Data show that the “normal” mode of speciation—the process in which one lineage divides into two or more species—involves the geographic isolation of populations of a single species. Over time, natural selection (and genetic drift) causes those populations to become more and more genetically different. When the genetic differentiation has gone to the extent that the separate populations evolved features that make them unable to produce fertile hybrids when they come back together in the same area (i.e. regain “sympatry”), then these populations have become separate species. They are now groups on distinct evolutionary trajectories, and their inability to exchange genes because of the evolved “reproductive isolating barriers” between them (e.g., behavioral differences in mating, preferences for different host plants or microhabitats, different times of mating, different pheromones, or the sterility or inviability of hybrids), is what makes nature “lumpy” rather than a continuum. The lumpiness of nature—the fact that, in a single geographic locality, in most groups you readily see distinct clusters of plants or animals (look at the birds outside your window, or look at a field guide)—is an important fact that can only be explained by connecting the formation of those “lumps” with the reproductive barriers that keep them from forming a continuum.
Geographic isolation is thought to be important because gene flow between diverging species tends to keep them from diverging. In our own species, humans in different places began the process of genetic divergence, as witnessed by the traits that distinguish human populations (these are correlated with geographic isolation, as the theory predicts), but this process was nipped in the bud by both population growth and the invention of forms of transportation that allow people to move much farther than they used to. There is now gene flow between many populations, and Homo sapiens is an example of a “polytypic” (variable) species that, if the populations had remained isolated for a million years or so, might have become more than one species of Homo.
One thing that biologists have discovered since the advent of DNA sequencing, though, is that gene flow between species is more common than previously thought. Reproductive barriers aren’t always complete (although they are now between our species and all other living species), and so sometimes hybrids are formed and genes can sneak between different species. In the group I used to work on, the closely related species Drosophila santomea and Drosophila yakuba, we and others discovered that the entire mitochondrion, with all of its own DNA, invaded D. santomea from D. yakuba, and there’s been a bit of other gene flow as well. (In most of the genome, however, the species remain distinct.) This could only have been due to hybridization, and it happened because although the species tend to live at different altitudes, there are areas of overlap where they can meet and hybridize, and the female hybrids (but not the males) are fertile.
So we know that genes sneak between species more often than we used to think.
Some biologists, however, have gone farther, and postulated that hybridization between two species can itself cause the formation of a third species, a process called “hybrid speciation.” This is somewhat common in plants, occurring through a special genetic mechanism called polyploidy. There are two forms. Allopolyploidy involves the hybridization of two species having different chromosome numbers, and since the different chromosomes can’t pair in the hybrids, those hybrids are sterile. However, if the chromosome number doubles in the hybrids, so that a new individual is formed with a chromosome number equal to the sum of the numbers in both parental species, one can get an “allotetraploid” populations whose members are fertile among itself but sterile when they mate with either parental species. (See any evolution textbook for an explanation.). This would, then, be a new biological species. A similar process can occur if chromosome number doubles within a single species, producing an autotetraploid. Further hybridization and chromosome doubling can lead to entire polyploid series of plants with hundreds of chromosomes, as in ferns.
As I said, polyploidy, both auto- and allo-, is a fairly common mode of speciation in plants. As Allen Orr and I noted in our book Speciation (read chapter 9), roughly 2-4% of speciation events in flowering plants involved polyploidy of one sort or another, and maybe as many as 7% of speciation events in ferns. This is a rough estimate, and the real frequency could be higher. But polyploid speciation in animals is much rarer, and I won’t go into the suggested reasons for it (see pp. 333-337).
There’s another form of hybrid speciation called “homoploid hybrid speciation” or “recombinational speciation.” In that process, a hybrid is formed between two species, and then, if it is at least partly fertile, the genes from the different parental species can sort themselves out into new combinations of genes or chromosome arrangements from the parental species. If the new sorted-out population is reproductively isolated from the two parental species that produced it, we have a new homoploid hybrid species.
Many biologists (I won’t name them) have posited that this kind of speciation is rampant in nature, so that it’s not just the occasional sneaking of genes between species that’s important, but also the wholesale formation of new species after hybrid formation. Lots of suggested examples of such species have been given.
However, it appears that most of the evidence for non-polyploid hybrid speciation is weak. That, at least, is the conclusion of Molly Schumer, Gil Rosenthal, and Peter Andolfatto in a 2014 paper in Evolution (link and free access below), a paper that I only learned about at CoyneFest. Schumer et al. argue that good evidence for a non-polyploid hybrid speciation event requires satisfying three conditions, and I quote:
To demonstrate that hybrid speciation has occurred given this definition, three criteria must be satisfied: (1) reproductive isolation of hybrid lineages from the parental species, (2) evidence of hybridization in the genome, and (3) evidence that this reproductive isolation is a consequence of hybridization. By contrast, a large number of empirical studies have simply used genetic evidence of hybridization (Criterion 2) as support for hybrid speciation. . .
The authors argue that there are many ways that a species can look as if it’s a hybrid without actually being the result of full-scale hybridization (or any hybridization); that in some cases a hybrid lineage hasn’t been tested to show that it’s interfertile with other members of that lineage and reproductively isolated from the parental species, and, especially, there are almost no demonstrations that the genes or chromosome arrangements of parental species have sorted themselves out in a way that has created a reproductively isolated homoploid hybrid. That is, few people have shown that the reproductive isolation of a putative hybrid species involves genes that came from the parental species rather than, say, genes that evolved via natural selection after hybridization.
You can read the paper for details, but Schumer et al. conclude that despite the big noise from some biologists, there are only four cases of homoploid hybrid speciation that meet their criteria. Three of them are in one genus: the wild sunflower Helianthus, which has formed three diploid species—all adapted to novel environments—by hybridization of pre-existing species and the sorting out of chromosome arrangements that, with their divergent genes, reproductively isolate the hybrid population from the parents. That superb work was done by Loren Rieseberg and his colleagues.
The other case is the butterfly Heliconius heurippa, which genetic evidence shows almost certainly resulted from hybridization between the species Heliconius cydno and Heliconius melpomene, H. heurippa has a hybrid wing pattern, which you can see below, and it’s been shown that each species, as well the “hybrid”, are reproductively isolated from the others because males mate almost entirely with females who have their own wing patterns. Thus H. heurippa (shown below with its parents) satisfies all three of the authors’ criteria, for the genes causing reproductive isolation are precisely the color-pattern genes derived from the two parental species.
The upshot is that while the movement of individual genes between both plant and animal species is more common than evolutionists assumed before the gene-sequencing era, there is still scant evidence that entire new species of animals form via hybridization. Hybrid speciation is more common in plants, but only through the unusual mechanism of polyploidy, and homopoloid hybrid speciation (without an increase in chromosome number) doesn’t appear common in either plants or animals.
Schumer, M., G. G. Rosenthal, and P. Andolfatto. 2014. How common is homoploid hybrid speciation? Evolution 68:1553-1560.