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I've talked to a couple of researchers today and read recent articles on the subject of hybridization of Darwin's finches. I'm confused about the consensus in the scientific community.
First, I was reading about hybridization in systems where adaptive radiation occurred. I came across this Honeycreepers paper: Knowlton, J. L., D. J. Flaspohler, N. C. R. Mcinerney, and R. C. Fleischer. 2014. First Record of Hybridization in the Hawaiian Honeycreepers: 'I'iwi ( Vestiaria coccinea) × 'Apapane ( Himatione sanguinea). The Wilson Journal of Ornithology 126:562-568.
In this paper they are saying:
The Galápagos finches, for instance, hybridize frequently both within and across genera (Grant et al. 2004, Grant and Grant 2008).
But in the resources they cite, here is what I found:
Rare hybridization between the species, occurring partly as a result of misimprinting on song (Grant and Grant 1998)
From Grant, P. R., B. R. Grant, J. A. Markert, L. F. Keller, and K. Petren. 2004. Convergent evolution of Darwin's finches caused by introgressive hybridization and selection. Evolution 58:1588-1599.
Hybridization was rare at the level of individuals: only a few percent of all offspring are hybrids
From Bell, G. 2015. Every inch a finch: a commentary on Grant (1993) 'Hybridization of Darwin's finches on Isla Daphne Major, Galapagos'. Philosophical Transactions of the Royal Society B: Biological Sciences 370:20140287-20140287.
Hybridization was always rare.
From Grant, P. R. 1993. Hybridization of Darwin's Finches on Isla Daphne Major, Galapagos. Philosophical Transactions of the Royal Society B: Biological Sciences 340:127-139.
Hybridization occurs rarely (less than 2% of breeding pairs) but persistently across years, usually as a result of imprinting on the song of another species.
In Grant, B. R., and P. R. Grant. 2008. Fission and fusion of Darwin's finches populations. Philosophical Transactions of the Royal Society B: Biological Sciences 363:2821-2829.
Did I miss something, or is hybridization rare and not frequent? Maybe frequently is so vague that it includes, rare?
I actually just had to read a Grant & Grant paper, so hopefully, I can get this right. My understanding is that hybridization events have repeatedly seen to occur, but the events themselves usually do not repeat within a lineage, i.e. once a lineage becomes partially hybridized, it is unlikley to continue to breed with adjacent populations.
Due to the reproductive barriers that come from song patters of hybridized individuals (Grant & Grant, 2009), Repeated hybridations are unlikely to occur within a lineage, even when in the presence of a species with which hybridization can occur.
Grant, P.R. and B.R. Grant. 2009. The secondary contact phase of allopatric speciation in Darwin's finches. PNAS 106 20141-20148
Hybrid speciation can be broadly defined as the hybridization between two or more distinct lineages that contributes to the origin of a new species. More specifically, hybridization must result in a hybrid population that is at least partially reproductively isolated from the parental species. Recently, three criteria were established to demonstrate hybrid speciation: (1) there must be evidence of hybridization between species, (2) the hybrids must be reproductively isolated from the parental species, and (3) there should be evidence that hybridization is the cause of the isolation ( Schumer et al., 2014 ). Studies of potential examples of hybrid speciation in nature and the lab over the past 100 years have shown that there are several ways that hybridization can lead to origin of a new hybrid species.
The most well-known route to hybrid speciation is through the doubling of chromosome numbers in hybrids (allopolyploidy), so hybrids have twice the number of chromosomes as their parents. These allopolyploid hybrids can be reproductively isolated from the progenitor species that have a different ploidy, due to improper chromosome pairing during meiosis ( Grant, 1981 ) and genetic incompatibilities between the hybridizing genomes ( Abbott et al., 2013 ). Therefore, the doubling of chromosomes offers a rapid route to hybrid speciation. Examples of allopolyploidy provide some of the clearest evidence of hybrid speciation, since the increased ploidy of hybrids resulting from hybridization directly causes reproductive isolation.
