Guest Essay by Kip Hansen
Biology has a ‘new’ problem: Speciation Reversal. One recent paper on the topic declares:
“We argue that extinction by speciation reversal may be more widespread than currently appreciated. Preventing such extinctions will require that conservation efforts not only target existing species but identify and protect the ecological and evolutionary processes that generate and maintain species”.
Another paper worries that climate change is hastening the loss of landscape heterogeneity thus encouraging “Interspecific hybridization [which] is …. an evolutionary process that is (i) highly susceptible to human influences, and (ii) very fast” and that “The most probable proximate outcome of such hybridization will be a collapse of hybridizing species and subsequent loss of biodiversity.”
A third paper laments the speciation reversal seen in two previously separately identified raven species in California, the non-sister lineages of ‘California’ and ‘Holarctic’ ravens, which underwent a fusion and formed the Common Raven. This “represents a case of ancient speciation reversal that occurred without anthropogenic causes.” This same paper holds that “Under certain circumstances, hybridisation can cause distinct lineages to collapse into a single lineage with an admixed mosaic genome. Most known cases of such ‘speciation reversal’ or ‘lineage fusion’ involve recently diverged lineages and anthropogenic perturbation.”
What in Darwin’s Name is going on here? Whole species going extinct by speciation reversal — an existential threat to biological diversity on Earth? Or just a threat to the concepts of modern biology?
The biological concepts are: “Speciation is the evolutionary process by which populations evolve to become distinct species.” However, “introgressive hybridization erodes differentiation until species collapse into a hybrid swarm. A special case of introgressive hybridization is speciation reversal, in which changes in selection regimes increase gene flow between sympatric species, thus eroding genetic and ecological differences. Speciation reversal may be particularly important in adaptive radiations with recently diverged sympatric species that lack strong intrinsic postzygotic isolation.” [quote link]
Let’s see if we can sort some of the terms out: [in all senses here, we are talking of natural interactions in the biota and we will exclude any consideration of the possibilities of human directed genetic manipulation such as CRISPR-Cas9 techniques.]
Introgressive hybridization, in genetics is the movement of a gene (gene flow) from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species. Purposeful introgression is a long-term process; it may take many hybrid generations before the backcrossing occurs.
Interspecific hybrids are bred by mating individuals from two species, normally from within the same genus. The offspring display traits and characteristics of both parents. [Many interspecific hybrids are sterile, preventing gene flow between the species. An example is the mule, a sterile cross between donkeys and horses.]
Sympatric species are species that occupy the same or overlapping territories — sympatric and sympatry are terms referring to organisms whose ranges overlap or are even identical.
Species: Oh boy — we have a problem here. Let’s try the old high school standard: “A species is often defined as the largest group of organisms in which two individuals can produce fertile offspring, typically by sexual reproduction. “
If that definition were the one adhered to, then introgressive hybridization and interspecific hybrids would be impossible by the definition of species that excludes reproductively-viable inter-species hybrids.
If that definition is adhered to, the Speciation Reversal is also impossible and we can relax — no threat to species then.
But, interspecific hybrids are popping up all over the taxonomic map. That leads us to:
The Species Problem: “The species problem is the set of questions that arises when biologists attempt to define what a species is. Such a definition is called a species concept; there are at least 26 recognized species concepts.”
The species problem is not new — Darwin spoke of it in his 1859 volume “On the origin of species by means of natural selection” in which he wrote:
“… I was much struck how entirely vague and arbitrary is the distinction between species and varieties.”
We see this in the domestic dog [Canis lupus familiaris]. Domestic dog sizes, physical forms, coloration of fur (and lack of fur), behaviors and even intelligence and “personality” vary fantastically for a single species. Despite this, based on the ability of dogs to inter-breed between varieties, dog breeds, they are considered a single species. There are some practical problems with inter-varietal breeding (crossing various breeds) — Great Danes cannot physically breed with Chihuahuas — but if they did, the offspring would be viable.
As with the domestic dog, it is highly uncertain how many “species” as currently designated are truly “the largest group of organisms in which two individuals can produce fertile offspring” rather than simply local breeding populations that might be better described as “varieties” — such as varieties of sparrows, varieties of wolves, varieties of bears etc.
