We’ve been talking about branches on the Tree of Life for weeks now, so I figured it was time to talk about what a species even is. And what the hell does it have to do with conservation? Why does it matter if there is one species of River dolphins in the river basins of Brazil or three? Why do we care how many species of humpback dolphins there are in the world—especially if you need genetics to distinguish them?
When I started creating the Climbing the Tree of Life series, I didn’t fully comprehend just how much flux there is in taxonomy. I knew that my teacher’s views were outdated, sometimes by decades, but I didn’t know how quickly even some of my information would become outdated. Last week, I uploaded a video about bacteria and viruses, explaining the main differences between prokaryotes, so eubacteria and archaea, and our eukaryotic side of the tree. One main feature was that prokaryotes don’t store their genetic material in a protected membrane, but have it floating around their cell. Well, the next morning, I read an article about a bacterium that isn’t only almost an inch long (2 cm) but also has a primitive membrane around their genetic material. Clam. Outdated a day after publishing? Luckily, an exception doesn’t make the rules invalid when it comes to biology. It still holds true that prokaryotes are typically characterized by what I explained. If you want to include every edge case, getting anywhere in biology and taxonomy is more or less impossible.
While studying for our exams last semester, we came across the definition of the term “fish.” What’s a fish? Sounds simple enough to answer, right? Well, let’s try it. If I ask you to tell me what a fish is, you’ll likely come up with something along the lines of this: an animal that lives underwater with fins and gills. Maybe you’ll add that they have scales.
The definition on our lecture slides was that they are aquatic vertebrates, so animals with a spine, with gills. Another lecturer defined them as aquatic vertebrates with jaws. Well, all three of these are wrong.
Eels, for example, are fish, but many eels don’t have fins or scales. Lampreys don’t have jaws. Lungfishes don’t have gills. And another issue with the last definition—aquatic vertebrates with jaws—is that it would include marine mammals like whales, and we all know that whales aren’t fish. Not that easy, right?
Even the most complex definitions include caveats like “when present” or “usually.”
So, to get to the point, exceptions cannot be seen as a falsification of the definition, or we’d never get anything defined. I know, I live in a country where everything has a norm, from power outlets to Christmas trees. But in biology, we need to give some leeway for nature—and for change.
And with many of these discoveries, definitions have to change, branches on the tree of life have to shift, merge, or be divided. Often, species have to be renamed to update where they belong. During my first semester of biology, I learned about the bacterium Agrobacterium radiobacter, or rather I learned about Rhizobium radiobacter, as our teacher called it, a name that has technically been outdated for decades. The bacterium currently has nine synonymous names listed on Wikipedia. And that’s not uncommon. Many species names change over time with updates to the Tree of Life.
Just in case you give a crab: A. radiobacter is the bacterium that causes these knobbly growths on trees that look like tree cancer. They are one of the few examples of horizontal gene transfer, so transfer from one species to another instead of from parent to offspring. They transfer genes into the trees and abuse the tree to grow their food. Sneaky bastards. They get into trees when they are injured by something, so think twice next time you want to be idiotically romantic by carving initials into a tree.
But I digress. Let’s return to our species. A long time ago, someone came up with the weird taxonomic levels of Domain, Kingdom, Phylum, Order, Family, Genus, and Species. I had to memorize those when I first started studying. There’s a stupid mnemonic for it: “Dear King Phillip Came Over For Good Soup.” This system is called the Linnean system.
But the thing is, these levels are plainly not enough to get the diversity of life categorized, and some scientists think the system should mostly be scrapped. Students and teachers joke that the system is taught mostly, so the next generation can teach it to their students. And while the system is not completely useless, it’s not as meaningful as your biology teacher likely made you think. To quote Andreas Hejnol, a comparative developmental biologist at the University of Bergen in Norway: “When a student has to learn it, it also suggests to the student that there is something special about these groups.”
Well, like an article in Quanta Magazine explains, that’s just not the case. The problem they see with the system is that these ranks mean different things across the different groups. Someone studying mollusks doesn’t use them the same way as someone studying ornamental plants. And someone who works with bacteria and such is probably using them completely differently again.