Hybrid speciation can also occur with no change in chromosome number, which is referred to as homoploid hybrid speciation (HHS) (also known as recombinational speciation, see Grant, 1981 ). In HHS, viable, true-breeding hybrids evolve that are reproductively isolated from the parental species. Homoploid hybrids do not have the advantage of being immediately isolated from the parent species, like allopolyploids do. Therefore, HHS requires the evolution of reproduction isolation while gene flow is ongoing with the parental species, which may explain why HHS is unlikely or more difficult to demonstrate than allopolyploid hybrid speciation ( Barton, 2001 Coyne and Orr, 2004 ).
It is also important to understand what hybrid speciation is not. Hybrid speciation is not simply the production of F1 or backcross offspring between distinct species, as it requires the establishment of a third, distinct species. Therefore, interspecific hybrids such as ligers and geeps, which are sterile like mules and cannot persist beyond a single generation, are not examples of hybrid species. Similarly, hybridogenesis, a reproductive strategy that involves backcrossing between hybrids and a parental species with different ploidy (as seen in edible European frog Rana esculenta ( Tunner and Nopp, 1979 )), does not qualify as hybrid speciation since the hybrids are obviously not reproductively isolated from the parental species. Additionally, hybrid speciation should also be considered distinct from reinforcement where natural selection against deleterious hybridization drives the evolution of increased reproductive isolation. Although reinforcement involves both hybridization and speciation, it does not necessarily involve the origin of a new, hybrid species.
The concept of hybrid speciation may seem counterintuitive to some. If species are defined as reproductively isolated groups, then by definition, hybridization should not occur. Hybrid speciation requires that the reproductive barrier between the parental species is either incomplete or has been lost, so that hybridization can occur. When considering hybrid speciation, it may be more useful to define species as distinguishable groups of individuals with clusters of shared genotypes that remain distinct in the face of gene flow ( Mallet, 1995 ). Importantly, this definition, referred to as the Genotypic Cluster Species Concept, acknowledges that divergence and speciation can occur even with hybridization and gene flow.
Evidence: what did they find?
In order to study the different species and hybridization, the scientists had to first classify all 468 finches in their sample into the groups. They recorded the sex of the birds, either by looking at their feathers or by molecular methods, and they took several different measurements of the birds:
- Beak to back of head
- Beak depth and width
- Tip of beak to the feather line
- Taurus length
- Tip of beak to naris
- Wing length
Once they had taken all the birds’ measurements they analyzed nine locations in the birds’ genome. Based on the morphological measurements and genetic information, the researchers were able to categorize the birds into three groups: C. parvulus, C. pauper, and a mixed group of hybrids (admixed). Scientists observed a pattern of mating: C. pauper females were heterospecific when mating, while C. parvulus and the admixed populations mated within their group/species. Looking at the DNA analyses, Peters et al. (2017) discovered that genes were flowing unequally between the two species, into the third admixed group. The admixed group showed a higher level of genetic similarity, as well as morphological, to the small tree finches, compared to the medium tree finches (fig. 3). The researchers therefore grouped the admixed hybrids and the small tree finches into the same mating group. Based on observation and genetic and morphological data, Peters et al. (2017) showed that although C. pauper females would mate outside of their species, C. parvulus and admixed birds would mate within their group, potentially due to their striking similarity in appearance and nearly identical songs, that males use to attract mates. Finches learn their songs from their fathers, which for the hybrids would be small tree finches.
Figure 3. Genetic similarities between the two finch species and the admixed group, figure from Peters et al.
Peters et al. (2017) made three large observations that support the asymmetrical introgression between species:
- C.parvulus have more specific genes that are only found in their species cluster compared to C. pauper
- Females of the small tree finch paired with males more similar (small tree finches and hybrids)
- Females of the medium tree finches and hybrids had no preference
Since there were higher amounts of small tree finch DNA found in the hybrids, it provides evidence that higher rates of backcrossing are occurring between hybrids and small tree finches (fig. 4). The asymmetrical introgression occurring on this island is widely thought to be driven by sexual selection through female choice. Ultimately, preference of the female birds, regardless of the species is directing gene flow between the groups.
