A new paper in The American Naturalist by Carl Rothfels and his colleagues (reference and free download below) shows that fern species that diverged in the very distant past can still form hybrids. That’s an interesting result, but I’m not sure about the authors’ conclusion that this revises our notion about how fast species can form in some groups, and I’m absolutely sure that the way creationists have distorted the paper’s findings is bogus. But let’s look first at the science.
The authors found a single fern in the French Pyrenees that looked as if it were intermediate in appearance between two distantly related genera of ferns, Cystopteris and Gymnocarpium. These genera were so diverged that they had been placed in different families. Suspecting that this individual (which was sterile) was indeed a hybrid between species in those long-diverged genera, they subjected it to DNA and chromosome analysis. (The hybrid individual was give the name “xCystocarpium roskanianum,” with the “x” denoting the likelihood that it resulted from a cross.)
Sequencing a single nuclear gene (gapCp “short”) in the parents, the hybrid, and many species in both putative hybrid genera, the authors wanted to see whether the hybrid had a mixture of genes from these putative parents, as we’d expect if the hybrid were indeed a hybrid. They also dated the branches of the tree using five genes that had been sequenced previously. The gene sequences were further used to construct a phylogeny, or evolutionary tree, of the two groups, so they could determine how long ago they diverged.
Finally, Rothfels et al. made meiotic chromosome squashes of the putative hybrid and of species in the parental genera, which showed how the chromosomes of the hybrid paired up during meiosis (the process of gamete formation). That could also show if the chromosomes of the putative hybrid appeared to be a mixture of chromosomes from the two parental groups, and if anything went wrong in the hybrids to make them sterile.
The results:
- The hybrid did indeed appear to have a mixture of alleles (gene forms) from the parental genera: it had four versions of the gapCp gene, two closely related to Cystopteris and the other two to Gymnocarpium. This verifies the putative hybrid as a genuine hybrid between genera. (The two Gymnocarpium alleles were themselves long diverged, indicating that one parental species was itself an “allotetraploid” produced by an ancient hybridization between two species within Gymnocarpium.) The similarity of these two gene sequences to that in a living species suggested that one parent was the cosmopolitan species G. dryopteris, which has 160 chromosomes. (Ferns have huge chromosome numbers!) The other parent was tentatively identified as a member of the European C. fragilis complex, which is a mixture of polyploids with chromosome numbers ranging from 168 to about 336 (it’s hard to count a lot of tiny chromosomes in a squashed three-dimensional cell).
- The chromosome analysis showed about 160 chromosomes in the hybrid, which is close to what one expects in a hybrid between these species (½ x 160 + ½ x 168 = 164). The exact number of chromosomes is hard to determine because of course chromosomes pair during meiosis (remember freshman biology?). Here’s a picture of prophase in the hybrid, when the chromosomes begin to pair up and are already themselves duplicated. Some of the chromosomes are paired, but others are not, which is expected if some pairing occurs between “homologous” chromosomes within one parental species’ genome but not between chromosomes from different genera.
Below you can see another picture of meiosis during metaphase, when the chromosomes move to the “metaphase plate” to divide into the daughter cells. You can see that some chromosomes are properly aligned here, but others are floating around in the cytoplasm. That means that there will be a willy-nilly division of chromosomes to each daughter cell that forms the haploid spore (fern reproduction is complicated; see here for more details). And the spore, which forms the ‘gametophyte’—the haploid plantlet that, producing sperm and eggs, produces a diploid plant—will not contain a full complement of genetic material. It’s as if a human male or female produced a sperm or egg that lacked several chromosomes: that individual would be sterile because any “zygote” that formed via fertilization would be inviable in the early stages. No fetus would be formed.
So although the hybrid fern was indeed a hybrid, and it was “viable,” because it grew into an adult plant, that plant was nevertheless sterile, and therefore a one-off anomaly that was not in itself a member of a new species. Further, because the two parental species were reproductively isolated, producing a viable but completely sterile hybrid, they are clearly members of different biological species and cannot exchange genes. .