We used to have only five divisions on the kingdom level of the Tree, but we now know that there are more. Depending on who you ask, there are five, six, seven, or even as many as 32. When I went down the rabbit hole of algae not all being plant, I realized that it is suggested the non-plantae algae should actually be their own thing entirely. You’d think the basis of the tree, at least, would be settled, but there is a boatload of debate. But no matter how many branches there are at that base of the Tree of Life, one thing is certain: there are more than eight levels. Some people add subkingsdoms and subphyllums and suborders and such, others accept that this just gets complicated quickly with sub-sub-subphyllums and such, and have gone with the more general terms of clade and taxon for the different levels. And considering how little the levels have to do with each other in the different areas of life, I think it’s time to accept this as the best solution.
Before we move on to the actual species, let’s return to that article in Quanta Magazine and Hejnol for one last quote: “Claims like ‘arthropods have the most species of any phylum’ don’t really tell us anything about evolution if the ranks aren’t objectively defined. When ecologists describe the variety of species in a given region by using families and orders as surrogates, this not only conflates rank with diversity but ‘prevents us from discovering the true biodiversity of animals and other organisms.’”
In other words: to have a measure for biodiversity, one needs to look at something that is much better defined than the bottom layers of the Tree of Life. Might species be the answer?
While I think there is much benefit to the two ranks of genus and species—after all, they make up the names of species—I don’t see much value in the arbitrary ranking system at the base of the Tree.
And it is true that there are different definitions of what makes up a species, too, but they are much less vague than the definitions of the bottom ranks. So, what exactly is a species, and why does it matter for conservationists?
There are many species concepts, but the two majorly applied concepts are the biological species concept and the phylogenetic species concept, which are often used in combination rather than competition.
First, let’s have a quick look at the biological species concept, which is still dominant despite major issues. For the biological species concept, it’s most important if organisms of two populations can duck and create offspring that is fertile. This concept has its limitations. If you keep it that simple, lions and tigers would be the same species because there are hybrids called ligers that can mate (and produce offspring) with lions, tigers, and other ligers. The thing is, though, that they don’t do this in real life. While I can’t say that there was never a time when lions and tigers ducked in the wild, it’s highly unlikely. Nowadays, they don’t even share the same territory. The only ligers we know about are in zoo settings and human-made.
But there are plenty of examples were distinct species do mate or populations of the same species don’t. In an article about issues with this concept, the University of Berkeley brought up two great examples:
First, if a road separates a frog population, so they are no longer able to interact and reproduce, should we separate them into two species? At least not immediately. If they are separated, it’s possible they’ll evolve away from each other and become distinct, but a human-made road is not a barrier that should count as of itself as a species divide.
Second, there are lizards in California that form something called a ring species. Their species, E. klauberi started at one point and explored a wider range over time around a physical barrier in the middle. Subtle changes built up. Over time, they reached back to the original point as E. eschscholtzii, forming a ring. These subspecies of lizards interbreed at the edges of the different ranges, but the two meeting at the original point don’t. They have evolved too far away from their origins to interbreed. But as the lines are blurry, the changes gradual, where would you draw the line?
Botanists who work with a lot of hybrid species, as well as anyone studying asexual organisms, has long rejected this concept. How would you decide if there is a new species of bacteria if they don’t breed to begin with? And that’s without mentioning the fact that we know very little about the actual breeding patterns of many species, so there’s a lot of guesswork involved, too.
You see, it’s not that simple. I promise, we’ll get back to the ocean in a moment. I have dolphins to discuss, after all. But first, the second species concept: phylogenetics.
The phylogenetic species concept cares little about sex. They care about characteristics. Wheeler and Platnick, who have written an entire book about species concepts, define a species as ‘‘… the smallest aggregation of (sexual) populations or (asexual) lineages diagnosable by a unique combination of character states.” That sounds complicated. More simply put, species cannot be divided into smaller units with shared, inherited characteristics. So, there is nothing you can use to describe the population to distinguish it from another with any certainty.
To finally get around to dolphins, let’s use two real-life examples: river dolphins in Brazil and humpback dolphins all over the world.
