The Science of Birds - What Is a Species, Really?

Episode Date: November 29, 2020

The definition of ‘species’ isn’t as simple as you might think. In this episode, we look at the methods ornithologists use to identify and name bird species. And we discuss the challenges they face in this work.We begin by highlighting the reasons that scientists need to classify birds as species.Next, we get into several definitions of ‘species’ and the pros and cons of applying these.We then look at several case studies of birds to see how they’ve been classified.Link to this episode on the Science of Birds websiteSupport the show

200 miles off the west coast of Mexico, in the Pacific Ocean, you'll find Socorro Island, Isla Socorro. This small isolated landmass is the peak of an old volcano, reaching almost 4,000 feet above the surrounding sea. The island's scrubby upland forests are home to the Socorro Parakeet. This lovely bird has a bright green body, a yellow beak, reddish skin around its eyes, and a long pointed tail. It eats fruit from several tree species in the forest. The saccoral parakeet is found only on its eponymous island. In other words, it's an endemic species here. Notice I said species.

I can call this bird a species with some confidence. That's because ornithologists have recently bestowed this statured. the status of species on the sacoral parakeet. It's been assigned the scientific name of Sitakara brevapis. By the way, the root word there, Sita, P-S-I-T-T-A, comes from the Latin word for parrot. So there's a silent P, just FYI. Until very recently, this bird was considered a subspecies of the similar-looking green parakeet, Sitakara holoclores. The green parakeet lives on mainland, to Mexico, far, far away. The classification of the

succoral parakeet has bounced around ever since it was first discovered in the late 1800s. First, it was treated as a subspecies of the green parakeet. Then it was elevated to species status in the early 1900s. Then it was demoted back down to being a subspecies. This flip-flopping happened over

many decades. But now, thankfully, we can all relax because the identity crisis of the succoral parakeet is settled, right? Everyone can agree that this bird is a bona fide species and it will be treated as such forever and ever. Not so fast there, smoky. It turns out that not all authorities in the world of ornithology agree that the succoral parakeet is actually a species. There are several official checklists for the world's bird species, and some still treat our parakeet as only a subspecies. So the story isn't over.

Maybe this bird will be reclassified again by one authority or another. And maybe more scientific data on the parakeet will help solidify our understanding of what we should call it. Why has it been so difficult for scientists to decide whether or not this bird is actually a species? And while we're at it, what is a species, really? I mean, we all know what a species is, right? We learned that in elementary school. Lions and tigers are two different species of cats. A bald eagle is one species, and a golden eagle is another species.

Easy peasy. In many cases, yes, it's easy to tell species apart. But there are countless situations where things are less clear-cut. When two critters look very similar, it can be a challenge to figure out if they belong to two species or one. The story of the Socorro Parakeet illustrates that even whole committees of bird experts sometimes disagree on which of our feathered friends should be treated as species. And this isn't an unusual case. Hundreds of debates like this rage among ornithologists year after year.

Those official lists of the world's bird species are updated annually. Changes to our bird taxonomies eventually make it into your field guides. This is why some species appear, disappear, or get renamed from one edition of a given guidebook to the next. At the hearts of all these debates and the ever-shifting species names of birds is a fundamental question in biology. What exactly is a species? And how do we know one when we've got one? Hello and welcome. This is the Science of Birds.

I am your host, Ivan Philipson. The Science of Birds podcast is a lighthearted. guided exploration of bird biology for lifelong learners. In this episode, we're going to talk about what a species is and how scientists distinguish one species from another. This topic is bigger than just birds, but we'll try to keep our focus on the avian world. Also, this episode is not really about how new species form. We're not going to go into the process of speciation. We'll touch on it, but speciation in birds deserves a form. full episode of its own.