The question of speciation reversal becomes policy-relevant in light of the Endangered Species Act (ESA). The ESA does not use the usual biology definition of species (if one can call what is in use a definition at all) when designating “species” to be protected, Instead it uses something quite different, as explained in “The Meaning of Species under the Endangered Species Act”:
A group of organisms can be listed under the ESA only if the group constitutes a species. Although the ESA uses the term “species,” it does not use “species” in the common biological sense. In the field of biology, “species” refers to a taxonomic category consisting of “groups of interbreeding natural populations that are reproductively isolated from other such groups.”
In contrast, the ESA currently defines “species” as follows:
(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.”
I hope that you can see the problem this presents. Not only does the ESA allow Endangered Species designation for “reproductively isolated” populations, which may not actually be species in the stricter sense, in that, if brought together, they would interbreed with viable offspring. The ESA goes much further and allows the designation of “subspecies” — another word without a scientific definition — AND “any distinct population segment of any species”. This virtually allows the designation of nearly any small, isolated population of any vertebrate fish or wildlife.
For example, such a designation could be made for a particular lizard population isolated on one of the Channel Islands of California even though the species is an extremely common lizard found up and down the coast of California.
What do we do about Speciation Reversal and protections under ESA?
The first paper mentioned in this essay stated: “Preventing such extinctions will require that conservation efforts not only target existing species but identify and protect the ecological and evolutionary processes that generate and maintain species.” It demands that conservation efforts combat the forces of evolution itself — that somehow we must prevent designated species from interbreeding with…well…themselves which would force biologists to acknowledge that the species involved were not species at all, but only varieties of the same species.
There is at least one situation in which the ESA requires that biologists run a breeding program to cross-breed two separate species, Coyotes and Grey Wolves, to produce the species labeled the Red Wolf to “keep it from going extinct” — the Red Wolf is not really a species at all but a hybrid between two “species” that are probably biologically varieties of an overlaying Canis species. You see, in the past, when wolves and coyotes both roamed the lands east of the Mississippi, the coyote [Canis latrans] and the grey wolf [Canis lupus] interbred, producing a hybrid known as the red wolf [Canis rufus or Canis lupus rufus] which has not only been incorrectly named as a distinct species but declared an officially-designated Endangered Species. For more about this interesting story, see The Gray, Gray World of Wolves. While I don’t normally recommend Wikipedia for anything more than quick references, the discussion there on the Red Wolf question is pretty thorough — at least as far as demonstrating how inadequate our current definition of species is. Carl Zimmer at the New York Times wrote about this in 2016 in a piece titled “DNA Study Reveals the One and Only Wolf Species in North America”, and highlights Bridgett vanHoldt et al.’s finding that “Whole-genome sequence analysis shows that two endemic species of North American wolf [the Eastern Wolf and the Red Wolf] are admixtures of the coyote and gray wolf”.
When grey wolves were extirpated from most of the Eastern US, interbreeding slowed to a standstill, and there has been an apparent “species loss” due to the normal processes of evolution — not, however, the Golden Age version of evolution, where everything runs in one direction — not the Beatles version “I got to admit it’s getting better, little better all the time…”.
Of course, it isn’t that simple. Again, the Species Problem — biology does not have a standardized definition of species based on similarity or differences in DNA sequences either. Despite this fact, the new techniques in DNA sequencing and whole genome sequencing have prompted a flood of studies of the genomes of various species comparing them to related species and making pronouncements about the need to combine or split species. Like the paper mentioned above on North American Wolves, another recent paper declared the need to re-speciate giraffes.
If wolves keep inter-breeding with coyotes, we will end up with one big hybrid population, undifferentiated into separate species. Is that a “loss of species diversity” that threatens wolf species with extinction, thus making them all qualified for Endangered Designation under the ESA? What about the ravens in California, should Holoarctic Ravens and California Ravens be designated Endangered because of the past speciation reversal that brought about the Common Raven — and if this trend continues, they will all end up as Common Ravens, and we will lose two species. How would we protect the ravens and wolves and coyotes from themselves?