- The calibrated molecular data show that two parental genera diverged roughly 58 million years ago (confidence interval between 40-76 million years). That is the oldest known divergence between any pair of species that can form hybrids. To show how long that is, whales and hippos diverged only 54 million years ago, and humans and lemurs about 74 million years ago. This observation is the equivalent, then, of getting a viable hybrid between hippos and whales, or between humans and lemurs! But there are a few cases of animals that are almost as long diverged producing hybrids. In our book Speciation, Allen Orr and I give the record for animals: a viable hybrid between the domestic chicken and the African helmeted guinea fowl, species that diverged abut 54 million years ago (the hybrid was anomalous, and there’s no record of its fertility). So it is possible for anciently diverged groups to form viable hybrids in animals, and possibly in flowering plants.
- Finally, by looking at the genetic divergence between forms of the gapCp gene in parental species and hybrids, we can judge how long ago the hybridization even took place. The genetic differences are very small, so the hybrid must have formed fairly recently. Even though it’s a single plant, it can propagate clonally through the growth of rhizomes, so it’s at least possible that the hybrid could have been around for a very long time. It wasn’t, though.
The authors hypothesize that ferns may be uniquely able to form viable hybrids in nature between long diverged species because of a feature of their reproductive system: sperm and eggs join simply through meeting in a film of water on the gametophyte, so there’s no need for the complicated forms of sexual behavior in animals, or the use of pollinators in many flowering plants. Those forms of fertilization raise the possibility of other forms of reproductive isolation that couldn’t evolve in ferns—for example, a difference in mating pheromones in animals or of pollinators in plants. This, say the authors, may slow down the rate of species formation in ferns because “genetic incompatibilities may form more slowly.” In other words, if some of the features that cause reproductive isolation in other species can’t evolve in ferns, then ferns may speciate more slowly.
The problem is that while the hybrid fern is viable, it’s also completely sterile, so there are at least enough incompatibilities (perhaps in chromosome number) to make the parents full biological species. Thus, we don’t yet have enough evidence to show that the process of species formation is slower in ferns than in other species. Perhaps the evolution of “prezygotic isolation” (reproductive barriers arising before fertilization, like ecological or sexual isolation) is slower in ferns than in other species, but we don’t yet know how much of a contributor such isolation is to speciation, and we certainly don’t have enough data to draw general conclusions about the rate of species formation from a single hybrid.
Nevertheless, the paper is a good one, documenting with high confidence the formation of a viable hybrid between anciently diverged species, and producing a Guinness-style record for ability to form viable hybrids. (It’s a pity that Guinness doesn’t confer records like this!)
*******
Over at the Baptist Press (the news site of the creationist Southern Baptist Convention), Discovery Institute hack Casey Luskin used this paper to argue that the ferns violate the theory of evolution. The article is called “New fern discovery a ‘problem’ for evolutionists,” Needless to say, there’s not a problem. The entire paper of Rothfels et al. fails to mention any problem with evolutionary theory. What they found, after all, was simply a surprising ability to form viable hybrids between long diverged species. But that doesn’t at all “fail to affirm the evolutionary paradigm,” as Luskin claims, for evolutionary theory says nothing about how long species can be separated and still form viable hybrids. And remember, these are still distinct parental species—the hybrids are viable but (to use a phrase from drosophilist Duane Jeffrey) are “as sterile as crowbars.”
Here’s another criticism by Luskin:
The researchers concluded the fern’s discovery implies the diversity of species that exists today may not be wholly accounted for by adaptation, but may also be the product of varied rates of the speciation clock, the rate at which a type of organism develops reproductive incompatibility with other related species.
This is completely false. The authors simply suggest that the “speciation” clock may run more slowly in ferns than in other species, though I suggest we need more data before concluding that. But even if it did, that wouldn’t constitute a violation of the “evolutionary paradigm”, since we expect some groups to speciate at different rates. Orr and I talk about this in our book. If speciation is, by and large, a byproduct of adaptation, and different groups adapt at different rates (again, completely expected), then of course different groups will form species at different rates. In the last chapter of Speciation, we deal with this exact issue, analyzing what selective forces may lead to different rates of speciation.