Another note before we dive in, you can hear that I pronounce the word as species. As I understand it, that's the earlier and arguably more correct pronunciation. But just as birds evolve, so too does language. I hear a lot of people saying species nowadays. So it's all good. Either pronunciation is fine. This reminds me of the old tongue twister that goes, She Seas Species by the Seashore

Probably ever since humans started to use language, we've had a desire to name things and put them in categories. We might have started with the kinds of fruits we could eat and the different animals we hunted. Then there were the types of trees to distinguish based on their utility for making tools and shelter. There were good reasons to name different kinds of rocks, medicinal plants, dangerous insects, and so on. These days, we categorize things like cars into classes, makes, and models.

And we've decided it's vital that we distinguish the sub-genres of heavy metal music, such as death metal, speed metal, Viking metal, sludge metal, neoclassical metal, and hippopotamus metal. I just made that last one up, but I kind of wish it was a real thing. So naming stuff is what we do. We can't help ourselves. This primitive impulse combined with our more recent motivation to classify all the things brought us to the monumental task of identifying every unique form of life on Earth. Birds being the most important, of course.

This is what countless scientists around the world are hard at work on. Why are they going through all the trouble? Why is it so important that we identify and name species? There are several practical reasons from the perspective of science. First off, the species is considered a fundamental unit of taxonomy and biological diversity. When we assign a genus and a species name to a bird, we're placing that bird within the greater framework of the great tree of life. A species classification is our best guess about its evolutionary. history and relationships to other critters.

The species as a taxonomic unit is the focus of much evolutionary theory and ecological research. If we're going to study these aspects of biology, it's pretty helpful to have names for our species. A lot of what we've learned about life on our planet comes from viewing it at the level of the species. Having identities for each of our planet's species is also super important for effective communication about them. communication among scientists for sure, but also educators, conservation workers, government officials, and nature enthusiasts like birders. Naturally, we can best communicate about a species when we all agree on its distinctiveness and what to call it.

Before we came up with any kind of standardized names for species, we had only local or common names to work with. This could be messy, because, let's say, the name given to a time. type of kingfisher by villagers living in a remote valley might be the orangey blue fish murderer bird. But in the next valley to the west, that exact same kingfisher might be called the bobble-headed laser beak. You can see the problem there. The more widespread a bird or other animal, the more local names it's likely to have. Swainson's hawk, for example, has a range that spans vast portions of North and South America. Its scientific name is

But it goes by several other common names, depending on which country or culture you're talking about. Busardo Chapulinerro, Aguiluccio Langostero, or Gavilland de Swainson. Local or common names for birds are still used in many places. And that's all fine and dandy. But thankfully, we also have our science. scientific names for birds. At least in principle, scientific names are standardized and recognized

across the boundaries of countries, languages, or cultures. So even if you and I live in different parts of the world, you know which bird I'm talking about when I say, did you see the size of that zyphorincus flavogaster? Who-wee! You don't see zyphorincus flavogasters like that every day. Another big reason we identify and name species has to do with conservation. How we classify a bird is often what determines whether or not it can be protected by conservation laws, laws like the Birds Directive in the European Union and the Endangered Species Act in the United States. Sometimes a bird or other organism will get legal protection only if it's considered a full species rather than just a subspecies. Thinking again about our little buddy, the Socorro Parakeet in Mexico,

As I said, this bird is now considered a full species by some authorities. Surveys suggest that only a few hundred socoral parakeets are left. Because this species has such a small population size and a tiny geographic range, ornithologists are making the case that it needs protection. But until very recently, the succoral parakeet was considered a subspecies of the green parakeet. There are many thousands of green parakeets across Mexico and San Francisco, Central America. It's not considered a threatened species, at least not at the global level. So, if the Socorro parakeet were classified as a subspecies, some people might think it doesn't

need any special protection, legal or otherwise. These cold-hearted people could argue that since the parakeets of Socorro Island are really just a small population of the abundant green parakeet, it would be no big deal if they got wiped out. Yeah, let's hope it never comes to that. at this point you might be thinking okay okay that's all groovy ivan but seriously dude what is a species that's the big question we're considering in this episode you've been patient so let's get you an answer here we go you ready a species comprises groups of acts of or potentially interbreeding natural populations, which are reproductively isolated from other such groups.

