How important is hybridization?

September 16, 2010 • 7:36 am

Because I work on speciation, I’ve been inundated with emails this week (okay, really just one email) asking about Sean B. Carroll’s New York Times piece on hybrid speciation, “Hybrids may thrive where parent species fear to tread.”  I won’t summarize it here because it’s short and you can read it yourself, but the upshot is that “hybrid speciation” may be much more important than evolutionists believe.  I think Carroll’s probably wrong, and wanted to say why.

The topic of hybrid speciation comes up surprisingly often. Indeed, when I talked to a group of high school kids in Houston via Skype this Monday, one of them asked if a new species could arise from mating between two other species.  There are two ways this could happen.  The first is simple fusion: members of two different species hybridize, and their gene pools fuse back into a “new” species, supplanting the parents.  This probably happens pretty often in evolution, since geographically isolated populations that are at least on their way to becoming different species might lose their geographic isolation in the fullness of time.  This may be what is happening now with polar bears and grizzly bears.  Their ranges are changing, probably due to global warming, and they’re coming into contact.  Although they’re considered different species (they were previously limited to different areas because of their differing ecologies), hybrids—called “pizzly bears“—have been found in the wild.  Here’s one:

Fig. 1.  A slaughtered pizzly bear with patches of brown on its fur.  Its hybrid status was confirmed by DNA testing (from National Geographic.)

We’re not sure whether polars and grizzlys will fuse into one species in the next few centuries, or whether hybrids will remain only a sporadic occurrence.  But certainly fusions like this have occurred over evolutionary history.

But there’s another way a new species can form by hybridization.  Two species can hybridize, and a few individual hybrids can give rise to a third species that is distinct from the two parents and can coexist with it.  This depends on hybridization being more than a once-in-a-lifetime occurrence, and the hybrids sorting out the two parents’ genomes into a new, genetically different population that can’t reproduce well with its two parental ancestors.

This seems to occur fairly often in plants through a process called polyploidy, where an inter-species hybrid doubles its chromosome number.  I describe this in WEIT, and won’t repeat it all here.  This form of hybrid speciation may be responsible for as many as 4% of new plant species.

But the form of speciation Carroll discusses is called diploid hybrid speciation, which doesn’t involve doubling of chromosome number.  Two species simply hybridize, and the semisterile hybrid then sorts out the parental genes and chromosomes into a new genome that is reproductively isolated—and ecologically different—from those of its two parents.  Carroll describes a few cases, mostly in plants, and concludes:

The discovery of hybrid species and the detection of past hybridizations are forcing biologists to reshape their picture of species as independent units. The barriers between species are not necessarily vast, unbridgeable chasms; sometimes they get crossed with marvelous results.

I work on speciation, and although we’ve known about these cases for a while, I haven’t reshaped my picture of species—nor have many others.  The problem is that Carroll’s few examples—three sunflowers and a fly—pretty much exhaust the known cases of diploid hybrid species (there’s a North American butterfly, too, but he didn’t cite that one).  And it’s not that biologist haven’t looked for hybrid species: in many groups we have, and simply haven’t found them.  There are several thousand species of the fruit fly Drosophila, for example—it’s probably the most heavily-studied group of animals on Earth, at least from a genetic point of view.  And not a single species is a hybrid between two others.  Indeed, we have only a half-dozen or so cases of any interspecific hybrids at all being seen in the wild, and almost all of those are not only one-offs, but the animals are completely sterile (and hence evolutionary dead ends that cannot produce new species).

Ditto for birds. It’s been estimated that 10% of bird species are known to hybridize, but this should not be taken as showing that hybrid speciation is important.  For one thing, most of those hybrids are one-offs, very rare occurrences that don’t lead to anything.  And not only are they rare, but they are probably either physiologically sterile or unable to find mates.  Finally, we know of not a single bird species that is a diploid hybrid between two others.  Diploid hybrid speciation in animals is likely to be quite rare, and certainly not something that’s going to “reshape our picture of species.”  That’s not to say it’s not interesting, but just that it’s probably not very common—and hence not world-shaking.

My friend Loren Rieseberg and his colleagues discovered the three diploid species of sunflowers, and that work is deservedly famous.  But even he admits that hybridization between plant species in nature is rarer than most people suppose, and that diploid hybrid species of plants are quite uncommon (although polyploid hybrid species are not).

Fig. 2.  The diploid hybrid species Helianthus anomalus studied by Rieseberg and his colleagues.  The species derives from hybridization between H. annuus and H. petiolaris. Its novel combination of genes enabled it to colonize sand dunes where the parental species can’t live.

