By Jerry Coyne and Greg Mayer
Yesterday Carl Zimmer wrote a piece for the NYT on species concepts and conservation. Both Greg and I, who discussed the piece and are coauthoring our take on it, found that while Zimmer does not take a position on species concepts (which is good thing), it does have a theme. And the theme seems to be this: that conserving biological diversity depends critically on what biologists decide a “species” is. Now this argument is not, in our view, correct, because you can conserve biological diversity regardless of your species concept, even though some biologists seem to feel that we must be conserving species. If you take that latter point of view, which we see as misguided, then you’re screwed, as there are, as Zimmer notes, dozens of species concepts, and each will lead you to a different decision about populations of an animal or plant. Is a population of owls long isolated on an island a different species from its relatives? That is largely a subjective judgment.
Click to read:
As Jerry emphasizes in WEIT, the species concept one uses depends on what question one is asking. To evolutionists, the main question about the diversity of nature is this: “Why is it lumpy?” That is, why do animals and plants appear not as a continuum, but in pretty discrete groups. Look at the birds out your window and see if you have any problem telling which is which. And so it is with most animals and plants—so long as they live in the same place, i.e., are “sympatric”. (For populations that are not sympatric, but live in different areas—i.e., “allopatric”—problems arise. and these are the problems that Zimmer describes in his piece.
At any rate, the explanation for the lumpiness in one area began to be solved when biologists adopted what we call the “biological species concept”, or BSC, devised by several biologists in the 1930s, notably Ernst Mayr. (Zimmer describes him as a “German ornithologist” but he really was a German-American evolutionist—the “Darwin of the 20th century”—who held forth on far more things than birds.) The BSC is basically this:
Two populations are members of different species if they live in the same area in nature but do not produce fertile hybrids in that area.
That is, they do not exchange genes because of what we call “reproductive isolating barriers” (RIBs) that prevent genetic interchange. These barriers keep populations distinct, and allow them to undergo evolutionary divergence without being held back by gene flow. It is this feature—reproductive isolation—that leads to nature’s lumpiness, and it is the origin of these barriers that explains, to an evolutionist, the origin of species.
It turns out that these barriers usually form when populations evolve in different places. Then, when the evolutionary divergence has proceeded to the extent that there is reproductive isolation between the populations when they come back together in sympatry, we now have evolved two species from a single ancestor. RIBs come in many forms: hybrid inviability, hybrid sterility (the mule), ecological isolation (related species prefer to live in different sub-areas of the environment, or are confined there, and thus do not meet), temporal isolation, so that populations mate at different times (common in marine organisms), and differences in mate preference, so members of each species prefer to mate with individuals of their own “kind,” forming substantial barriers to gene flow.
If we can understand how one ancestral species forms two populations that cannot exchange genes, then we’ve solved the problem of the origin of species—a problem that, despite the title of his great book, Darwin didn’t come close to resolving. Now most evolutionists realize that the answer is the origin of RIBs. In fact, neither of us have ever found a scientific paper on how species form that doesn’t involve the origin of RIBs: a tacit but telling admission that the BSC is the answer to most questions about speciation.
The problems that Zimmer outlines largely involve animal populations that are geographically isolated from one another, so the BSC can’t really be applied: the populations don’t coexist. Some of them, like the giraffe populations, breed with each other like gangbusters in zoos, but that’s a very weak test of conspecificity, because some species that live in the same area without interbreeding have their RIBs broken down in captivity (this is true of many fruit flies and of species isolated by ecological preferences). One thing we can say is that if two populations in captivity produce hybrids, but that those hybrids are inviable or sterile, then, yes, they are members of different species. But breeding in captivity, something usually impossible to test, is at best a one-way test.