Hybridization of Darwin’s finches on Isla Daphne Major, Galápagos
There has been much debate in the past about whether Darwin’s finches hybridize in nature, and if they do whether hybridization could account for the intermediate appearance of certain forms. To resolve these issues the breeding of all finches on the small Galápagos island of Daphne Major was studied in every year from 1976 to 1992. The island supported breeding populations of Geospiza fortis (harmonic mean of 198 breeding individuals), G. scandens (H = 80), G. fuliginosa (H = 3) and, in the past 10 years, G. magnirostris (H = 6). Morphological criteria for defining species were developed in a study of the finches on the neighboring large island of Santa Cruz. These were then used with modification on Daphne to classify members of the first few generations to species. Observations of breeding birds showed that in a few cases species interbred. G. fortis hybridized with G. fuliginosa in 11 out of the 13 years in which both species bred. G. fortis and G. scandens hybridized in six of the years. Hybridization was always rare. Hybridizing individuals constituted 1.8% of breeding G. fortis, on average, 0.8% of G. scandens, but 73.0% of the much rarer G. fuliginosa. F1 hybrids were viable and fertile. They rarely bred with each other to produce an F2 generation. Much more frequently they backcrossed to the common species, G. fortis and G. scandens. In all these cases hatching and fledging success were high, giving scarcely any indication of genetic incompatibilities in the F1, F2 or backcross generations. The demonstration of natural hybridization answers some questions and raises others. It shows that introgression of genes could be a small factor contributing to the interm ediate appearance of G. fortis on Daphne Major: that is between typically larger forms of this species elsewhere in the archipelago and the smaller G. fuliginosa. However hybridization with the larger G. scandens has the opposite directional effect on G. fortis. Hybridization and introgression sometimes complement the effects of natural selection, sometimes they are opposed by it. Introgression also contributes to the large morphological variation displayed by this and several other populations in the archipelago. Hybridization raises questions about how species of Darwin’s finches (and other organisms) should be defined and recognized. In terms of the broad biological species concept there are four species of finches on Daphne Major, neither completely independent evolutionarily on the one hand (except for G. magnirostris, nor approaching panmixia on the other hand.
Understanding the effects of climatic factors on interannual patterns of hybridization is necessary to uncover the temporal and ecological factors that promote the formation of hybrids. This study reveals marked annual shifts in hybridization in Darwin’s Camarhynchus tree finches on Floreana Island that were not correlated with year of sampling but were strongly positively correlated with rainfall. Specifically, with data for male birth year spanning a 20-year period, we found that the number and percentage of hybrid males recruited into the population increased with increasing annual rainfall, when finch productivity and food resources are high ( Holmgren et al., 2001). This positive relationship between annual rainfall and the percentage of hybrid recruits was not found in the parental species and was independent of the number or percentage of hybrids captured per sampling year (i.e. eight years of data). Considering effects of year, we mist-netted a lower proportion of critically endangered C. pauper compared with C. parvulus and hybrid birds across the decade ( O’Connor et al., 2010), underlining concern for monitoring the fate of C. pauper in the face of introduced threats from biotic and abiotic factors. The finding of a positive effect of rainfall on hybrid recruitment lends strong support for climate-associated selection for hybrid birds in years of high rainfall.
Indeed, the outcome of hybridization can be favoured by selection ( Kagawa & Takimoto, 2018), as evidenced by its large role in the adaptive radiation of Darwin’s finches ( Palmer & Kronforst, 2015 Almén et al., 2016), as found for other taxa such as cichlid fish ( Meier et al., 2017) and monkeyflowers ( Rieseberg, 1997 Stankowski & Streisfeld, 2015). A recently documented evolutionary outcome of hybridization in Darwin’s ground finches was speciation in situ (Grant & Grant, 2017), where on Daphne Major in 1981, a resident female Geospiza fortis paired with an immigrant male G. conirostris and produced a lineage of reproductively isolated birds within three generations ( Lamichhaney et al., 2018). On Floreana, we have documented the opposite phenomenon of ‘species collapse’ due to hybridization among Camarhynchus species ( Kleindorfer et al., 2014 Peters et al., 2017, 2019), and the current study finds that high rainfall may be facilitating this process.
