A tale of two crows

A famous puzzle about classifying animals involves the abutting distributions of the hooded crow and the carrion crow in Europe. The two crows are considered members of different species, Corvus cornix and Corvus corone, respectively, and have been classified that way because they not only have different color patterns, but tend to mate with others of like pattern, as well as differing in their dominance behavior. Here’s what they look like:

Hooded crow:

Hooded Crow ok

Carrion crow:


4517854764

The crows’ distributions abut abruptly along a line from north to south through Europe—the red line shown in the picture below, taken from the second reference at the bottom of the page. The caption gives information about where the birds were captured (one taken from each locality) and the genetic relationships between them:

Screen shot 2014-07-24 at 3.45.29 AM

Speciation battleground. On either side of the narrow hybridization zone (dark brown), the carrion crow (Corvus corone) (dark area) and hooded crow (Corvus cornix) (pale area) ( 2) maintain their marked phenotypic differentiation, despite apparent lack of genetic differentiation. Genome-wide admixture analyses (inset at bottom) show that German carrion crows most closely resemble (80%) hooded crows, and are quite distinct from Spanish carrion crows. Sampling sites for the present study ( 6) are shown as circles. Sp, Spain; Ge, Germany; Po, Poland; Sw, Sweden.

Yet they are otherwise very similar, and do hybridize from time to time, so some biologists have considered them subspecies rather than species. The “biological species concept” (BSC) uses the idea of reproductive isolation as the criterion of species distinctness: if two groups inhabit the same area, but do not produce fertile hybrids (i.e., are “reproductively isolated”), then they are considered separate species.

Under that criterion, the presence of some fertile hybrids between these groups means that they aren’t strictly “species” according to the BSC, but in our book Speciation, Allen Orr and I use reproductive isolation as a relative criterion: the more two populations are reproductively isolated from each other, the more “species like,” they are, up to the point where there is no genetic interchange, at which point they can be called “good biological species.”

Under this criterion, these two crows are “species-like” but not “good species,” and how you name them becomes somewhat arbitrary.

But how much genetic difference is there between the groups? If there are extensive genetic differences, spread throughout the genome, that suggests that although there is interbreeding, the genes from one species don’t become incorporated easily into the genome of the other’s, so reproductive isolation is stronger (perhaps hybrids can’t find mates, or don’t survive as well as the parents). On the other hand, if the groups differ by only a few genes, one could more easily see them as subspecies, or “races,” or “ecotypes.”

To resolve this question, a large group of researchers used a number of scientific methods, including sequencing of most of the genome as well as looking at differential expression of the genes, in both groups of crows. The results, by J. W. Poelstra et al., were published in a recent issue of Science (reference and link below, but no free download), and were highlighted in a “news and views” piece in the same issue by Peter de Knijff (reference and link also below).

Here are the salient results, and I’ll try to be brief:

  • The hooded and carrion crows were barely different genetically. Of over 8 million DNA positions that were variable within the two species, they were “fixed” between them in only 83 places (i.e., one could diagnose with certainty the two species by looking at only 83 places out of millions in the genome, while the rest of the DNA bases gave no diagnostic information about whether a crow was hooded or carrion.
  • Similarly, gene expression (as measured by the gene product, messenger RNA) barely differed between members of the two species. The percentage of all genes expressed differently ranged from 0.03% to 0.41% depending on the tissue—less than half a percent difference between hoodeds and carrions. As one might expect, most of the genes that differed in expression were those involved in plumage color, the most obvious difference between the species.
  • Surprisingly, 81 of those 83 fixed differences between the “species” mapped to one small region of a single chromosome: chromosome 18. That region represents only 0.28% of the total genome: about one quarter of one percent. The figure below shows the fixed genetic differences between the species (indicated with red arrows) across the genome, with each chromosome given by either a light blue or dark-blue color. You can see that most of the differences cluster on chromosome 18.

Screen shot 2014-07-24 at 3.39.40 AM

  • The genes in that very small region included “transcription factors” (genes that control the expression of genes elsewhere in the genome); these factors appeared to control pigmentation and vision.
  • The genes that differed in that region were probably contained in an “inversion”: a section of the chromosome that has been turned around in one of the two species since the common ancestor.  That is, if the common ancestor had gene order ABCDEFGHIJKLMN on its chromosome, one of the two descendants had an inversion that made the gene order something like ABCDEKJIHGFLMN, with the region F-K turned around. This happens when a chromosome breaks in two places and re-forms, but with the broken bit inserted backwards.
  • A few genes on other chromosomes also affected differences in pigmentation and probably vision as well.
  • Finally, as shown in the diagram at the top of the species’ ranges, carrion crows from Germany were more genetically similar to hooded crows from Poland and Sweden than to crows of their own species (carrions) from Spain. This probably reflects a historical phenomenon: the Spanish crows were isolated during the last glaciation, while the German crows expanded their ranges eastward out of a glacial refugium to eventually contact the hooded crows that were west from their refugium. Genetic interchange between the adjacent populations have made them more genetically homogeneous, despite their differences in the inverted region of chromosome 18.