In 2016, Jerry wrote about the giraffes here: the populations, which look different, live in complete geographic isolation, but breed like crazy in zoos, producing viable and fertile offspring. What do we call them? We don’t know, but we’d say that they’re subspecies rather than full species. It’s a judgment call. The non-BSC people have simply raised the rank of all the traditional giraffe subspecies to species. Nothing prevents people from wanting to conserve subspecies– we sure do! People tried desperately to conserve the two subspecies of white rhinos, well before it became fashionable to raise the subspecies to species.
In recent years, Christophe Dufresnes, a herpetologist at Nanjing Forestry University in China, has used this concept to classify different species of frogs in Europe.
Some of the groups of frogs interbred a lot, whereas others had no hybrids at all. By analyzing their DNA, Dr. Dufresnes found that groups with a recent ancestor — that is, those that were more closely related — readily produced hybrids. He estimates that it takes about six million years of diverging evolution for two groups of frogs to become unable to interbreed — in other words, to become two distinct species.
“This is very cool,” Dr. Dufresnes said. “Now we know what the threshold is to deem them species or not.”’
Well, Dufresnes is still using a proxy for the BSC, but his concept of conspecificity: the “ready production of hybrids”, is a bit off. In fruit flies, species can readily produce fertile hybrids in vials in the lab yet they don’t do so in nature. Still, Dufresnes’ approach is better than just judging by genetic distance alone, or, worse, by the degree of morphological difference between isolated populations, which may be the worst way to make a species call.
Zimmer describes the intriguing finding that polar bears and brown bears have had several episodes of genetic exchange over the last 120,000 years even though they split from a common ancestor about half a million years ago. What do we call them? Our view is that they are biological species that have had their ecological isolation (polar bears “prefer” to live in colder habitats) broken down several times by climate change. The fact that there is historical gene exchange doesn’t mean that reproductive barriers don’t exist, for speciation can be either fully or partly reversible if RIBs change—in this case by changes in ecological isolation caused by climate change.
But our point is that we don’t have to make a a strict call about whether brown bears and polar bears are different species before we can decide whether to protect them as separate entities, or only protect one of them. Conservation decisions shouldn’t rest heavily on a particular species definition; rather, we have to decide exactly what we want to conserve: nature’s lumps (biological species), geographically isolated populations of a single species, like the giraffes, or even just populations of a single species that differ by one or a few traits, like color. As Zimmer quotes:
“They [the two bears] clearly demand separate strategies for conservation management,” Dr. Shapiro said. “It makes sense to me to consider them distinct species.”
But separate management strategies do not demand that they be considered distinct species– US law allows protection of subspecies and “distinct population segments” of vertebrates. From the ESA: “(16) The term “species” includes any subspecies of fish or wildlife or plants, and any distinct population segment of any species of vertebrate fish or wildlife which interbreeds when mature.” In other words, conservation strategies don’t depend on fixing on a hard definition of “species.”
Zimmer writes this on barn owls:
Even a common species like the barn owl — found on every continent except Antarctica, as well as remote islands — is a source of disagreement.
The conservation group BirdLife International recognizes barn owls as a species, Tyto alba, that lives across the world. But another influential inventory, called the Clements Checklist of Birds of the World, carves off the barn owls that live on an Indian Ocean island chain as their own species, Tyto deroepstorffi. Yet another recognizes the barn owls in Australia and New Guinea as Tyto delicatula. And a fourth splits Tyto alba into four species, each covering its own broad swath of the planet.
This is no big deal: it’s just the standard difficulty of ranking allopatric populations. We can just call all the populations members of a “superspecies” and then try to keep all the populations from going extinct. This strategy will of course conserve both genetic diversity and the presence of endemic wildlife.
Zimmer mentions a botanist who is using a “triage” method:
Thomas Wells, a botanist at the University of Oxford, is concerned that debates about the nature of species are slowing down the work of discovering new ones. Taxonomy is traditionally a slow process, especially for plants. It can take decades for a new species of plant to be formally named in a scientific publication after it is first discovered. That sluggish pace is unacceptable, he said, when three out of four undescribed species of plants are already threatened with extinction.