Research increasingly documents the link between hybridization and climate change effects with studies of novel contact zones between previously allopatric species ( Hewitt, 2011 Mallet et al., 2011 Ottenburghs et al., 2015). Introgression or hybridization is often found in these zones, potentially due to the breakdown of reproductive barriers via niche modification or expansion ( Wellenreuther et al., 2010 Mallet et al., 2011 Keller et al., 2013 Gómez et al., 2015). Such species ‘mixing’ can be an important source of new genetic variation that may allow adaptive traits to be introgressed into a population via alleles that increase a species’ fitness ( Lewontin & Birch, 1966 Arnold, 2004). Indeed, adaptive alleles with high fitness benefit may rapidly spread throughout a hybrid population when reproductive barriers are weak or absent ( Brand et al., 2013).
For Darwin’s finches, rainfall is tightly linked to resource availability and, in turn, to survival of Darwin’s ground finch hybrids (Geospiza spp.) ( Grant & Grant, 1996a). Apart from the role of climate, anthropogenic selection pressures may be further driving the increased recruitment of Camarhynchus hybrids on Floreana. The endemic Scalesia forest present on a few of the Galapagos Islands is where Camarhynchus finches predominantly nest, and this habitat has been degrading across the archipelago ( Rentería et al., 2012 Gardener et al., 2013 Cimadom et al., 2019), as well as in our study area (our personal observations) due to introduced weeds, pests and agricultural practices. This habitat degradation may reduce the abundance of native seeds and insects, leading to selection for a broader foraging niche breadth through hybridization via an intermediate bill size ( Boag & Grant, 1981 Grant & Grant, 1996b Langton & Kleindorfer, 2019).
A significant natural selection pressure on Darwin’s finches is the introduced blood-feeding parasitic fly Philornis downsi ( Kleindorfer & Dudaniec, 2016 Common et al., 2019), which was first discovered in Darwin’s finch nests in 1997 ( Fessl et al., 2018). We previously found that nests of hybrid birds possess fewer P. downsi than nests of parental species ( Peters et al., 2019), suggesting that hybridization confers some resistance to P. downsi. It is possible that selection resulting from P. downsi on Floreana Island favours greater genetic introgression between parental species ( Peters & Kleindorfer, 2015, 2018 Peters et al., 2019), and may help to explain why hybrid birds are much more common on Floreana than on Daphne Major where hybrids occurred in just 2–5% of breeding pairs ( Grant & Grant, 2008). Overall, our analysis of increased hybrid recruitment relative to parental species suggests that hybrid offspring are at a selective advantage in years of high rainfall, with interacting factors that are acting in concert to sustain a growing hybrid swarm on Floreana Island, which may be better equipped under anthropogenic pressures.
A notable difference between patterns of hybridization in the tree finches on Floreana and the ground finches on Daphne Major is that on Floreana, there was some admixture (genetic assignment ranging between 0.75 and 0.96) between parental species detected in all individuals we sampled ( Supporting Information, Fig. S1 see also Peters et al. 2017), whereas on Daphne Major, the majority of G. fortis were genetically assigned to G. fortis with a probability at or very close to 1.0 ( Grant & Grant, 2010). These different patterns across islands are unlikely to be the result of poor marker resolution given that similar microsatellite datasets were used in both cases. Furthermore, preliminary analysis of the Floreana population using
15 000 single nucleotide polymorphisms indicated some genetic admixture within the parental species (R. Y. Dudaniec et al., unpublished data).