What does this mean? Are they good species or not? Well, it’s still a judgement call. There are some fixed genetic differences between hooded and carrion crows, but it looks as if hybridization has homogenized most of their genes, so reproductive isolation is far from complete. In fact, if you just classify species by overall genetic similarity, you’d call Spanish carrion crows a different species from German carrion crows!

What we have here are two partially isolated populations: interbreeding is limited by the fact that there are color differences between the types, and each type tends to mate with others of its color. That clearly means that there are differences in at least two types of genes: color genes, and genes for how one responds to the colors, which makes you more likely to mate with a bird having a color similar to yours. (There are probably differences in visual sensitivity to the patterns as well, which may explain the fixed differences in the DNA of “vision” genes.) The “response” or “preference” genes could be active in females (who do most of the mate choosing) males (who may decide which species to court) or both sexes.

The importance of the inversion is that it keeps these types of genes together, because inversions keep genes tied up in blocks. If an ABCDEFGHIJKLMN bird mates with an ABCDEKJIHGFLMN bird, there can be free genetic interchange (“crossing over” between the chromosomes, except in the inverted F-K region, because a cross-over in that region will produce sperm and eggs that yield inviable zygotes (they will have duplications of some genes and absences of others, leading to inviability).

Therefore, if one species has the first configuration, and the other the second, genes in the F-K region will tend to stay together.  So if that inversion contains, in one species, genes for the hooded pattern as well as genes for preferring the hooded pattern, while the region in the other species has genes for the non-hooded pattern and genes for preferring the non-hooded pattern, the system will be stable.

If crossing-over were allowed, and the species-specific genes were not in inversions, you’d get hybrid birds having, say, a hooded pattern but a preference for a non-hooded pattern, and the species would soon lose their integrity for pattern and mate preference. Population geneticists have shown that, because of this, genes that are involved in speciation will tend to accumulate in inversions if there is gene flow between the populations during speciation. Since we know that there is gene flow between these groups, and they are not yet “good” species (they may never be), this observation is a striking confirmation of population-genetics theory.

So what do we call these things? Are they species or not? My preference is to consider them subspecies, as many biologists have before. Most of the genome is being exchanged between the hooded and carrion crows, so reproductive isolation is far from complete. But that is a judgment call using my definition of biological species as something of a sliding scale. Others will disagree, for no species concept will always work, if for no other reason that species are dynamic entities that begin as populations and only gradually become species. There will always be cases of species in statu nascendi, or gray areas that defy classification under any definition of species. (See chapter 1 of Speciation by Coyne and Orr if you want to see why we prefer to use the BSC.)

Although deKnijff’s piece tends to concentrate on the semantic problem—what do we call these birds?—I find the genetic results more interesting: that they maintain their distinctness due to only a very few genetic differences, and those differences are largely bound up in rearranged sections of one small chromosome.

So when my European readers (and I am, for another few days, a writer in Europe) see a hooded or carrion crow, be aware that you’re looking at a remarkable case of recent evolution, and a puzzle that has largely been solved by work published in just the last two weeks.

__________

Poelstra, J. W., N. Vijay, et al. 2014. The genomic landscape underlying phenotypic integrity in the face of gene flow in crowsScience 344: 1410-1414.

de Knijff, P. 2014.  How carrion and hooded crows defeat Linneaus’s curse. Science 344:1345-1346.

76 Comments

  1. Posted July 24, 2014 at 10:39 am | Permalink

    Very cool, very interesting. One thing puzzles me: I’ve seen plenty of hooded crows in Ireland. Does the diagram just indicate the birds that were studied (probably you said that but I read to fast and sloppily.)

    • Posted July 24, 2014 at 10:40 am | Permalink

      Wrote too fast too

    • Erik Verbruggen
      Posted July 24, 2014 at 10:58 am | Permalink

      If you look at wikipedia that is indeed acknowledged. It is the only exception to the perfectly straight line!