At the end of his piece Carroll cites the recent discovery that the genome of modern humans carries a small amount of DNA from Neanderthals.  This suggests (although there is dissent) that humans coming out of Africa in our most recent migration, about 60,000 years ago, hybridized with Neanderthal populations that derived from an earlier migration.  But this says little about diploid hybrid species, for it’s merely hybridization between different human populations. (Recall that both Neanderthals and the ancestors of modern humans are both considered members of the species Homo sapiens.)

The entire recent history of our species involves hybridization between previously isolated populations.  Those populations may have been on the way to becoming different species, but they never got there because they weren’t isolated long enough to become reproductively incompatible, and our species also devised means of transportation that linked the populations together, causing interbreeding.  Bit by bit, the human gene pool is melding.  About 20% of the genes in African-Americans, for example, came from matings with whites after they were brought from Africa as slaves.  This is precisely the same phenomenon observed in Neanderthals/modern humans, except that those populations had been isolated a bit longer.  It’s still “hybridization”, but it’s of no relevance to speciation.

Hybrids continue to fascinate both biologists and laymen alike. I’m not sure why—perhaps because they’re seen as a violation of the “natural order.”  And they are certainly interesting, and of some evolutionary importance.  But for the nonce I think biologists go overboard when claiming that hybridization will completely revise our view of nature, or of evolution.

50 thoughts on “How important is hybridization?

  1. Hybrids continue to fascinate both biologists and laymen alike. I’m not sure why—perhaps because they’re seen as a violation of the “natural order.”

    No, it’s because a spider crossed with an elephant is totally awesome.

    Not to mention a gorilla crossed with a scorpion, a crocodile crossed with a porcupine, and a frog crossed with a shark.

    So… get on that, scientists!

  2. In vertebrates, parthenogenetic species usually (if not always) trace back to some sort of hybridization event. For those who want to understand the origin of that means of reproduction, the answer would be that hybridization is incredibly important.

    1. Indeed. I forgot to mention those, but parthenogentic populations aren’t really “species” under the biological species concept because they’re not a group of interbreeding populations–there’s no interbreeding. They’re mor like clones. But they nevertheless represent evolutionarily significant events.

      1. The biological species concept was created for sexually reproducing organisms, so it cannot be used for the asexual organisms in a strict way. However, since these parthenogentic populations are reproductively isolated from both parental species, according to part of the biological species concept, they are justified to be called different species, particularly if these populations also possess distinct morphological, ecological, or other phenotypic characters.

    2. Beat to the punch. The best examples of this are the several lineages of parthenogenetic whiptail lizards in SW N America. The hybrid nature of these clones was worked out decades ago because–incredibly–the parental species often have very different karyotypes.
      There are suggestions out there that these lineages are generally recent–a few hundred years old–and may have resulted from habitat change from overgrazing.
      Very cool stuff.

      1. Which came first – a lizard with eggs but no sexual reproduction or the lizard with eggs and sexual reproduction?

  3. I live in Calgary, Alberta and back in the early 20th century people transplanted Brook Trout (an Eastern species) into some of our local streams. They frequently breed with native Bull Trout and a local biologist confirmed that some of offspring are fertile and have produced new generations of Brook/Bull hybrids. New species?

    1. What do you get when you cross Ray Comfort with a banana? … Kirk Cameron! If you don’t think that’s funny, don’t blame me – it’s god’s joke.

      I’m just waiting for Ray to say that there is no crocoduck hybrid therefore god is true and not evolution.

      1. More likely, Comfort will be saying “Darwin was WRONG because Sean Carroll says so in the New York Times. Therefore, Jesus.”

        I stay away from deliberately dishonest preachers, so someone else with a stronger stomach than me will have to check and see if that prediction comes true.

          1. (if this is a duplicate post I am sorry, my internet has mad cow disease)

            Crocoduck here too. All Hail Crocoduck.

  4. There are examples of hybrids in Whales – an article from MARINE MAMMAL SCIENCE, 14(1):82-98 (January 1998)by Berube & Aguilar decsribes a Fin/Blue Whale hybrid & looks at 5 others in the literature, but that was found in 1984, presumably under great population stress due to whaling reducing potential mates of the same species. I think it is unknown if they were fertile.

    Some interesting gamebird hybrids on Wikipedia including a pheasant/chicken –

    Human action is no doubt behind quite a few of these…

  5. “Hybrids continue to fascinate both biologists and laymen alike.” I’m not sure why either, but taking a cue from the birding community here in the US, the prevailing attitude seems to be that the ‘good birders’ find hybrids, so everyone goes looking for them in the hope of someday being considered one of those ‘good birders’. As anyone can imagine, this leads to serious overreporting of hybrids, and a complete ignorance of other factors that might make a bird appear different from the pictures in the book. You know, like that cornerstone of evolutionary theory: intraspecific variation. If the bird looks a bit ‘off’ then it must be a hybrid.