It is well-established that what brings about evolution — in either ‘direction’ — is change: genetic changes (either from normal genetic mixtures or genetic mutations), behavioral changes (such as mating and feeding preferences), spatial changes (displacement of species and introduction of species) and environmental changes (changing climates, changing biota, changing landscape, volcanoes, hurricanes, etc) . Some of these changes can be anthropogenic and others due to natural forces. It is, of course, possible that when local and micro-climates change it will affect local populations of animals — which may (or may not) result in changes in breeding patterns (etc.) which could be evolutionary in effect. We see that these changes can tend towards differentiation and speciation or, in the other direction, towards hybridization and collapse of two or more species or varieties into one, or the creation of what appear to be new varieties or species.
Bottom Line: It is fairly certain that Mankind cannot defeat the forces that drive evolution — which will run on, despite our efforts, in either direction.
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Author’s Comment Policy:
The topic of Speciation Reversal brings up so many questions that it is hard to focus on a direction for further discussion. My inclination is to let you, the readers, propose the follow-up questions and then we can jointly try to get some answers to those questions.
If you have a comment or question for me personally, begin your comment with “Kip…” and that will help me see it.
Darwinism and Evolution both tend to be “triggering” topics which evoke a lot of emotion and strong opinions. Let’s try to keep the discussion just to these two narrower topics: The 1) Species Problem, 2) Speciation Reversal and the ESA. Thank you.
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This may be a drastic oversimplification, but it seems to me that Speciation Reversal is an evolutionary process, so the complaint is that “evolution is happening!”
Of course, there would be a similar complaint if new species were replacing existing ones. Anything but stasis is bad.
TonyG ==> “the complaint is that “evolution is happening!”” — yes, precisely right!
Thank you, tty, for providing much good explanation and evidence.
(Sorry if I repeat others – long thread!)
Although this article is more open-ended and balanced than many, there’s still a gap between it and reality. The most glaring error is the idea that the ESA demanded crossing grey wolves with coyotes to get red wolves. (Where did this come from??? It’s nuts!) From a 2017 ESA document:
https://www.gpo.gov/fdsys/pkg/FR-2017-05-23/pdf/2017-10551.pdf
“In 1975, it became apparent that the only way to save the red wolf from extinction was to capture as many wild animals as possible and place them in a secured captive-breeding program. This decision was based on the critically low numbers of animals left in the wild, poor physical condition of those animals due to disease and internal and external parasites, the threat posed by an expanding coyote (Canis latrans) population, and consequent inbreeding problems. ”
Breeding with coyotes is seen as a problem, not a goal!
The genetic relationships among domestic dogs, coyotes and the various species/varieties of wolves is still not settled. A more recent publication about wolf DNA suggests:
“These analyses indicated that it was not coyote genetic material which led to the close genetic affinity between red wolves and eastern Canadian wolves. …The data are not consistent with the hypothesis that the eastern Canadian wolf is a subspecies of gray wolf as it is presently designated. We suggest that both the red wolf and the eastern Canadian wolf evolved in North America sharing a common lineage with the coyote until 150 000 – 300 000 years ago. We propose that it retain its original species designation, Canis lycaon ”
https://www.researchgate.net/publication/229189481_DNA_profiles_of_the_eastern_Canadian_wolf_and_the_red_wolf_provide_evidence_for_a_common_evolutionary_history_of_the_gray_wolf
From Kip’s article:
“The ESA goes much further and allows the designation of “subspecies” — another word without a scientific definition — AND “any distinct population segment of any species”. This virtually allows the designation of nearly any small, isolated population of any vertebrate fish or wildlife.
For example, such a designation could be made for a particular lizard population isolated on one of the Channel Islands of California even though the species is an extremely common lizard found up and down the coast of California.”
As I stated in another comment, this disregards the ESA definition of “distinct population segment” as well as ESA priorities, and suggests that the ESA actually might treat populations of a common species as list-worthy just because they are distinct, which is BS. Then there is the assertion that “conservation efforts combat the forces of evolution itself.” This doesn’t recognize that humans have already manipulated evolution through hunting and habitat change, and that’s the problem. The ESA is trying to rectify human influence on evolution, creating the conditions that will allow natural evolution to proceed.