Phrased another way, a species is a group of organisms that can successfully interbreed and produce fertile offspring. Now, before you nod with knowing satisfaction, push the stop button, and go make a sandwich, please hang with me because we definitely aren't done here. What I gave you a moment ago is just one way to define a species. It's called the Biological Species Concept, and it's the definition you're most likely defined in a biology textbook. There's a good chance you've heard it before. The Biological Species Concept was formally introduced in 1942 by Ernst Mayer, one of the

20th century's most famous evolutionary biologists. He also happened to be an ornithologist. cool. At the heart of Mayer's species concept is this idea of reproductive isolation. Simplistically, it works like this. If two organisms can mate to produce healthy, fertile babies, well then happy day, they are members of the same species. But if their offspring aren't fertile, that means the original parents are not of the same species. The classic example is a female horse mating with a male donkey to produce a mule. Mules are almost always infertile. So if we apply the biological species concept, we must conclude that horses and donkeys are separate species. Mules then are hybrids between the two. For a couple hundred years leading up to the introduction of the biological

species concept in 1942, biologists and naturalists had been swarming across the planet, collecting, and naming species. I imagine they were like kids in a candy store, wide-eyed and feverish with the thrill of discovery. Those early naturalists looked mostly at the physical, the anatomical differences between organisms when deciding which were different enough to be treated as separate species.

And since nature is positively bristling with diversity, they found lots of opportunities to divide and subdivide groups of organisms into more and more species. With respect to birds, the naming frenzy brought the global number of species to a staggering maximum of 18,937. That was by 1909.

I say staggering because remember that today we recognize only 10 or 11,000 bird species. So what happened? Did 9,000 bird species go extinct since 1909? Thankfully, no. What happened was Ernst Mayer and the biological species concept, that's what. Only a few years after the scientific community embraced Mayer's idea, enthusiastic taxonomists had rearranged the world's birds into a mere 8,616 species.

The trend had become to lump variable geographic forms of birds into single species, reassigning many former species to the subspecies level. The main criterion behind these decisions was Reproductive isolation, yes or no. If no, lump them and call them subspecies. In recent decades, the number of bird species has been creeping upward again. This is happening in part because we've recognized some limitations of the biological species concept. Some species previously lumped under that concept have been split once again. Also, the data we can collect from birds has become more varied and rich.

A lot of this is genetic data. These days, it's relatively routine to sequence whole genomes from multiple birds to work out their evolutionary relationships. And biologists have come up with other species concepts. The biological species concept is no longer the only game in town. We'll talk about some other species concepts in a few minutes. For more than a hundred years, our lists of bird species have been expanding and contracting, accordion-like, as we lump species together or split them apart.

But there's a chance ornithologists will someday recognize many more bird species than they do right now. But if this happens, it will be a slow grind. Scientists are conservative and skeptical, as they should be. They don't want to just name species all over the place willy-nilly. Well, at this point, do you feel like you know exactly what a species is? Does the biological species concept make you feel all fuzzy inside, and does it lay to rest all your uncertainties? Yeah, me neither.

You and I aren't alone in feeling like there has to be more to identifying bird species, more than just determining or making assumptions about which birds are. are reproductably isolated. Biologists have struggled to define species for a long, long time. Over 80 years before there was a biological species concept, Charles Darwin 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. This uncertainty became known as the species problem. Note that this is different from the

feces problem, the one faced by zookeepers and the parents of small children since time immemorial. The species problem hasn't gone away. In 2001, the evolutionary biologist Jody Hay wrote, The species problem is the long-standing failure of biologists to agree on how we should identify species and how we should define the word species. The biological species concept was a helpful step forward in resolving the species problem.