    I routinely bird with several different professional biologists, and, predictably enough, they rarely even mention the ‘h-word’ until they’ve thoroughly investigated every other possibility.

  6. I got into an argument with a friend over the whole neanderthal/Homo sap sap cross breeding issue. He was over the top stoked about the impications. My response was…neat, but really, so what?
    It obviously happened early enough from the divergence that you really couldn’t say that speciation had occured. Well that was my take. He swears that they would have been entirely different species at the time of mating…Or at least a sub species (I hate that term, how on earth is it quantifiable in any meaningful way?)Reproductively viable offspring is pretty much the only “proof” you need. I think I’m right on that…

    1. From what I know I’d have to say that speciation occurred a long time before either species arose.

      Home georgicusH. ergasterH. antecessorH. heidelbergensisH. neanderthalensis.

      homo georgicusH. ergasterH. erectusH. RhodensisH. sapiens.

      Of course, we could be talking about sub-species of Homo ergaster

  7. This is precisely the same phenomenon observed in Neanderthals/modern humans, except that those populations had been isolated a bit longer. It’s still “hybridization”, but it’s of no relevance to speciation.

    Point taken, and maybe I’m mistaken, but I read the logic of that to be defining the complement set, i.e. there is some relevance to speciation by defining what a viable population is.

    1. @Torbjorn.

      ” there is some relevance to speciation by defining what a viable population is.”

      Hadn’t thought of that!

  8. A bit off topic but, how can you identify, out of thousands of base pairs, a segment of DNA that is “Neanderthal”? I’m not implying that it can’t be done but how is it done. Any books? Websites?

    1. The link in the article leads up to that. The short version is that “Neandertals shared more genetic variants with present-day humans in Eurasia than with present-day humans in sub-Saharan Africa”.

    2. Green RE, et al. Analysis of one million base pairs of Neanderthal DNA. Nature. 2006; volume 444: pp. 330-336.

      There are others; but that one is available for free download (somewhere, I’m not sure where I got my copy).

      Cool stuff.

  9. I agree that Carroll overstated the importance of hybridization leading to a third new species. But I think that Jerry tends to understate the importance of gene flow between species.

    A good example is equating hybridization between H. sapiens sapiens and H. sapiens neadertalensis with hybridization between African populations and White populations today. I agree that the latter is between two sub populations of H. sapeins sapiens, but the difference between the populations in the former was much greater (~100,000s of years vs ~10,000s of years, which is a big difference for a mammal). Whether they were actually separate species or not is more of a subjective call than you’re implying.

    Closely related species probably hybridize all of the time at low rates. The resulting hybrids may be sterile, and even if they aren’t sterile, they may be less fit. But if they aren’t sterile, they probably are a source of a small level of gene flow between separate species. If the gene flow is kept to a minimum, the populations can stay separate or further diverge. If not, previously separate species may converge again.

    I don’t know of examples of this, of course, but I’m sure it’s more common than we know. Heck, we don’t even have a good appreciation for the numbers of cryptic species that are out there. This may be happening right under our noses all the time, and we just don’t appreciate it because they look to us like the same species.

  10. Our research group does a lot of work on hybrids; escaped domestic animals, flying squirrels, Canids etc. The Canids are a pretty interesting story in this respect. Due to over-harvest and predator control eastern wolf populations (smaller than grey wolves with food habits that reflect that smaller size) in our region were decimated. Following the coyote range expansion eastern wolves and coyotes hybridized. On top of that these hybrids will also mate with grey wolves too creating a 3 species hybrid swarm. No pure eastern wolves exist but there are still large portions of their genome floating around within animals in this population. Within this Canid soup it now seems that we have 3 somewhat reproductively isolated populations (a syngameon). There are eastern wolf x grey wolf animals in the north eating primarily large prey like moose, eastern wolf x grey wolf x western coyote hybrids in the central region which seem to be evolving to fill the ecological role once played by the eastern wolf in taking medium sized prey, and we have an eastern coyote, which is a new term as there were never coyotes in the east, that are eastern wolf x coyote hybrids.

    So that’s 3 morphologically, genetically, and ecologically distinguishable groups, each of hybrid origin, that have formed in the absence of hard geographical barriers to gene flow. There certainly still is gene flow although it seems that some pre-mating barriers are beginning to form as new hybrids (hybrids of the new hybrid group) are rare. Proto diploid hybrid speciation perhaps? Fun stuff that is difficult to sort out and taxonomic designations of the groups are also a bit contentious.