      • Dave
        Posted July 24, 2014 at 11:20 am | Permalink

        The map is also slightly inaccurate in showing the Great Glen as the boundary between the two types in Scotland. In reality, hooded crows occur well south of that line, and I think throughout the Scottish highlands.

        The very first hooded crow I ever saw was on the west coast of Anglesey, North Wales. Carrion crow territory, but also not far from Ireland, as the crow flies (sorry, I couldn’t resist that!), so I assume that is where the bird came from.

        • Lars
          Posted July 24, 2014 at 2:13 pm | Permalink

          Yes, I was going to comment on that myself – I’ve seen them well south of the Great Glen.

        • gravelinspector-Aidan
          Posted July 24, 2014 at 3:12 pm | Permalink

          Same as for Lars : they’re well south of the GG. In fact, I’d say that they, hoodies, may be in the majority as far south as the border, if not down into Geordie-land.
          In 30-odd years of travelling the length of the country, I’d say that the hoodies are moving south, at quite a pace. If I knew any twitchers, I might be able to get a handle into their literature to back that up (or refute it).
          At least they’re more distinguishable than LBJs.

          • Lars
            Posted July 24, 2014 at 6:40 pm | Permalink

            Actually I think it was in the Grampians, but it was over 40 years ago so the details are a bit fuzzy. But we didn’t get further north than Long Loch on that trip, and we saw them well south of that – on the way north to Glasgow, actually.

          • Dominic
            Posted July 25, 2014 at 2:55 am | Permalink

            They will migrate in the winter I think with Scandinavian birds crossing the North Sea, & perhaps carrion crows migrate to Ireland in the winter, so if you live in these areas observe WHEN you see them – it would be very interesting to keep a note.

            • gravelinspector-Aidan
              Posted July 25, 2014 at 7:44 am | Permalink

              Hmmm, migration. Hadn’t thought of that.
              TBH, since I see crows all year round, I hadn’t considered that there might be a cryptic migration going on.
              I keep on getting these invites to “twitcher day out” type events ; I’m going to have to keep my eyes peeled for one that’s within cycling distance. “Twitch”, “twitch”!

        • HaggisForBrains
          Posted July 25, 2014 at 2:54 am | Permalink

          Agreed.

        • aljones909
          Posted July 25, 2014 at 3:33 pm | Permalink

          The RSPB site also indicates that hooded crows are mainly north of the great glen. I see lots of carrion crows in south east scotland. It’s rare to see a ‘hoodie’.

  2. Posted July 24, 2014 at 10:46 am | Permalink

    This is quite fascinating, last year I saw a crow that might have been a hybrid. It’s colouring was mostly carion crow type, but it hat some distinct patches of grey plummage on its breast and one wing was greyish on the underside. It was the only one.

    I live in Hamburg, quite close to the hybrid line.

  3. Mobius
    Posted July 24, 2014 at 10:48 am | Permalink

    I think this all goes to show that while the concept of species is useful, it is fuzzy at best. The real world does not play along with our desire to neatly pigeon hole things.

    This is something that most creationists just don’t get. With their black-and-white thinking, the idea of one “species” blending into another just doesn’t fit. So the fact that one can look at a species and its a-million-years-later descendants and conclude they are two distinct species means, to them, that there had to be a discontinuity somewhere, that one species could not change gradually into another over a vast amount of time.

    • Hempenstein
      Posted July 24, 2014 at 8:08 pm | Permalink

      Most of them (or so it seems, anyway) don’t even buy into the concept of a vast amount of time, tho, so all bets are off where they’re concerned.

  4. TJR
    Posted July 24, 2014 at 10:49 am | Permalink

    Using the BSC, but acknowledging that in cases like this its a sliding scale, seems obviously the right thing to do IMHO.

    I agree entirely that the genetics is far more interesting than arguing over the semantics of how to apply a binary classification in a case where it is clearly inappropriate.

    Mind you, I’m looking out of my office window at a large black bird with a yellowish beak, and for all I know it could be a pigeon…..

    • Posted July 24, 2014 at 12:44 pm | Permalink

      I, too, like the idea of abandoning a purely quantized classification of speciation. The discontinuities are essential to understanding the process, but the process itself is continuous. In those regions where the continuity starts to break down, one shouldn’t be too eager to insist one one perspective or the other.

      b&

      • Posted July 25, 2014 at 9:48 am | Permalink

        When Pluto was “demoted” I was following Phil Plait’s stuff at the time and asked whether or not it would make sense to introduce the idea that planethood comes in degrees. I wonder if that would work in the species case; it seems to me (as a naive outsider, of course) that it would be especially useful for non-sexually reproducing organisms.