The reasons for such marked differences between the two systems are not clear, but we propose the following. (1) Camarhynchus pauper and C. parvulus appear to have diverged from each other more recently than G. fortis and G. scandens, which may have facilitated low rates of hybridization and gene sharing. This is indicated by the phylogeny given by Lamichhaney et al. (2015), which shows that C. pauper and C. parvulus (from Santa Cruz) share their most common recent ancestor whereas relationships for Geospiza are variable and often not shared. (2) The documented loss of the large tree finch C. psittacula on Floreana may have promoted long-term, ongoing hybridization between ‘larger and smaller birds’, resulting in a hybrid swarm-like pattern on Floreana, with a gradient of genetic assignment probabilities from larger- to smaller-bodied birds ( Kleindorfer et al., 2014 Peters et al., 2017). Finally, (3) selection pressures from habitat fragmentation in the Floreana highlands, which is severe and ongoing, combined with mortality impacts associated with P. downsi, may further promote hybridization. Indeed, we found that hybridization did increase after 1997, when P. downsi was first discovered in Darwin’s finch nests ( Fessl et al., 2001).
Genetically distinguishing the occurrence and frequency of hybrids formed by sympatric and recently derived species can be challenging alongside the ecological and genetic processes that define species’ boundaries ( Gow et al., 2006). Here, we partially overcome this difficulty with a validated hybrid genetic assignment test for the study system ( Kleindorfer et al., 2014 Peters et al., 2017) and having the capacity to age males and ascertain hybrid birth rates, and therefore, annual hybrid recruitment. Most avian hybridization studies have been unable to quantify hybrid birth rate due to an inability to assign hybrid age ( Jackson et al., 1992 Ebels et al., 2001), and so potentially provide misleading estimates of hybrid recruitment and the drivers of hybridization. Notably, in the current study we do not assume an equal sex ratio of male to female hybrid birds and make conclusions for male hybrids only, due to the possibility of sex-specific fitness differences ( Langton & Kleindorfer, 2019). Furthermore, we were only able to confidently assign male age up to
4 years based on plumage coloration, and with an estimated longevity of 12–15 years ( Langton & Kleindorfer, 2019), we could not assess hybrid recruitment for older males. Therefore, our estimated values for hybrid recruitment provide a relative rather than actual measure for the population.
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Zombie Watch: Debunked Finches Re-Emerge to Validate Darwin
Peter and Rosemary Grant are the Princeton pair who have spent their careers on the Galápagos Islands trying to tease out the slightest bits of evidence to support the iconic myth of Darwin’s finches. Having received the Royal Medal in Biology last summer, they’re at it again. That is despite having been soundly refuted by Jonathan Wells in his book Zombie Science. Now that the Grants are passing the baton to younger researchers, we will undoubtedly be treated to more parades of this zombie icon.
In “Rapid hybrid speciation in Darwin’s finches” in the journal Science, four other lead authors, accompanied by the Grants, try to sanctify neo-Darwinism with a melodrama about three “species” of finches that can all interbreed. Mind you, they are all finches. They are all Galápagos finches. They are all family.
Any differences among the groups are tiny changes in beak size and shape, and changes in the songs one group sings. Science Daily has a cartoon version of the story, complete with a lineage called “Big Bird”:
The arrival 36 years ago of a strange bird to a remote island in the Galapagos archipelago has provided direct genetic evidence of a novel way in which new species arise.
In this week’s issue of the journal Science, researchers from Princeton University and Uppsala University in Sweden report that the newcomer belonging to one species mated with a member of another species resident on the island, giving rise to a new species that today consists of roughly 30 individuals.
The study comes from work conducted on Darwin’s finches, which live on the Galapagos Islands in the Pacific Ocean. The remote location has enabled researchers to study the evolution of biodiversity due to natural selection. [Emphasis added.]
The first question is obvious: If they can interbreed, how can they be called different species? Darwin’s book was about the Origin of Species, not the origin of varieties. As Wells points out, “If they continue to breed and exchange genes, they are usually regarded as varieties of the same species, even if they are morphologically different (as is the case with dog breeds)” (Zombie Science, p. 68).