Nowhere do Rothfels et al. suggest that speciation may not be accounted for by adaptation. What they do suggest is that the adaptations of different groups (i.e., the reproductive system of ferns versus other taxa), may make them speciate at different rates. This may be a form of “species selection,” in which the diversity of groups, a higher-level property, depends on their lower-level properties. Orr and I discuss this issue in our last chapter, but it’s certainly no flaw in Darwinism, for it’s possible for natural selection to operate on different levels of the biological hierarchy.
Finally, Fazale Rana, a “biochemist” at Reasons to Believe (Hugh Ross’s old-earth creationist organization) says that the new paper “attests to creationism in two ways”:
- “First, ‘it represents a problem for evolutionists because it shows there are things being discovered that fail to affirm the evolutionary paradigm,’ Rana said. ‘You wouldn’t expect that hybridization to take place. It’s the work of a creator used to create a novel organism.'”
As I said, there is no a priori expectation on how fast hybridization can disappear over time, so it’s not a problem for evolutionists.
- “Second, a species that can adapt to the environment and give rise to a sister species demonstrates God’s design in giving living things the ability to respond to changes in the environment.”
We already have a way for species to respond to changes in the environment without the need for a god. It’s called “natural selection.” Rana appears mired in the pre-1859 era—the last time that rational people saw design as prima facie evidence for God.
h/t: Bob F.
Sub
This is a very cool paper, although it would have been nice to see them sequence more than a single gene. Thanks for the writeup, Dr. Coyne.
The creationist response to this was, of course, laughably pathetic.
Here’s a takedown of Luskin from 2009.
LinkText
or this LinkText
Am I interpreting Fazale Rana correctly? God used this opportunity to create a sterile fern from other ferns? What a bummer! With all the sex slavery and homicide and childhood cancers in the world I would hope sterile ferns we much lower on the to do list.
What’s the difference between a mule and a creationist? One is a stubborn animal much like a jackass, and the other is the sterile offspring of a male donkey and a female horse.
I don’t know if there is a word for this, but is it possible ferns are a flexible? malleable? primitive? sort of organism and that the organization of their genes is a factor in their strangeness? I ask because they have always struck me as sort of alien and creepy, the way their branches unfurl and just sort of live on in my garden as long as there is shade and a smidgeon of water. Not quite a weed, and obviously not an algae or a fungus, just sort of Jurassic and creepy.
They are seedless vascular plants, descended from early plants that never evolved seeds. Their sperm still has tails (which ‘higher plant sperm lacks), and fertilization requires conducting the sperm to the egg in water, like an amphibian. That is one of many examples of where they are ‘primitive’ as far as plants go. I know quite well that saying things are primitive is a bit loaded.
Pre-Jurassic, really.
I think ferns are beautiful, but as a NZer I’m probably biased.
Oh so do I. And I appreciate the lack of care involved in keeping the dark places green in my yard. Not sure why they creep me out – maybe forgotten childhood trauma.
“It’s the work of a creator used to create a novel organism.”
Bit of a slip-up on the creator’s part to make it sterile, though.
I admit that I haven’t read the paper, but who in this day and age draws phylogenetic conclusions from a single gene?
They’re just using that one to show that the hybrid is a mixture between the genomes of two other species. Given now long diverged the parental species are, that’s probably good enough, although of course the more genes the better.
I was ready to make that criticism but then read on: calling it a ‘single gene’ might be a little misleading, considering they obtained four different sequences for the one locus in the hybrid.
I’m thoroughly impressed with the technology available to biologists. Comparing genes is just so much more satisfying than morphology alone. Good work.
Amazing how quickly the creationist is to publish the stupidity. Most humans, like me just say stupid things but they like to get it out there. Reminds me of republicans.