  5. Erik Verbruggen
    Posted July 24, 2014 at 10:55 am | Permalink

    Nice story. Coming from the Netherlands, “carrion crow” territory, and now living in berlin, “hooded crow” territory, I was already puzzled why there is this pattern, with both species having the same ecology and can easily reach territory occupied by the other. One minor quibble: if there would be one gene reponsible for color, and another prescribing to “only mate with own color”, then reinforcement would only require one gene not two. But really amazing how genetically similar these species are!

    • Posted July 24, 2014 at 11:13 am | Permalink

      But notice that I didn’t say anything about reinforcement; we don’t know if that is part of this story. Usually, however, there are separate genes for recognition and response, as in the corn borer moths.

      • Erik Verbruggen
        Posted July 24, 2014 at 11:58 am | Permalink

        Okay, point taken.

  6. Posted July 24, 2014 at 11:02 am | Permalink

    The hooded crow coloration is probably neotenous. Juvenile carrion crows show grey coloration and are easily mistaken for hybrids.

  7. eric
    Posted July 24, 2014 at 11:27 am | Permalink

    Stupid question, but in the figure shown, the “narrow hybridization zone” consists of the reddish brown line separating the two colors, right? The text confused me at first, because I was interpreting that line as an ‘artists highlighting’ of the two main regions, not an actual, geographic region in and of itself.

    • gravelinspector-Aidan
      Posted July 24, 2014 at 3:16 pm | Permalink

      That is what I took the line to mean. As it says in the picture caption.
      Note also the comments above that, at least in Scotland, the line is too far NW and not broad enough.

  8. Mark R.
    Posted July 24, 2014 at 11:46 am | Permalink

    In the US, many people ask “what is the difference between a crow and a raven?” There are many differences, but a main differences is their calls. “Caaa”, “Kraww”, “Aaw” etc. So my question about the hooded and carrion crow: do they have different calls?

    • Erik Verbruggen
      Posted July 24, 2014 at 11:57 am | Permalink

      To me they sound the same. But to me a magpie and an ara also sound amazingly similar! :-) but really!

    • bonetired
      Posted July 24, 2014 at 12:56 pm | Permalink

      We have a similar question in the UK: “How can you tell the difference between crows and rooks?”

      “If you see many crows then they are rooks, if you see a solitary rook then it is a crow!”

      (BTW: I know that crows actually can flock together …)

      • Diana MacPherson
        Posted July 24, 2014 at 1:04 pm | Permalink

        A murder of crows.

      • Erik Verbruggen
        Posted July 24, 2014 at 1:22 pm | Permalink

        Rooks are also more ugly and look like someone farted :-)

        • Jonathan Wallace
          Posted July 25, 2014 at 12:46 am | Permalink

          I don’t think rooks are ugly!

      • gravelinspector-Aidan
        Posted July 24, 2014 at 3:19 pm | Permalink

        to quote “Sparrah”, a gamekeeper of my acquiantance in my youth, “a craw in a crowd is a rook, and a rook on ‘is own is a craw” ; clear now.
        Lovely guy, “Sparrah” ; hard to hear what he said, ever. Mustard gas.

    • sgfreimansgfreiman
      Posted July 24, 2014 at 7:40 pm | Permalink

      Crows and ravens are more different than that. The shape of their tails, the shape and size of their beaks, the size of their bodies.

  9. Mark R.
    Posted July 24, 2014 at 11:47 am | Permalink

    “main difference” damn typos

  10. Shwell Thanksh
    Posted July 24, 2014 at 11:59 am | Permalink

    I have just been comparing the definitions of species … It is really laughable to see what different ideas are prominent in various naturalists’ minds, when they speak of ‘species’; in some, resemblance is everything and descent of little weight — in some, resemblance seems to go for nothing, and Creation [is] the reigning idea — in some, descent is the key — in some, sterility an unfailing test, with others it is not worth a farthing. It all comes, I believe, from trying to define the undefinable.
    – Charles Darwin, letter to Joseph Hooker dated December 24, 1856

    • Posted July 25, 2014 at 4:30 am | Permalink

      Darwin was wrong, and if you’re implying he was right, you’re wrong, too. We’ve made great progress in understanding not only what species are in organisms that reproduce sexually, but how they form. If we didn’t, I wouldn’t have been able to write an entire book on speciation with Allen Orr.

      It’s ironic that the title of Darwin’s greatest book gave no insight into the origin of species.