But it does have some limitations. Let's give those a quick look, shall we? As a reminder, the biological species concept goes like this. A species comprises groups of actually or potentially interbreeding, natural populations, which are reproductively isolated from other such groups. What are some problems with applying this concept? Well, the test of species hood here is whether or not two birds or whatever can reproduce successfully, right? It might not be too hard for ornithologists to apply this test to birds that have overlapping distributions and that share the

same habitat. Careful observation alone might be enough to figure out which birds can or can't interbreed. But there are many, many scenarios where applying the test of reproductive isolation is either, A, super challenging, B, probably unethical, or C, just plain impossible. Let's say you have two populations of birds, each living on a separate island. As far as you can tell, all these birds are identical, except that their tail feathers are blue on one island and green on the other island. The islands are close enough that the birds might be able to fly back and forth between them, even if only rarely. Now you want to figure out if the two populations represent two species or just one. How do you determine if a bird from one island can, at least potentially,

breed with a bird from the other island to produce fertile offspring. There are several ways you might confirm this, most of which will take lots of time, hard work, and probably money. More often than not, ornithologists have opted to just make educated guesses about the reproductive isolation of birds in situations like this. Another problem has to do with hybrids. Back in the day, zealous supporters of the biological speech, concept would lump together any two types of bird into a single species if those birds could

produce fertile hybrid babies. Even the tiniest amount of successful hybridization, they argued, was enough to prove that two bird populations belong to the same species. They just lumped and lumped like there was no tomorrow. Is that approach reasonable? I mean, to consider two birds as separate species, how much hybridization between them is too much. It's hard to say. In recent decades, we've learned a lot about natural hybridization in wild birds. First off, it turns out that about 10% of all bird species hybridize with other species. This is way more common than we once thought. Sort of like Romeo and Juliet, it seems birds sometimes disappoint their parents in the name of romance and forbidden love.

between species. Anyway, we now have more data and better tools like genetic analysis to look closely at how hybridization really plays out in birds. It turns out there are a lot of nuances to hybridization, and we just don't have a black and white way of looking at hybrids when applying the biological species concept. There's another scenario where this species concept fails to be helpful. Say we have a widespread bird whose populations freely and happily interbreed with their neighboring populations. Except that this isn't true for populations at the far ends of the bird's geographic distribution. Birds from each end don't interbreed with each other. Maybe they can't even stand the sight of each other, and if they do mate, half-heartedly, their babies might be

genetically inferior. Do birds living at the two ends of this vast, continuous distribution belong to one species, or two? We can't easily answer that question with the biological species concept. Granted, a scenario like this may be uncommon in nature, but it does happen. So those are some big limitations of the biological species concept when applied to birds. But you should know that it also fails when we try to use it. with prehistoric animals or asexual critters like bacteria. One final and very serious problem with the biological species concept is its name. I mean, biological is not a helpful descriptor, is it?

It kind of implies that there are other species out there that are somehow not biological. Is there a geological species concept that I'm unaware of? Or perhaps a mythological species concept? Very smart biologists have been trying for years and years to come up with a better species concept, maybe one that can be applied to every living thing, unlike the biological species concept. Like flavors of toothpaste, we now have literally dozens of different species concepts to choose from. Let's spend the next five hours discussing each of these concepts in mind-numbing detail. Or let's not and say we did.

Thankfully, most ornithologists these days rely on just a few well-supported species concepts as they go about the business of identifying and naming bird species. The biological species concept is still one of the most widely used, despite the problems we just discussed. it works pretty well in many situations. Before applying this or any other species concept, a biologist will probably have reasons to suspect that two seemingly similar organisms might be different species.

Despite all our modern progress, the starting point for identifying bird species is generally the same as it's been for hundreds of years. An ornithologist looks at birds from two populations and notices that they seem different somehow. Maybe they have some subtle differences in plumage, or their songs sound a bit different, or their feeding behavior. Intrigued, our ornithologist might bust out a measuring tape and some calipers to collect hard data from birds in both populations. If the two populations appear to be identical across all the measured dimensions, that might be where the investigation ends.