We’ll talk a lot about whales and dolphins when we reach the mammals in the Climbing the Tree of Life series, but for now, let’s focus on the so-called Amazon river dolphin—that’s the pink ones. Pink dolphins that live in freshwater? If I didn’t know it was true, I’d say this is made up. And just imagine what will happen if we drive these to extinction: children will think they are somewhere in the realm of the tooth fairy or a unicorn kiwi bird.
But, at least for now, they are very real. But how many species of these Amazon river dolphins are there? They can be found throughout the Amazon and surrounding basins. The problem is that there are only small channels connecting between some of these basins. Some of them are natural rapids that make swimming through tough, others are human-made barriers such as hydroelectric dams. And yes, we’ll get into renewable energy at some point soon, too.
If we go with the biological species concept, it’s pretty simple: the dolphins can’t interact and don’t interact, so they should be separate species, right? Well, but what if they could interbreed if they’d be thrown together by humans? Would that make them the same species again? How far have these distinct populations evolved from each other after their separation? That’s where phylogenetics come in.
I read a study from 2012 where a team of researchers set out to figure out exactly that. They applied both the traditional biological species concept and the phylogenetic one. They used nuclear and mitochondrial DNA to determine if there are distinguishable characteristics that divide these populations into separate species. The results were pretty clear: there are three distinct species of the Amazon river dolphin.
Before I tell you why this matters, let’s have a look at our second example: the humpback dolphins, which are pink-gray, as well but marine. This time, the study is from 2013 and another team of researchers. They, too, used the biological and phylogenetic concept in combination. Their aim was to figure out how many species of dolphins there are in the Sousa genus because some people say there is just one and others claim there are four.
Like the river dolphin researchers, they used both mitochondrial and nuclear DNA but also had a look at the dolphins to see if there are morphological differences such as more pronounced humps. They found that there are at least four, probably even five, different species of the humpback dolphins, including a yet-unnamed one off Australia. Really interestingly, there are two of these species in the same range, but they behave differently and look differently, so they don’t reproduce. So, it’s not just territory that keeps species apart?
So, why do we give a crab?
Well, it’s pretty simple. Just think about the last time you tried to plan a party: it’s impossible to get everyone on the same page. And the same is true for conservation: if we combine species into one, we make a plan that is a compromise—if we even make one at all.
Let’s return to our river dolphins for a moment: Three of the four species on the list are on the IUCN Red list. The fourth hasn’t been assessed yet but has low genetic diversity, a restricted habitat, and is even further fragmented because humans don’t care and build dams through their already small range. Oh, and there just aren’t enough of them. But each of these species of river dolphins fights their own battles—and needs their own conservation plan. Lumping them together means helping none of them properly.
Similarly, the humpbacks can’t get one shared plan. This should already be obvious from their wide range of distribution, where multiple countries would need to work together.
Another issue is that populations might get misjudged and get the wrong label on the IUCN list—thus not getting protection at all. For example, if one species in one area is close to extinction while another species in another area is doing well, they might not get the attention they urgently need when combined.
So, when you read about a paper that wants to argue about the number of species of river dolphins in the Brazilian river basins, don’t laugh at them. It might seem nitpicky and irrelevant. To the dolphins, it could make the difference between going extinct and getting the protection they need.
Next time, we’ll return to our Tree of Life and talk about crabs, shrimp, and other crustaceans. I’ve got a few more really cool episodes planned, like an episode about queer animal relationships and another one about modern zoos. Well, and the rest of the Tree of Life, of course. After the crustaceans, we’ve got insects, sea stars, urchins, and cucumbers, and soon even fish. I’m super excited. These episodes are so much fun to create! And I’m working on getting more hands-on with episodes. Soon, there’ll also be some VLOGS about moving because we finally found an apartment. Exciting!
If you want to help me make these research-intensive episodes, consider becoming a Patreon supporter for as little as 2 bucks a month. You make this possible. I appreciate all your donations, likes, and comments. As always, a special thank you to my two patrons, Paul and Robert.
Thank you! See you next time, friends.
Yours weirdly
Kate Hildenbrand