      • Posted July 26, 2014 at 12:41 pm | Permalink

        Uh-oh…I feel a Cretinist quote-mine coming on.

        Of course, the only reason they’d think such would be effective is because they think Darwin is our Jesus and On the Origin of Species our Bible. What nonsense! I mean, would you want to grope Darwin’s guts? Of course not — it doesn’t even aliterate!

        b&

        >

  11. Frank Bartell
    Posted July 24, 2014 at 12:17 pm | Permalink

    The 4 NA species of Canids C. familiaris, C. latrans, C. rufus, & C. lupus are generally considered species in that without human disruptions they are behaviorally reproductively isolated, but with us screwing up environments we find hybrids of all. Of course the Inuit have been breeding their dogs with wolves for a long time, but now coyote-dogs, red wolf-coyotes, etc. are found. In fact the existence of hybrids helped bring red wolves back from the brink of extinction.

    • John Scanlon, FCD
      Posted July 25, 2014 at 9:51 am | Permalink

      Here in Australia we have Canis lupus dingo and Canis lupus familiaris, officially conspecific with the northern wolf (and introgressing like mad with each other). Species-vs-subspecies for threatened large carnivores is obviously a political question as much as a biological one.

  12. mecwordpress
    Posted July 24, 2014 at 12:29 pm | Permalink

    Dr. Coyne

    I have a question for you (and/or your readers).

    You say; “Allen Orr and I use reproductive isolation as a relative criterion: the more two populations are reproductively isolated from each other, the more “species like,” they are, up to the point where there is no genetic interchange, at which point they can be called “good biological species.” Ever since I read Speciation, I have interpreted the BSC to be rephrased from “two populations are from different species if they can’t produce fertile hybrids” to “two populations are from different species if they don’t produce fertile hybrids”.

    In the cases where two populations of closely related organisms occupy the same niche (the same geographical area, say) but are reproductively isolated by differing mating schemes (different mating seasons, for example), would they be more to the side of “species like” or closer to “good biological species”.

    One example I’m thinking of might be the Jonah crab (Cancer borealis) and Cancer irroratus, a species of Rock Crab that lives in the same area as the Jonah crab and are nearly identical but are mainly distinguished by when they reproduce – the Jonah crab in the spring and the Rock crab in the fall (I may have got that wrong way round, but it’s something like that). In the lab, I believe, they produce fertile hybrids, but they don’t in the wild simply because of timing of their reproductive cycles. It seems to me these would be like your “species like” organisms because they can produce fertile offspring but they don’t. On the other hand, because of the timing of their cycles they also have no (apparent) genetic interchange, thus they are also “good biological species”.

    Maybe that quote by Darwin about trying to “define the undefinable” applies to my silly question here. Anyway, any insight or comment would be appreciated.

    • mecwordpress
      Posted July 24, 2014 at 12:32 pm | Permalink

      Oops

      Forgot the darn question mark. Paragraph 3 is the question.

      Sorry

    • aspidoscelis
      Posted July 24, 2014 at 6:32 pm | Permalink

      IMO, reality (they don’t interbreed) is more important than hypothetical situations (they -can- interbreed -if-…). These sound like species.

      • aspidoscelis
        Posted July 24, 2014 at 6:39 pm | Permalink

        Ultimately, any pair of species that are reproductively isolated in fact could interbreed if whatever prevents them from interbreeding were removed. This is no less true of post-zygotic isolation than of pre-zygotic isolation.

    • Posted July 25, 2014 at 4:06 am | Permalink

      What I mean by “good” biological species are species that DO NOT produce fertile hybrids in nature that can and do mate with each other or with the parental species. Some good biologial species that exist in nature without interbreeding can nevertheless produce fertile hybrids in zoos (e.g. lions and tigers, which used to coexist in parts of India), but that is because the barriers that keep them apart in nature are broken down in captivity.

      It doesn’t matter what keeps species from exchanging genes in nature; if it’s different times of mating, or different pheromones, or hybrid sterility or whatever, if that feature is biologically based and completely prevents gene flow, the populations are good species.

      The crabs you mention are GOOD biological species because they don’t produce fertile hybrids in nature where they coexist, although they can in captivity. The criterion, which I perhaps failed to make clear in my haste, is that the BSC deals with gene interchange between populations that live in the same area in nature. Different mating periods that prevent interbreeding, which are surely based on genetic differences, are in your case reason to regard them as good biological species.

  13. mecwordpress
    Posted July 24, 2014 at 12:32 pm | Permalink

    Oops. Forgot the darn question mark. Paragraph 3 is the question.