Dr. Wells and his colleagues are developing a new method to speed up the process. They are taking photographs of plants both in the wild and in museum collections and using computer programs to spot samples that seem to cluster together because they have similar shapes. They’re also rapidly sequencing DNA from the samples to see if they cluster together genetically.
If they get clear clusters from approaches such as these, they call the plants a new species. The method — which Dr. Wells calls a “rough and ready” triage in our age of extinctions — may make it possible for his team to describe more than 100 new species of plants each year.
A triage approach is fine– there are many approaches to trying to document and preserve biodiversity quickly. But the clear implication that debates about species concepts delay publication is just wrong. The delays discussed by Wells are all about collecting decent samples of specimens, which takes time! We have both written about the importance of museum collections, including continued collecting, for understanding and conserving biodiversity. So, we are all for accelerating collection and description of biological diversity — before it’s gone, and to try to prevent its loss.
We’ll come to an end now, but we find Zimmer’s discussion somewhat incomplete, and for the reasons we mentioned at the beginning. First, conservation need not depend on what biologists call a “species”. Second, for populations that are geographically isolated, any decision on species status will usually be arbitrary, and so we can leave aside applying fixed species concepts and instead decide what it is, exactly, that we want to conserve. We might want to save as much genetic variation as we can, or perhaps conserve morphological traits (based of course on genetic variation) that affect how a species looks or lives (e.g. coat color in mice), or even evolutionary history as reflected in genetic distance. But none of this relies particularly heavily on adhering to a particular species concept.
“Our own view would be to save all the populations, regardless of whether you call them species, subspecies, or simply different populations.” Simply and powerfully stated! My view, too.
The problem is that with limited resources and high human population pressure, we simply can’t save everything. I face this problem every day, where we at Fundacion EcoMinga have to choose which tracts of forest to save and which to leave for destruction. Some relatively objective criteria are needed for making these decisions. I tend to favor tracts that hold locally endemic unambiguous species, because if these are lost, they are lost forever and cannot easily be replaced. Preserving a mere color morph, with negligible genetic distance between its closest relative, has a lower priority than saving a BSC species. In fact I would go further and say that not all BSC species are equivalent; ecologically “keystone” species deserve higher priority.
Beyond that, I rank species in terms of phylogenetic distinctiveness. All else being equal, a species in a monotypic genus gets a higher priority than a species which is part of a very recent evolutionary radiation and which hardly differs from a score of its congeners.
Our goal then is not to preserve diversity at the species level but to maximize the preservation of phylogenetic diversity. My colleagues and I have developed the mathematics to quantify this. In this approach we aim to conserve as much of earth’s unique evolutionary history as possible. Species which are on long branches of an age-calibrated phylogenetic tree embody a lot of unique evolutionary history, whereas a species which is just one member of a short bushy cluster of twigs contains very little unique history.
Thanks for your detailed perspective, Lou. Best of luck to you and your colleagues!
Good point about phylogenetic distinctiveness. Whenever I think of this, I wonder what would have happened to the tuatara if it hadn’t made it to a few offshore islands.
Meiolaniid turtles are another example – an ancient and very distinctive chelonian lineage that held on in Vanuata until human beings settled there.
I was just in southern Ecuador on a bird tour with a focus on endemics. I was very happy to see the Pale-headed Brushfinch (Atlapetes pallidiceps), a bird I became intrigued with over twenty years ago when I first read about it. My understanding is that it was at the very brink of extinction, with a tiny number of individuals left, but has been brought back from the edge (at least a little ways) with successful conservation measure that are still very much ongoing. It is, however, just one of many species in the genus Atlapetes, thus a twig in a bushy cluster to use your analogy. Do you have any thoughts as to whether too many resources are being devoted to this bird?
Tough question. There are not very many birds as close to extinction in Ecuador as that one. In fact this is probably the most threatened one of all. The foundation that protects this bird, Fundacion Jocotoco, is dedicated to protecting endangered birds in Ecuador. That foundation was already protecting the other endangered Ecuadorian birds that are more phylogenetically distinct than that species. So it makes sense for them to invest a small proportion of their resources to prevent the extinction of this bird.