The “strange bird” that showed up was a lone male who had a slightly different song. He found a mate, they had chicks, and the family decided to live in the same community away from the others. This is called “reproductive isolation” and is considered by Darwinians as a step toward speciation. But people do that. How many stories are told of a wayfaring stranger appearing from a far country, finding a bride, and, over the objections of her family, taking her to start a new life together in a different place? Are they now “reproductively isolated”? Are they emerging as a new species? As Wells says in his charitable way, “Indeed, it is far from obvious why we should consider them separate species at all.” He gives an example:
The Ainu people of northern Japan and the !Kung people of southern Africa are separated not only physically and linguistically, but also (for all practical purposes) reproductively. Are they therefore separate species? Or are they all human beings? Of course the Ainu and the !Kung are all members of the same species.
Since the Galapagos finches regularly interbreed, why should we call them separate species, other than to make them appear to be evidence for evolution?
The BBC News tries to have it both ways:
In the past, it was thought that two different species must be unable to produce fertile offspring in order to be defined as such. But in more recent years, it has been established that many birds and other animals that we consider to be unique species are in fact able to interbreed with others to produce fertile young.
They’d better not push that idea too far, or else they will be calling Japanese a different species from Germans. That’s no joke to evolutionists, human beings fit in the category “other animals.”
The cartoon version accentuates the differences between the birds to make them look as different as possible. Science Daily continues:
The offspring were also reproductively isolated because their song, which is used to attract mates, was unusual and failed to attract females from the resident species. The offspring also differed from the resident species in beak size and shape, which is a major cue for mate choice. As a result, the offspring mated with members of their own lineage, strengthening the development of the new species.
Humans do this, too. Think of cases where an immigrant population kept to themselves, because they had their own culture and music. This affected their “mate choice,” as well.
The paper in Science makes a big deal of hybridization (see here about how rampant hybridization is scrambling Darwin’s tree). Science Daily explains:
A critical requirement for speciation to occur through hybridization of two distinct species is that the new lineage must be ecologically competitive — that is, good at competing for food and other resources with the other species — and this has been the case for the Big Bird lineage.
But again, the human analogy gives the lie to this idea. If an Ainu woman married a !Kung man, we wouldn’t, needless to say, call their children hybrids. In addition, human tribes in many places on Earth are reproductively isolated, yet successful. They can even be reproductively isolated in the same country, preferring to marry ones that have the same tastes or looks. The idea that they must be competitive comes from Malthus and Darwin, not from real life.
Here’s another glitch in the story not apparently noticed by the researchers:
Researchers previously assumed that the formation of a new species takes a very long time, but in the Big Bird lineage it happened in just two generations, according to observations made by the Grants in the field in combination with the genetic studies.
They sound delighted to find that speciation occurred fast, but think of what that means. Those islands have been isolated from the mainland for at least 8 million years — maybe 90 million. Unless the evolutionists believe the Big Bird incident was extremely rare or unique, such hybridizations should have been frequent. If so, wouldn’t the gene pool be scrambled beyond recognition? If rare, the story begins to look like a case of special pleading. Neither option is particularly helpful to Darwinian theory. The paper appeals to “rare and chance events” to explain Big Bird. Isn’t it odd that such a rare event happened while the Grants just happened to be watching? What’s the probability of that?
Why do the Darwinians make so much of so little? The reason: the Galápagos Islands are holy ground. Researchers will work for years to honor the founder of their worldview.
Photo: Medium ground finch, Galápagos Islands, by Charlesjsharp (Own work, from Sharp Photography, sharpphotography) [CC BY-SA 3.0], via Wikimedia Commons.
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The main finding of the study is the role of G. fortis as a conduit species, which is demonstrated by the occurrence of two types of hybrids. The first group of hybrids are G. scandens × G. fuliginosa admixtures (dihybrids), since these two species are not known to interbreed on Daphne. G. scandens and G. fuliginosa differ strongly in the traits used in mate choice—beak size and shape, body size, and song (61, 62, 72)—whereas morphological difference between them are smaller on San Cristóbal island, where they do interbreed (personal observations, 1997 and 2018). The second group of hybrids are admixtures of three species (trihybrids).