    Sorry

    • mecwordpress
      Posted July 24, 2014 at 12:34 pm | Permalink

      Is there an echo in here?

      (apologies again. don’t know what I did or how to remove this redundancy)

      • gravelinspector-Aidan
        Posted July 24, 2014 at 3:25 pm | Permalink

        Is there an echo in here?

        No, there’s a comment editor application which is a bit quirky and doesn’t handle disconnects with the server too well. You’re not the first to feel it’s … quirks.

  14. Diana MacPherson
    Posted July 24, 2014 at 12:47 pm | Permalink

    We should introduce hooded crows to North America if they aren’t here. Europeans brought over other birds so what could go wrong? ;)

    Seriously though I do wish those hooded crows were here.

    • Posted July 24, 2014 at 12:50 pm | Permalink

      Hooded crows are cool…but we already have ravens. What more could one ask for in a corvid?

      (If you want to get your fill of ravens, visit Flagstaff. There’re as many ravens in Flagstaff as there’re grackles here in the Phoenix area.)

      b&

      • Diana MacPherson
        Posted July 24, 2014 at 1:02 pm | Permalink

        Yeah Ravens don’t live where I live. I saw them in BC or at least I thought I did. I started just calling anything that was crow or raven looking, “craven”.

        • Mark R.
          Posted July 24, 2014 at 2:52 pm | Permalink

          Yeah, first time I visited the Grand Canyon in 2006 was the first time I saw Ravens…at least that I remember. “What more could one ask for in a corvid?” Not much…they have a pretty good football team too ;)

          • Posted July 24, 2014 at 4:16 pm | Permalink

            I’ve watched ravens in Joshua Tree Park hover in the updraft coming up a cliff. Yes, they really are show-offs with a mischievous sense of humor. They migrate through the Phoenix area, at least…once or twice a year I’ll hear them making a racket outside, and I might get lucky and catch sight before they’re over the next rooftop.

            b&

        • Posted July 24, 2014 at 4:13 pm | Permalink

          That’s not the most courageous stance to take in nomenclature, is it?

          b&

  15. stevenjohnson
    Posted July 24, 2014 at 12:51 pm | Permalink

    The genetic divergence between the Spanish and German crows seems to be the most interesting. But is it really because the German crows interbred with the Polish crows? (I presume since the report highlights the German/Spanish difference there’s not so much difference between Swedish and Polish crows?) What evidence rules out that all these crows relatively recently developed from a a common ancestral population, but the Spanish population diverged from from genetic drift and a somewhat different biome?

    Is it feasible to interpret the course of events then as a population of the ancestral crow (which we might see as a carrion crow) develops the inversion during a period of isolation to a particular biome during a glacial. In the interglacial the ranges of what we now see as carrion and hooded crows expands. The imperfect reproductive isolation imposed by sexual selection leaves at the intersection a limited number of hybrids.

    You write “there are differences in at least two types of genes: color genes, and genes for how one responds to the colors, which makes you more likely to mate with a bird having a color similar to yours.” Is it possible the populations have the same gene(s) “for” color response, with the effect that most strongly prefer the colors they grew up with? (If so, the hybrids would be largely the offspring of partners rejected by their preferred choices.)

    I think in that case, if some flock of hooded crows were transported far west, they would hybridize. And by the BSC, this kind of potential interbreeding would indeed define the two populations as subspecies? But if there is a genetic basis for sexual selection? I suspect that would mean a flock transported into the other range would most likely die out as a distinct group, not propsering even as a hybrid. In that case, would the BSC criterion then dub them separate species, albeit nascent ones?

    I’m not sure why the overall genetic similarity is regarded as so important. The sexual selection, whether directly due to genetic differences or not, seems to be creating new populations that will over time inevitably diverge. It is hard to imagine a way in which hybrids will be perpetually viable. Speciation which is driven by sexual selection instead of natural selection may seem kind of random but it is still underway. These crows, whether hooded or carrion are still just subspecies or imperfectly separated species seem to be a fascinating example of this.

    • Erik Verbruggen
      Posted July 24, 2014 at 1:08 pm | Permalink

      Just thinking out loud in response to the last part of your question: if hybrids are attractive to eachother, and to some extent to either or both of the crow species (and fertile), there is no reason the hybridization zone wouldn’t expand to the detriment of the other species. They have more mates to choose from! So it IS a real problem that requires an explanation. I could also imagine territoriality being a thing….. but maybe I am just too much of an ecologist…..