Lou– I thought of you as we were writing this, as you are right on the frontline of conservation. It was good to get your sensible and wise take on conservation prioritization.
GCM
I think taxonomy is a bigger bottle neck and problem than stated above. Bill Bryson wrote about how overwhelmed the field of taxonomy is throughout the world, particularly in Africa where, arguably, it is needed most (due to more genetic diversity?). He was in Kenya at the time.
The sheer numbers of new species vs number and methods of taxonomists is the problem.
Ignorant as I am of both fields, I can’t help thinking that this problem could be helped by AI processes. (?)
Just throwing it out there… probably looking stupid but that never stopped me before. 🙂
D.A.
NYC
https://whyevolutionistrue.com/2020/06/10/photos-of-readers-93/
(looking stupid but with doggie)
A good introduction to the ontology of species:
https://plato.stanford.edu/entries/species/
(Prof. Coyne’s book on speciation is mentioned in the bibliography.)
It may be of trivial interest that Buffon (1707-1788) had a species concept that reads very similarly the biological species concept attributed to Mayr. To Buffon: “A species is a constant succession of similar individuals that can reproduce together.”
Thanks for this topic. In the realm I inhabit, we’re conserving — and restoring (Stage Zero riparian with some upland forest) with conservation easements –habitat diversity and enrichment. The predominant aquatic charismatic vertebrates are fish – Coho, Steelhead, Chinook salmon. It’s a small contribution (65 acres) in a wider watershed-wide conservation effort. Our goal is to nurture as much habitat complexity as possible as micro-nano-(pico?)-refugia for what can survive and evolve in the developing anthropogenic holocaust. No idea if it’ll make a shred of difference. It beats fretting about circumstances beyond control.
Thanks to the Professors for the explanations. Think I’ll pull out my hardcover of WEIT, blow off the dust, and re-read Chap 7, or maybe the whole book.
As an interesting aside, I met the S.O. of one of my nieces over the holidays, who told me of his months camping in the field, studying some nocturnal insects (and trying not to get too bitten by others), trying to identify specific species based on some feature I can’t recall (sounds, I think), for his Ph.D. I mentioned how I follow this website, and Prof. Coyne’s specialty, and he replied something to the effect of how often he cited Speciation in his dissertation.
Many thanks for the important discussion. I haven’t read Carl Zimmer’s article, but like others above I agree that conserving diversity is what is crucial and not whether that diversity is “labeled” with a species rank or not.
I have nothing meaningful to add to the discussion, but thank you for yet another fascinating science post.
Same here. I always appreciate these articles even if I cannot fully absorb all of the details or add anything to the commentary.
Yes.
Will read this post in more detail but just wanted to say….thanks Jerry for writing about this. Even though the comments on these types of posts may be less than other topics, a lot of us long time WEIT readers really appreciate them.
+1
+2
Like others, I have nothing of import to add, but wanted to comment that this was an excellent piece, informative, lucid and interesting. Thanks.
Interesting. Many thanks.
Adding the the chorus of thanks. Why I come to WEIT.
I had read Zimmer’s article yesterday, and kept thinking to myself that he was giving outsized importance to the various ways humans categorize life on earth (given that it was a lay article about conservation rather than an academic discussion). I am glad to see my thoughts validated.
Do cross breeding experiments, when feasible, provide a way to test an hypothesis that two geographically separate populations are or are not the same species?
In some genera this seems to yield interesting results (e.g. AxB->fully functioning gametes in offspring; AxC-> sterile offspring; AxD-> no viable offspring, etc.).
Given that these experiments may be difficult to do, can they be useful in answering the species question (for some populations), or do they just kick the semantic can down the road?