The actual, practical consequences of the ESA definition of species have not been addressed. One indication is the way it’s treated relative to other concerns:
“In our priority system, the degree or magnitude of threat is the highest criterion, followed by the immediacy of the threat and the taxonomic distinctiveness of the species (monotypic genus, then
species, then subspecies, variety, or vertebrate population). The ESA gives no preference to popular species or so-called “higher life forms.””
It’s very important to distinguish those hybrids that have occurred naturally vs. those manipulated intentionally by humans. Part of one common definition of species is that they don’t normally hybridize >>>in the wild.<<<< (Even if there is a little bit if hybridizing, if it's unusual enough it doesn't detract from "species" status. For instance, grizzlies and polar bears are still clearly separate species even if there was a case or two of interbreeding.) This makes the distinction among species a little easier..
The fact that so many ecosystems have been modified by humans means that we have influenced evolution, both creating and tearing down barriers to hybridization.. This may happen through habitat loss, moving species around accidentally or intentionally, or enabling population to grow and spread outside their historical habitat. A combination of these human factor likely led to hybridization between red wolves and coyotes, and it is certain that humans contributed to the demise of red wolves in the wild.
Speciation is a process that happens over the course of generations, and the rate can be highly variable depending on many factor. Whether populations can interbreed is not always a good criterion for species separation. Think of the difference between domestic dogs and gray wolves. One is "man's best friend," one is a top predator capable of killing livestock and humans that can never (or very rarely) be tamed. They can still interbreed. Does that mean they are interchangeable, one is as good as the other? No – they fill completely different roles.
I've don't remember ever studying reverse speciation per se, but it doesn't strike me as odd. Introgression is just one factor in evolution among many.
It seems to me that this article makes some claims that don't really hold water. It is assertions like these that make people think the ESA is ridiculous, illogical and unquestionably a huge waste of money. I understand this view, but it's simplistic. People are very quick to judge things they don't completely (or even vaguely) understand. This is not skepticism, it is a sign of a closed mind.
We all know of things we pay for through our taxes that we don't support or don't directly benefit from. The childless among us pay for the schooling of those who have children. We pay for subsidies to corporations we don't support, sporting venues we never enter, infrastructure we never use. This is all part of living in a very large community we call a nation. There are sacrifices and benefits, and we all have to accept that no one gets all they want and no one deserves more than any other to get what they want from a government that is supposed to represent everyone.
There is intrinsic value in biodiversity, the loss of which can lead to economic and human health problems that laymen may not understand, much less foresee. It's very easy to decide one's position when one has little understanding of the topic.
Personally, I don't know if the red wolf is worth spending the resources to save. I simply don't know enough about the circumstances or the role of the red wolf in the ecosystem.
True skepticism recognizes one's own limitations, and scrutinizes one's own beliefs. True skepticism doesn't play favorites. True skepticism of any value is curious – it searches for the truth even if it may never come to decision about it. It is the search that is enlightening. Once one believes one has found the truth, the search ends and the mind closes to alternatives.
The internet has opened the door to more information that anyone can absorb at the same time it has contributed to a kind of tribalism that leads to the death of intellectual pursuit.
Rebel!
Kristi ==> You apparently didn’t read the background data on the Red Wolf — see my essay on it and the NY Times article.
I’m afraid you don’t understand the whole situation. The genetics of the wolves are still not clear – even vanHoldt admitted that. The sample size was small, IIRC. There has been new DNA evidence since that paper. The fact that there has been hybridization does not mean they aren’t different species.
It is not possible to have one meaning of species that works in all situations. There is no scientific basis for any set meaning. Genetics won’t work either.
At any rate, you say,
“There is at least one situation in which the ESA requires that biologists run a breeding program to cross-breed two separate species, Coyotes and Grey Wolves, to produce the species labeled the Red Wolf ”
That is just completely wrong.
“The Red Wolf breeding program breeds “wolves” that have everything from nearly pure Grey Wolf genes to pure coyote genes.”
Even your essay doesn’t support this. Wolves used for breeding were chosen for their wolf-like features. The idea is to get the coyote out.
At some point the two were separate, or there never would have been divergence. Humans changed that. The ESA wants to reverse the changes. Whether it’s worth the resources, I don’t know – that is not something we know enough to decide. Large ecosystem imbalances can result following the removal of the top predator, sometimes resulting in economic losses. For instance, deer populations might get too high and hamper forest regrowth. See? It’s not always a simple question of “just another species.”