  16. Greg Peterson
    Posted July 24, 2014 at 1:12 pm | Permalink

    Fascinating.

    And Inverted F-K Region is my new band name.

    • Duncan
      Posted July 25, 2014 at 12:28 am | Permalink

      Not sure why, but your name works well in the extended band name. As in

      “Ladies and gentelmen please give a warm welcome to Greg Peterson and the Inverted F-K Region!”

      • TJR
        Posted July 25, 2014 at 2:26 am | Permalink

        Playing prog rockabilly.

  17. Jim Thomerson
    Posted July 24, 2014 at 2:20 pm | Permalink

    Suppose that two similar species who are syntopic, occur together, have strong premating isolating mechanisms:different spawning sites, different timing, etc. etc. Then there might be little or no selection for development of postmating isolating mechanisms. They do not hybridize in nature, so they are two good species. Never mind that if you can figure out how to break down their premating isolating mechanisms, they produce fertile hybrids.

    • John Scanlon, FCD
      Posted July 25, 2014 at 11:23 am | Permalink

      Sounds like pythons. Ancestors of Asian, African, and Australian lineages had diverged by late Oligocene (possibly quite a bit earlier) and will still mate and produce viable offspring if you put them in a box together; not sure about fertility of hybrids for the more distant crosses, but it’s plainly irrelevant to their taxonomic status because each branch has speciated multiple times within its own geographic region.

  18. Gregory Kusnick
    Posted July 24, 2014 at 4:02 pm | Permalink

    Slightly off-topic, but I’m curious as to how chromosomal inversion works chemically. My naïve intuition says it ought not to be possible. If you cut out a section of a bar magnet and try to invert it, the result won’t be stable, because you’ve put the wrong poles together. Similarly, if you imagine a long chain of electrical extension cords plugged head-to-tail, and try to take one out of the middle and invert it, it simply can’t be done because the plugs won’t match.

    But apparently the bonds between adjacent DNA base pairs are not polarized in that sort of way. Even so, I’m still confused, because if you cut out a single base pair, a G for instance, and splice it back in upside down, can it still be transcribed properly by a ribosome? How? Won’t the bond geometry be wrong?

    The Wikipedia article on inversion was unenlightening, as were my cursory attempts at Googling an answer. So if there are any biochemists present who can shed some light, I’d be grateful.

    • Gregory Kusnick
      Posted July 24, 2014 at 5:07 pm | Permalink

      Allow me to rephrase. Obviously it’s not the ribosome that transcribes DNA into RNA. (I blame the beer I had with lunch.)

      But the question stands: how is it possible for an upside-down DNA base to be transcribed into a readable mRNA?

      • Mark Sturtevant
        Posted July 24, 2014 at 7:11 pm | Permalink

        The polarity of a strand of DNA is labeled as 5′ to 3′, and the polarity of the two DNA strands run in opposite directions (hope this works):

        5′ ATCGATCG 3′
        3′ TAGCTAGC 5′

        I will flip the portion of DNA in bold. Note that it will not involve inverting just one strand, but of both the top and bottom strands of DNA.

        Just inverting the DNA left to right would create a problem in the polarity within each strand, as you say, but that is fully resolved if the inverted strands are also flipped upside down, so the piece of the top strand is on the bottom and vice-versa.

        5′ ATATCGCG 3′
        3′ TATAGCGC 5′

        There is no reversal of the 5′ to 3′ polarity within a strand, even though the sequence of bases are now flipped left to right, and top to bottom. DNA will replicate fine, and any gene expression where DNA is used to make RNA will also be ok — the latter will make a ‘wrong’ sequence, but it will still be made.

        • Gregory Kusnick
          Posted July 24, 2014 at 8:02 pm | Permalink

          That makes sense; thanks.

          In the extension-cord analogy, it’s as if each end of the cord had one prong and one slot, instead of two prongs at one end and two slots at the other.

          • Posted July 25, 2014 at 9:53 am | Permalink

            Also, though I don’t remember if it applies here, the “mereology” chemists use is sometimes a bit “loose”. For example, people say that proteins are composed of amino acids. More pendantically, they are *synthesized* from them, with the water “dropping out”.

            (Philosophers take note: this is the root of one of the problems that Thagard and I have with the twin earth thought experiments.)

  19. Jim Thomerson
    Posted July 24, 2014 at 4:22 pm | Permalink

    Your base pair is a G-C pair. Spliced in upside down, it is a point mutation. This is the kind of mutation which causes sickle cell anemia (I was surprised to learn there is more than one kind of sickle cell mutation). As I recall, the old familiar sickle cell mutation is a single A-T inversion.