A lot of people are like farmers: they see (or create) a pile of manure and can’t resist spreading it.
Disclaimer/qualifier: the USDA considers me a farmer (wheat/canola), so I feel a little less inhibited in casting this aspersion.
Said people won’t be happy with this development.
That is good news. So when I do a Google search about evolution or about GM crops I wont get a ton of crap near the top. Excellent.
In this sentence below illustration B,
“Further, because the two parental species were reproductively isolated, producing a viable but completely fertile hybrid, they are clearly members of different biological species.”
Should that be “infertile hybrid” ?
Yep, I’ll fix it.
Could the fact that each parent species is already tetraploid (and probably itself the product of a hybridization)explain the viability of the hybrid ?
Maybe. But interspecies hybrids can be viable from non-polyploid parents.
I remember that about 80% of fern species are polyploid.
I read about these ferns awhile ago, so this isn’t news to me (well, the cretinist, er, I mean creationist views are) but I will say that, as usual, Prof., your explanation of it made much more sense that the one I read. You have a nice way of simplifying without dumbing down.
I totally agree about Jerry Coyne’s posts on science topics. I consider this website a great resource.
Curious phrase. I’ve never thought of crowbars as particularly sterile…indeed, if I were ever to have a close encounter with one, one of my first thoughts is going to be of tetanus….
b&
Speaking of hybrids between long-diverged species, some vertebrate examples were discussed at this Tetrapod Zoology post a few years back, mostly in the comments.
So cool! This is all so very interesting, Professor CC, and also a coincidence for me. I was discussing the subject of highly divergent species producing viable offspring just before this hit the news, but it was about domestic cats and if they could produce viable offspring with any “wild” cat species like lynxes or bobcats.
Based on what I’ve read, domestic cats can’t reproduce with any “wild” cat species, though some sources disagree. After researching this, the question in my mind shifted to: “which are the most highly divergent species of any plant or animal capable of producing viable offspring?”.
So this recent news and your post helped answer this question for me, leading me to abandon my previously solid pro-evolution stance to embracing creationism(kidding!). Will definitely need to reread this post and links to fully digest it. I also totally appreciate the creationists providing comic relief well off on the sidelines of the sidelines real science.
I’m surprised that no domesticated cats can produce viable offspring with wild cats. I guess it’s because I lump d*gs and cats together, and since d*gs can create offspring with wolves and coyotes, I figured cats could too with their wild relatives. Perhaps dogs diverged more recently than cats? Must google.
They descended from a species of N. African wildcat, and they are hybridized with other wildcats. The Bengal cat is hybrid between domestic cats and a species of wild cat, if I recall.
Aha…thanks for the clarification Mark.
It seems to me, naive as I am, that these ferns are an argument against separate creation, not evolution. Clearly the two hybridizing species must belong to the same kind, as it would be weird if separately created ferns would be genetically similar enough to hybridize. Even baraminologists have used hybridization as evidence for common ancestry (within kinds, of course).
Luskin? Who cares when he haqs to say about evolution or genetics. He’s a Lawyer. why did I even capitalize that. His curriculum vitae would be more relevant if he listed 5 years at pit boss at Jiffy-Lube.
we need to stop even mentioning fools like Luskin.
YECs are always proclaiming that this or that ‘unusual’ finding is a catastrophe for Darwinists, or whatever pointy word they want to call us. ‘Evolutionists are baffled’. ‘This is a discovery that Godless scientists do not want you to know’.
This latest example is an example of where they think, deep down, that evolution should be somehow regulated along a progressive, clock-like schedule (overseen by what?). X amount of time should mean X amount of change, or else there is a crisis for evolution.
The earliest fern fossils are found in rocks that are 200 million years older than the earliest flowering plant fossils. Wingless animal fossils are in older rocks than winged animals; and so on.
It’s interesting how quick creationists are to jump on an interesting discovery such as this hybrid sterile fern. I guess either they forgot the fossil record or they’re hoping others pay no attention to it.