Thank you for the tip regarding posting.
Kristi ==> The Red Wolf breeding program is doomed, and I think you already see why — you seem to have a good grounding in the genetics of evolution. They are trying to recreate and maintain a interspecific hybrid through introgression. The Red Wolves will not self-maintain themselves as a species as they require natural backcrossing — read up on why the Feds and the States disagree about the program. Heck, write an essay on it, but only if your will really dig in and get to the bottom of the problems of maintaining the Red Wolf, a hybrid, as a “species”.
Kristi ==> You are coming along nicely here. Try to comment to each comment separately, rather than in these long rambles addressing several other people and ideas. You’ll have better luck communicating that way.
The Red Wolf breeding program breeds “wolves” that have everything from nearly pure Grey Wolf genes to pure coyote genes. They know full well that the red wolf is a hybrid and not a species in its own right, yet they keep banging away at it.
The units doing the breeding do not like to look at it that way, but Zimmer reports on the conflict between agencies over the issue.
The Red Wolf breeding program is no different than a breeding program for a champion Cockapoo. (a dog resulting from a cross between a cocker spaniel and a miniature poodle)
The “problems” of speciation and speciation reversal strike me as much ado about nothing. It is nature’s method of continuing life. Nature doesn’t care, so why should we?
Clearly yet another damaging effect of climate change is to make climate scientists forget science. Here they are forgetting Dollo’s law of evolutionary irreversibility:
https://en.m.wikipedia.org/wiki/Dollo%27s_law_of_irreversibility
Or more likely they never bothered learning it on the first place.
Except that evolution is reversible, in so far as specific traits are concerned.
Chimp
For a trait, yes it happens all the time.
Birds become flightless, land vertebrates return to the sea, etc.
However Dollo’s law relates to the whole organism – all its traits.
Richard Dawkins expressed it well when he says that the probability of a whole organism returning to a former ancestral state in its entirety, is negligible.
The phenotypic phase space of an organism with thousands of genes and traits is practically infinite, so the chance of revisiting a point on a past trajectory is practically zero.
I suspect researchers here are confusing a ring species (such as herring gull – black backed gull) or a species continuum, with reverse evolution. The state “returned to” was always there, it never disappeared.
My favorite example of reverse evolution is mollusks in African lakes. The layers show evolution and reevolution of traits depending upon wet and dry intervals. Also the beaks of Galapagos finches.
But Dollo’s “Law” is simply a trivial artifact of genetics. If enough generations have intervened, then, yes, other stochastic or directional changes will have made it highly improbable for a population to show exactly the same allele frequencies as before.
The first rule of logic is that “the definition cannot change.” The second rule of proganda is “those that define the terms win the argument.”
Sounds to me like these three papers are anti-miscegenation.
Trevor ==> We don’t talk about “miscegenation” anymore…..apparently, mankind may have finally recognized that humans are all the same species.
Of course. But my point is, if you apply the definition of “species” used in the ESA to human races, one could make the case that, at least a couple thousand years ago, there were multiple SPECIES of humans. But that’s considered racist, so let’s just say there are “distinct population segments” of the human species. Segments that are arguably no less different from each other than the California Raven was from the Holarctic Raven, or the Gray Wolf from the Red Wolf.
The authors of these three papers lament the loss of species (or “distinct population segments”) due to “speciation reversal”, which is simply the interbreeding of two “distinct population segments” of the same species (biological definition), which, for purposes of the ESA are “different” species.
My point is, if you apply the same logic to the “distinct population segments” of the human species that existed a couple thousand years ago, when the races were still largely geographically isolated and rarely interbred, these authors would appear to come down squarely on the WRONG side of the “issue” of racial mixing. They would appear to be advocating, as did the Nazis and the KKK, for keeping the races separate and pure. Humanity now almost unanimously rejects that idea, and shuns the few extremists who embrace it. So why do we accept the same logic when it is applied to other species?
You’re right, humans are all the same species. In the same way that the “distinct population segments” of other species are all the same species.
What is it that makes some people believe that nature is an idiot that cannot be trusted to do what it has done so successfully for billions of years without their personal supervision? Perhaps they ought to learn the definition of hubris.