As I believe I said in the post, cross-breeding experiments are useful in one direction: in telling us that two populations are NOT of the same biological species. If members of population 1 yield only inviable or sterile offspring when crossed with members of population 2, they must be different species, because it’s very likely you’d get the same result in nature, i.e., there’s complete postzygotic isolation. But when offspring are viable, or even fertile, it doesn’t answer the question with certainty.
Thanks! I should have read your post more carefully.
There’s a lot of natural experiments going on out there. True, they are less scientific but they do provide information.
Consider for example the Mallard, a very common duck. Apparently mallards will mate with anything that quacks, and the resulting hybrids can be viable. You can observe this by just going out and looking at a lot of ducks. Second-generation hybrid individuals can be identified as such. Other duck species are less prolific hybridizers but there are plenty of known hybrids.
And yet we don’t just throw up our hands and classify all of the ducks in one species. It’s obvious to the human eye what the species are, but it’s not just a human construction because it’s also obvious to the ducks’ eye too; the vast majority of duck offspring are not hybrids.
The taxonomic status of a population can be important if the conservation initiatives meant to protect it face legal challenges. Subspecies may not be valued as much as species, or at least may be presented as less valuable.
I’d like to add my vote to those promoting this post, and others like it. This one in particular would have been valuable when I was teaching a conservation biology course, and it’s not likely that I would have seen it or been alerted to it – the NYT isn’t a regular item on my daily reading list (but WEIT definitely is).
I’d bet it was fun getting back to science for a change! Thank you both for covering this topic.
Here in the Pacific Northwest, we have an intensive and popular effort to protect the local population of Orcas (Killer whales). There are actually two populations at issue (and several “pods”), one termed “residents” that live here year round and the other termed “transients” that are, let’s say, frequent visitors. As far as I know our resident Orca population is conspecific with other such populations along the west coast, even though they have the locally unique strategy of feeding on Pacific salmon, whereas the transients seem to prefer seals. (It sometimes seems that the transients are referred to with derision in the local media as they aren’t true residents of the Puget Sound region and they eat mammals—another strike against them. I find this somewhat amusing.)
The point is that it’s perfectly legitimate to try to protect our unique local population of Orcas, which number about 75 individuals and are probably the best studied Orcas in the world, even if the extinction of this population wouldn’t mean the end of the species. (Orcas are a cosmopolitan species and may number upwards of 50,000 individuals worldwide.*)
Population and species identification are important topics, but they can muddy the waters a bit where conservation is concerned.
*https://en.wikipedia.org/wiki/Orca
The phylogenetic component is just part of the conservation goal; protect species as they arrive from the past (phyogenetics), abide in the present (ecosystem based management), and project into the future (evolution). The last part is the hardest to define, but not impossible. Certainly genetic diversity is a big part of the toolkit to adapt to future challenges.
This strikes me as an insightful and cogent way to organize thought about the broad scope of conservation goals.
A question: how does the differing opinions between lumpers and splitters decide
how species are classified ?
This is SUCH a reasonable discussion about how species concepts interact with conservation goals! If only we could apply the same principles to how definitions of sex interact with goals about human sexuality…
it occurs to me, when thinking of the change in species, you mentioned mate preference but in a broad sense of not mating with different types from your self. Has any one tested whether the idea that females mate with males that remind them of their fathers been tested. I can’t remember who posited the idea in the first place, but if it is true, over time, this would lead to large differences within populations without the need for isolation and recombination of populations.
I just wanted to say that I have been inspired by this website for many years. I love it for the cats & perspective on the world but the reason I return here basically daily is to learn about the wonderful complexity of life. Science posts are what makes WhyEvolutionIsTrue.com the special place it is. I’m not qualified to comment on the posts but over the years they have educated me and I want to say thanks for that as I realise how much effort it takes to create them. Bravo Prof Coyne and Thanks.
Thanks. This was very interesting.
As always, I am late to any discussion and have little to add. But I really appreciate this post and others comments.
I am reminded of the ring of Larus gulls in the arctic. I believe they hybridize with neighboring/ adjacent species, but not with those further removed.