Thanks for this interesting paper- truly amazing. The only thing that would make it less amazing would be if G*d did it.
Some influential fundamentalist thinkers [sic] have advocated the creation of species by hybridization. The process can be made Scripture friendly by attributing hybridization to the Devil, and hybridization hurries up what happened after Adam and Eve to help paleontology fit with Biblical chronology. An interesting case is given by Robert L. Numbers (2004, p. 88, in Darwinian Heresies, Cambridge) concerning Adventist “Flood Geologist” George McCready Price who, following prophetess Ellen G. White’s idea that “[t]he confused species which God did not create . . . were the result of amalgamation,” proposed in a letter to Martin Gardener (of Scientific American fame) that (paraphrasing Numbers) Satan himself, “the great primal hybridizer,” had been “the real instigator of all the mixing and crossing of the races of mankind . . which God designed should remain separate.” Pretty disgusting stuff I’d say.
The problem with that (OK, one problem with that) is that there are two fairly clear rounds of whole-genome duplication — and that’s hybridization — in vertebrate history. So all vertebrates are the product of Satan.
Does whole-genome duplication necessarily involve hybridization? I haven’t seen that claim before. Sure, there are lots of known cases of allopolyploidy, but autopolyploidy can also occur. (And I just wanted to type those words)
OK, forget “necessarily”. Allopolyploidy is just much more common. This long after the fact, there’s no way to tell.
Is this quantified anywhere? I’m not seeing it in the abstract, and the paper itself is behind a paywall.
Suppose for the sake of argument that the hybrid had been around for a long time and occupied a fairly extensive range. At what point would we be comfortable calling it a new species? There are after all recognized species of lizards that originated by hybridization and reproduce asexually.
I am puzzled by this discovery, as I was sure that hybridization between distant species was already known to be somewhat commonplace among ferns. Am I completely confused? Or is this simply the latest (and greatest) example?
I’m reminded of a duck, paper produced talked about certain avian species fulfilling the same niche as ducks in the past. ICR took that and ran with it till half way through their piece when duck like changed to duck.
http://www.icr.org/article/5522/ and the actual article it was based on
http://www.scientificamerican.com/article/winged-victory/
Also another example of cross breeding in plants is Spartina anglica. A really cool plant which I always used to annoy creationists.
The duck in question is Presbyornis or at least a presbyornithid. It’s an odd, long-legged sort of waterfowl, but it’s more closely related to typical ducks than is the living magpie goose (Anseranas). Whether it’s a duck depends on your arbitrary definition of the term. It looks like a duck, but being so long-legged it didn’t walk like a duck, and even very few ducks quack like a duck (it’s a mallard thing, mostly).
The SciAm article is paywalled, but the ICR one isn’t. The ICR article is scary stupid. But I note that they cite me; thanks?
You’re welcome? I don’t know if that puts up the citation count of the paper, if not you can always have pie.
I think they do the same thing as DI and AIG. So I wouldn’t enter into email exchange as they claim any email sent to them as their intellectual property to do with as they please. So selective publication etc. would be possible.
Just the misuse of a publication reminded me of this particular article in ICR which did the same thing.
When I was more actively involved with arguing with creationists online this was thrown at me as proof that science agrees with the creationists. Poor guy really regretted it when I pointed out that the original had little to nothing to do with what ICR had said.
Jerry, you wrote “Finally, Rothfels et al. made meiotic chromosome squashes of the putative hybrid and of species in the parental genera, which showed how the chromosomes of the hybrid paired up during meiosis (the process of gamete formation).”
However, as alluded to in some of the bullet points, gametes in plants are produced by haploid gametophytes. Haploid gametophytes necessarily produce haploid gametes by mitosis, not meiosis. That plants produce gametes by mitosis, not meiosis, is true for plants generally. In plants, the process of meiosis produces spores, not gametes as in animals.
Rather than violating the theory of evolution, does this example not satisfy the likes of Ray Comfort’s demands for evolution in real time?