Conclusion, they're the same species. Boring, maybe, but oh well. But if there are measurable, quantifiable differences, well then, it might be worth a closer look. Once upon a time, back in the 1800s, finding a few morphological or behavioral differences was all you needed to distinguish new species. Hooray, they're different. You've discovered a new species. Time to turn off the kerosene lamp in your lab, ride the streetcar home, pop the cork on a bottle of champagne, and dance awkwardly to some tunes on your phonograph.

In those days, biologists and naturalists were applying what we would call a morphological species concept. This approach is still helpful in some cases, such as identifying prehistoric species, since the only data we have for them comes from fossils. As for identifying living species these days, our standards are more rigorous than they were in the 1800s. morphological and behavioral data often need to be scrutinized in combination with genetic data, geographic distributions, and evidence of breeding compatibility, or the lack thereof. So you've got a bunch of data like this, and it suggests that your bird of interest is distinct. This is the point where you'd need to decide on which species concept to apply.

If you don't want to use or for some reason can't use the biological species' concept, your best alternative is probably the phylogenetic species concept. This one was introduced in the 1980s. Some ornithologists would argue that we should always use the phylogenetic species concept and do away with the crusty old biological species concept. So, what is the phylogenetic species concept? A phylogenetic species is a single evolutionary lineage that stays cohesive, close-knit in both space and time. A phylogenetic species is one small branch on the tree of life. Its members all share

at least one unique, inherited feature that separates them from members of other groups. That trait might be a plumage pattern or eye color. It might be a song phrase, or it might be a sequence of nucleotides in a gene. The phylogenetic species concept has a strong focus on the history of a species. It's about how much genetic connection there was in the past. The biological species concept by contrast is more about what's going on today with respect to who can successfully breed with whom. Reproductive isolation in the present is mostly irrelevant in the phylogenetic species concept. I should point out that these two species concepts aren't always at odds with each other. They're not necessarily mutually exclusive and they

frequently point to the same answer. The phylogenetic species concept has its pros and cons, which shouldn't surprise you. One interesting challenge is the question of which unique inherited traits should we use to define species this way. If we're using genetic data from DNA sequences, for example, what is the minimum amount of genetic difference we're willing to use to separate species? These aren't easy questions to answer. Also, if you're liking the sound of the phylogenetic species concept, I hope you're ready

to say goodbye to all your beloved subspecies of birds. This species concept renders subspecies as obsolete as your old VHS tapes and CDs. Under the phylogenetic species concept, if a population of birds has a unique trait, bam! It's a species! Many of what we call subspecies right now might be reclassified as full species if ornithole around the world suddenly embraced the phylogenetic species concept. In such a hypothetical situation, the dark-eyed junco here in North America could go from being

one species to 15 overnight. Likewise, the European Robin could get split up into eight species, and the superb fairy wren in Australia could become six. Sounds like good times to me. As I mentioned earlier, there are several official checklists of the world's bird species. Four, to be exact. All of the lists have between 10 and 11,000 species. The biggest difference in length between any two of these lists is 956 species. Not only do the absolute numbers of species differ, but the actual identities and names of the species are sometimes different between one list to the next.

By now, you can probably guess why we don't have one glorious checklist of birds that all ornithologists hold aloft like some immutable document handed down from the heavens. It's because this is science, man, and science can be messy. What we know about nature is constantly changing and getting updated as we get more data as we improve our methods of analysis. And scientists, bless their hearts, can be stubborn curmudgeon, that bicker over every little data point. And so we have not one, but four bird checklists.

These guys say their list is the best, and those guys over there say theirs is the best. But that's okay, you know, because they're all working hard to get our birds sorted out. Through this slow and sometimes painful process, we're moving closer to understanding how things really are. Each of the four major bird checklists has a scientific committee or group of editors that make decisions about what species to add or

remove with each update. They consider peer-reviewed published research and other information as they make these decisions. Each committee has codified guidelines for their process. Some lean more heavily on one particular species concept than another. Some are more conservative with splitting species. Others are more liberal. Most, if not all, of these committees are moving towards a multidisciplinary approach to identifying and classifying species. It's called integrative taxonomy. Multiple sources of data are brought together to make decisions about species. Morphology, genetics, behavior, ecology, and geography can provide complementary perspectives in the integrative taxonomy approach. Are you ready to talk about some birds already?

