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:
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:
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.
- 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 crows. Science 344: 1410-1414.
de Knijff, P. 2014. How carrion and hooded crows defeat Linneaus’s curse. Science 344:1345-1346.