    Maybe you should look at some DNA diagrams, the linkage, across the molecule between G and C is different from the linkage between A and T, but the linkages down the molecule are all the same (I think).

  20. aspidoscelis
    Posted July 24, 2014 at 6:17 pm | Permalink

    My $.02:

    Forms that abut but do not overlap are almost always infraspecific variation.

    If two forms are identified by coloration, and almost all of the genetic differentiation between the two species is in coloration, you’re almost certainly looking at color forms of a single species.

    Regarding the second point, you’ve basically got a hypothesis (there are two species, A and B, that can be distinguished visually by character X) and a test of that hypothesis (let’s see if the putative species A and B are genetically distinct at the genomic scale). The results of that test appear to be that they are indeed distinct in character X, but that’s it. The hypothesis was that the color variation is a marker for something deeper–separate species, which in the genome should be marked by differentiation in randomly evolving characters across the board. The test results say, “Nope, it’s just color variation–oh, and it has a genetic basis!” Ergo, the hypothesis is falsified. The color variation is just color variation. It is not a marker for anything else.

    This also hits a problem I have seen before in statistical analyses of speciation. If species A and B are visually identified by character X and you use a classification based on X as the basis for further tests of distinctness, you are begging the question. You are, in essence, asking: “-Given that- we have these two groups, are they different?” The question you should be asking is: “What groups can we identify with the data available to us?” You really can’t take the classification as a given in the analysis if that’s what you’re supposed to be testing.

    And, of course, if you ask, “Given that we have these two groups, are they different?” and the answer you get is, “Yes, but only in the character used to classify the groups in the first place!” then, well, all you’ve shown is that if you separate groups by character X, yes, they are different in character X. It’s like putting a line at 5′ 6″ and asking, “Are people shorter than 5′ 6″ different than people taller than 5′ 6″?” and getting the answer: “Yes, people who are shorter than 5′ 6″ are shorter than people who are taller than 5′ 6″.” It’s a pointless tautology.

  21. Jim Thomerson
    Posted July 24, 2014 at 7:58 pm | Permalink

    When I first studied what are now known as Gnatholebias hoignei and G. zonatus, (Rivulid killifishes) I had two similar forms which were not known to occur together, and had consistent differences in preferred habitat. Perhaps just ecophenotypes. I found a character X which was consistently different between them. I began to think of them as separate species.

    I was able to readily breed and reproduce either one in the same aquarium setup. However, when I put heterospecific pairs together, they showed no interest in each other. I described them as separate species.

    Later we did karyotypes and no way they could produce viable offspring. Observation in the wild revealed quite different spawning behavior. We found a large disturbed area where they occurred together, with no evidence of hybridization. Even in the same pool in the disturbed area, if open water and shaded habitat were both available, they would segregate out into their preferred habitats.

    • aspidoscelis
      Posted July 24, 2014 at 9:08 pm | Permalink

      Now those are definitely good species. :-)

  22. sgfreimansgfreiman
    Posted July 24, 2014 at 10:48 pm | Permalink

    I need help!!! I cannot find how to forward material to Dr. Coyne.

  23. Dominic
    Posted July 25, 2014 at 3:01 am | Permalink

    Interestingly the “Birdwatch Ireland” website shows only the hoodie not the carrion crow…

    http://www.birdwatchireland.ie/IrelandsBirds/Crows/tabid/403/Default.aspx

  24. Diego
    Posted July 25, 2014 at 4:13 am | Permalink

    Would the Recognition Species Concept apply here? And since the differences are concentrated in visual markers and this results in nonrandom mating could this be an example of reinforcement? I wonder if there is a fitness cost to hybridization. That’s what I would like to see researchers look into next.

    • Posted July 25, 2014 at 4:17 am | Permalink

      Rather than write a long response, I direct you to the Appendix of Speciation by Coyne and Orr, which discusses the problems of the Recognition Concept. Yes, you could in principle use it here so long as one knows that assortative mating (and perhaps mating disadvantage of hybrids) are the only reproductive barriers.But as our discussion shows, there are general problems applying this concept more widely, so the BSC is more generally useful. As I recall, Rick Harrison has a related critique in the Howard and Berlocher volume.

  25. Jim Thomerson
    Posted July 25, 2014 at 1:12 pm | Permalink

    I think it is intrinsic to the BSC that hybridization leads to decreased fitness in those individuals who hybridize.

  26. Diane G.
    Posted July 28, 2014 at 11:50 pm | Permalink

    sub


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