How about a few real-world examples? Remember the socoral parakeet? Sure you do. Well, in a 2017 study published in the Journal of Ornithology, researchers used genetic data to construct a tree of evolutionary relationships between that bird and its closest relatives, the other so-called subspecies of green parakeet. This tree, this phylogeny, showed that the saccoral birds all belong to one branch of that tree,

to the exclusion of the mainland birds. The authors of this study make a case for the succoral parakeet getting full species status. The genetic data supports this. These island birds also have longer tails and lack the little orange blotches in their plumage that other green parakeets have. so there's some morphological evidence there, too. The authors suggest that by applying the phylogenetic species concept, the genetic and morphological data show that the Socorro parakeet should be treated as a species,

not a subspecies. They also remind us that this bird is geographically isolated on its far-flung island, so in effect it's reproductively isolated. For another example, take the wren. There's a tiny brown ball with feathers known as the Eurasian wren. It has an astonishingly loud and complex song for such a small bird. Here in North America, we have two wrens that are very much like the one in Europe. We have the Pacific Wren, which I have in my first.

backyard, and the winter wren, which lives east of the Rocky Mountains. Now, if I took 10 individuals from each of these three wren species and dropped them into one of those lottery machines you see on TV, you know, those clear plastic globes where air mixes the balls with lotto numbers on them, of course you do. So the air is blowing and these tiny brown birds are bouncing all around inside the globe, then the machine selects one ren and pop out it comes into your hand. For a prize of one million dollars, I ask you to tell me which species it is, Eurasian, Pacific, or Winter. You look at the bird, it looks at you, you have no idea. You make a wild guess. There's a one in three chance that your random answer will make you

a millionaire. These three species look so much alike that not long ago they were thought to be only one species. Troglodytis, troglodytis. Troglodytes. Troglodytes. the Winter Wren. But back in the early 2000s, a couple studies were published on the genetic relationships among populations of the bird formerly known as the Winter Wren. It turned out that there were deep genetic divisions between some of this Wren's populations, which were spread across the northern hemisphere. One of these studies focused on a field site in British Columbia, Canada. There, the western and eastern subspecies of North American wrens came into contact, sharing the same coniferous forest habitat. The researchers in that study measured the genetic

differences of multiple birds in this contact zone. But they also measured differences in the birds' songs. The results revealed major differences between the two subspecies, in both their genetics and their songs. So even where these subspecies lived side by side, their songs were so different that females of one subspecies probably had little interest in the males of the other subspecies. Interbreeding, it seemed, was rare. Long story short, in about 2010, the Checklist Committee overlords elevated the two American subspecies to full species status.

They became the Pacific Wren, Troglodytis Pacificus, and Winter Wren, Troglodite's Haimalis. Birds in Eurasia kept the original scientific name, and their updated common name is either Eurasian Rhen or Northern Ren. The biological and phylogenetic species concepts both came into play in the decision to split that one species into three. These little brown birds provide good examples of what we call cryptic species. At least to human eyes, these wrens are indistinguishable. We thought they were all one species, but good data and careful analysis allowed us to throw back the curtain, revealing the hidden diversity that was there all along.

This is kind of like when three little kids sit on each other's shoulders, totem pole style, inside a trench coat, pretending to be one adult so they can buy beer at the liquor store. And there might even be several more cryptic species masquerading as the Eurasian Wren. There's published genetic data that supports this. So in the future our checklists might split this Wren into four or more species.

With cryptic bird species, we have a single form or morphology that is in reality multiple species, multiple genetic lineages. But sometimes we have the reverses. situation, birds with strikingly different appearances that apparently belong to one species, a single genetic lineage. A great example of this multiple appearances one lineage scenario is seen in a couple of cheery little songbirds from North America, the blue-winged warbler and the golden-winged warbler. These two species are the same size and shape, but their colors are dramatically different and they sing different songs.