      1. Dogs are domesticated grey wolves as the latin suggests. Really tough to tell the difference genetically in many ways. But yes there are some bits of dog dna in there too!!

    1. Colin, where to ‘Red Wolves’ come into the equation? Are they really just a sub group (subspecies) of Canis lupus?

  11. There is also the interesting case of the Amazon Molly (Poecilia formosa)- an all female species resulting from a probable cross between a female Poecilia mexicana and a male Poecilia latipinna. They require sperm from related species to “fertilise” eggs, but there is rarely any genetic contribution from the male.

    More annecdotally, I keep cichlids from Lake Malawi and have the odd hybridisation problem. I have hybrids that have a lineage like Labidochromis caeruleus X Labidochromis perlmutt. The offspring of which have hydbridised with female “Maylandia” emmilitos, “Maylandia” zebra (Chilumba) and “Pseudotropheus” perspicax (Ndumbi). They just keep throwing out fertile hybrids (needless to say, I don’t release the hybrids into general circulation to contaminate “true” species). Again, anecdotally, my local supplier tells me that Lake Victoria Cichlids are hard to come by as they hybridise like crazy and the are few pure lines. Given the rapid recent radiation of cichlids, I wonder if this would be a good place to look for naturally ocurring hybrids.

    Hybrids are also cool because they ignore Leviticus 19:19 – the little sinners 🙂

  12. I have read three of Sean B. Carroll’s books and enjoyed them and learned from them. I am surprised that he said “The discovery of hybrid species and the detection of past hybridizations are forcing biologists to reshape their picture of species as independent units”.

    I hope he responds to this post.

  13. Mating between moderns and neanderthals was a lot less than it could have been given that the two lived side by side for tens of thousands of years. Both moderns and Neanderthals likely originated from a species close to H Heidelbergensis, but for moderns this happened in Africa and for Neanderthals in Eurasia. It likely means that hybridization is a rare phenomenon once such species start to diverge.

  14. Homoploid hybrid speciation is also known in freshwater fishes, most notably the Virgin Chub Gila seminuda and claimed (controversially) for the Red Wolf Canis rufus.

    If we scale out to evolutionary time scales the fact that such occurrences have only been detected rarely within living populations does not necessarily discount hybridization as an important factor in speciation, even if it is ancillary to more ubiquitous factors.

    While de novo speciation via hybridization may not be known in birds (which is important since they hybridize a lot) the Grants have argued that hybridization and introgression among incipient species of Galapagos Ground Finches may be (rather paradoxically) an important factor in promoting speciation within this radiation.

    The marked differences we see in the frequency and nature of interspecific hybridization between plants and animals or even among different groups of animals makes me wonder if we should not be extremely cautious in trying to generalize the situation observed in any particular group (whales, fruit flies whatever) to eukaryotes as a whole…

  15. Speciation through hybridization may have occured in birds. The Pomarine Jaeger Stercorarius pomarinus might have speciated via hybridization between the Great Skua S. skua and some other Jaeger (cite).

    1. True, Rob. The Pomarine Skua remains an enigma. But I note that the authors investigated three hypotheses regarding the possible source of that species, only one of which involves hybridism (between Catharacta skua and some unspecified Stercorarius sp.). Furthermore, their fairly eclectic (there are a lot of authors, with differing specialties) study failed to eliminate any of the hypotheses, all of which they termed ‘far-fetched’.

      Ultimately they concluded not to conclude, and recommended further study of the issue. This did lead to a handful of other papers, but interest seems to have died out–or, at least the publications largely dried up–in about 2000. The Pomarine Skua (Jaeger) apparently will remain enigmatic for now.

  16. Jerry do you know much about about Rift Valley Cichlids – ie fishes from Lakes Malawi, Tanganyika and Victoria?

    They seem to hybridise at the drop of a hat – much to the consternation of hobbiests who keep them.

    There has been some research on the subject see here:

    It’s all a bit impenetrable to the lay person though 🙁

  17. I’m curious to know why you do not include the butterfly Heliconius heurippa in your list of species arising via homoploid hybrid speciation.

  18. I am not qualified to add anything to the scientific discussion. But hybrids are, well, fun to talk about!

    We have a public health club and a paved walking/running path near the Illinois river where many geese congregate. Most are the Canadian type, but on occasion you’ll see some interesting hybrids (geese with some snow goose traits and some Canadian ones)

    Here is a post on that topic (photos are way better than mine)

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