we'll find the right clusters to put everything in. Others say that 'what is a species' is driven by the perspectives of individual scientists and the changing needs and desires of society.".

"But wait!" you may be thinking. "Isn't this also about sex?"

Pictured: A liger.

Back in junior high and high school, sex was probably a big part of what you learned about species. In order for two living things to be part of the same species they have to be able to get it on, and make a baby — and that baby has to be capable of becoming a parent.

That's not a bad rule of thumb to start off with, Michel and Siddall say. In fact, some of the earliest work Ellinor Michel did as a taxonomist involved taking many different snails from the bottom of Tanzania's Lake Tanganyika and trying to see which ones would mate together.

"I tried to set up these little breeding experiments. I had a bench covered with little dishes of snails at the University of Burundi," she said. "But we didn't know what to do to make the snails happy enough to mate. It was a complete exercise in futility."

Today, she says, scientists agree that ability to interbreed doesn't count if you have to force it. There are examples of captive lions and tigers having baby ligers (or tigons, depending on which species is the mother and which is the father). Ligers can even produce offspring of their own. But we don't say that lions and tigers are the same species, partly because there aren't any well-documented cases of the two animals reproducing together in the wild, without urging from humans.

The process of evolution also helps to make the sexual definition of species problematic. Living things adapt to their habitats. When the habitat changes, you start to see behavioral and biological changes that can end up leading to the creation of a whole new species, somewhere down the road.

Larus gulls are one of the big examples of this. Larus is a genus, comprised of several different species, some of which live in a circle around the North Pole. One species of Larus gull lives in Norway. Another lives in Russia. Others live in Siberia, Alaska, Northern Canada, and England. The Larus gulls that live in England can interbreed with the Larus gulls that live in Canada. But they can't interbreed with the ones from Norway. As the Larus gulls' common ancestor circumnavigated the pole, its descendants ended up more and more different from the original population that had been left behind. By the time Larus gulls met Larus gulls again, they were so different as to be unable (or unwilling) to produce chicks together. But scientists consider every step in that process to be a different species — not just the gulls at either end of the broken ring.

All of this is really about species as groups — hypotheses that mark the temporary boundaries between one group and another and help us understand how different groups are related.

But species are also individuals. Very specific individuals, in fact. Next week, I'll take you behind-the-scenes at the American Museum of Natural History in New York to meet a type specimen — an individual animal by which all other animals in the species are judged.