But unlike the wrens we just talked about, these two warblers hybridize frequently where they come in contact. Their hybrid offspring are fertile. The genomes of the golden-winged and blue-winged warblers show evidence that their interspecies trists have been going on for thousands of years. Hybridization is so common between these little dudes that two of their hybrid forms have their own names. Brewster's warbler and Lawrence's warbler. Given all this interbreeding, shouldn't we be treating the blue-winged and golden-winged warblers as one species? The answer is yes if we were to adhere to the old-school version of the biological species concept.

But, as far as I know, these birds have always been treated as two species. Why didn't those 1950s-era lump-happy ornithologists lump these birds into one species? Were they so bedazzled by the strong color differences in the warbler's plumages? Way to be inconsistent, guys. Now here's where this gets really interesting. Initial genetic studies of these birds couldn't find any real differences between the two species. Perhaps not surprising given how much they hybridize. However, there had to be some differences because we know plumage colors are controlled by genes.

Finally, a 2016 study of these birds published in the journal Current Biology compared their entire genomes. That's an enormous amount of data. This study found that there are indeed some genetic traits unique to each species, but this amounts to only a taincy-weensy bit of difference across the two genomes. The golden-winged and blue-winged warblers are 99.97% the same at the DNA level. It seems the few regions of their genomes that are different are, as we might expect associated with feather pigmentation. So before this recent study using genome data, the available genetic data combined with all that freewheeling hybridization might have impelled us to lump these birds as one species.

But now that scientists have found a few genetic traits which are unique to each of the two warblers, the phylogenetic species concept can come to the rescue and provide justification for keeping them separated. That's a good thing for the golden-winged warbler because its populations are declining rapidly due to habitat loss. Continuing to recognize this bird as its own species will help to keep its legal protections in the U.S. and Canada. I wish I could have given you a simple, easily digestible answer to the question of what a species is. But I've tried to give it to you straight.

That's how we do on the Science of Birds podcast. The species problem is real and there's no way we can solve it here today. If nothing else, I hope you have more appreciation for the challenges ornithologists face as they try to classify every bird species on the planet. The main reason there's no simple definition of species is that the evolutionary process of one species dividing into two is gradual and sometimes convoluted. It's not like there's a switch that gets flipped and, ta-da, you have a new species. That said, when bird populations have been reproductively isolated for millions of years,

they tend to become distinct in many ways. The biological species and phylogenetic species concepts work pretty well in these cases. Ornithologists can assign such birds to different species with confidence. It's in the early stages of species formation, when two populations are slowly splitting away from each other, generation by generation, that the boundaries between them can be blurry.

There's still a lot of mixing and interbreeding. No matter what species concepts we use and no matter how much data we have, we might not be able to find the breakpoints between newly forming species, because those breakpoints don't exist yet. We'll just need to wait a few million years and try again. We have good reasons to name and categorize things.

But with species, we're trying to give discrete names to things that aren't always discrete things. We're trying to impose our idea of order on the biological world that is, in some ways, a big mishmash of staggering complexity. And it doesn't always play by our rules. Thanks so much for listening. I really hope you enjoyed the journey today and learned some things. Please consider leaving this show a review. Apple Podcasts is a great place to do that. But if you don't use Apple Podcasts, you can leave a review on Pod Chaser.

A great review from you would be so appreciated. If you have thoughts or feelings about the show or about your favorite subgenre of heavy metal, shoot an email to ivan at scienceofbirds.com. I've been getting some great comments from listeners, and it's awesome to know that I'm making something here that people enjoy and value. So thanks to everyone who's sent me an email already. You can see the show notes for this episode,

which is number 15, on the Science of Birds website, scienceofbirds.com. This is Ivan Philipson, and I can't wait to nerd out with you next time. Ciao. Thank you.