We used neutral markers, microsatellites, to assign individuals to species or hybrids, using an arbitrary but often used and biologically justified criterion (36, 39, 43, 46) to separate them (SI Appendix, section 2). Assignments are subject to error (SI Appendix, section 2), and it is possible that the small number of loci results in an inflated number of erroneous assignments to hybrid classes. There are three reasons for believing this is not a source of strong bias. First, in a simulation study, most hybrids (0.85) were correctly identified (63). Second, each hybrid group has morphological features expected from its assigned genetic composition: intermediate means between the parental groups that are known or estimated to give rise to it, and increased variances. This would not be expected if many of the hybrids were not in fact hybrids. Third, there is no evidence of inflation in the samples from 1984 to 1998 when the pedigree allowed identification of first-generation backcrosses. Ten of 1,808 individuals with all four grandparents and both parents known have genomes with mixtures of G. fortis, G. scandens, and G. fuliginosa. Eight of the 1,808 individuals were assigned to trihybrids by microsatellites.
Existence of the two groups raises questions of genetics and fitness. How can a two-way or three-way mixture of genomes of different species function in individual organisms, what combinations of genes are compatible and which ones are not, how stable or transitory are their long-term dynamics, and what factors permit, promote, or hinder their existence? Answers to these questions can help to explain how barriers to interbreeding become strengthened in the process of speciation leading to complete reproductive isolation (1, 2, 15, 73, 74). Similar questions have been addressed in studies of interbreeding populations of Neanderthals, Denisovans, and ancestors of modern humans (75, 76), and the answers may portend future discoveries with finches and other organisms. Introgressive hybridization has been associated with a transfer of adaptive genes, for example those affecting skin color and immune function (75), deleterious genes affecting male fertility (76), and presumably an abundance of selectively neutral genes in other words, a mixture, subject to repeated selective filtering.
To address these genetic questions, a preferable approach would be to use whole genomes. Nonetheless, microsatellites are sufficient to show that, in combination, dihybrids and trihybrids were at no disadvantage in terms of survival or recruitment when compared with the species that gave rise to them during a period of favorable food conditions following the change in ecology caused by the 1982 to 1983 El Niño event. High fitness is surprising, and indicates that selection against novel gene combinations in this group of finches must be weak at most. The only disadvantage we could detect was low survival of the smallest SF hybrids. This should slow the rate of convergence of G. scandens on G. fortis however, we doubt if this happens because it is counteracted by relatively high survival of Sf hybrids (Fig. 5), which are also smaller in body size than G. scandens (Table 3).
We can identify two factors that help to explain how the genome of one species is apparently tolerant to invasion (77) by two others. The first is the phylogenetic youth of the group. G. fortis and G. fuliginosa are sister species according to a whole-genome phylogenetic analysis of autosomes (28). They shared a common ancestor ∼240,000 y ago, and they shared a common ancestor with G. scandens ∼260,000 y ago or earlier. These are minimum estimates because allele sharing makes them appear to be more similar and younger than would be revealed by their true history. Their youth means that relatively few genes have diverged in separate species they mainly affect morphological traits related to feeding and breeding ecology, and they cause no incompatibility and have little or no transgressive effects upon phenotypes. The second factor is the genetic similarity of G. fuliginosa and G. scandens, as indicated by their microsatellites. Although G. fortis and G. fuliginosa are the most similar pair of species (Nei’s D = 0.28), G. scandens is more similar to G. fuliginosa (D = 0.46) than it is to G. fortis (D = 0.76). G. scandens is also more similar to G. fuliginosa in beak shape, although not in body size (Fig. 3D).
Triad hybridization, with one species acting as bridge or conduit between two noninterbreeding species, is valuable for what it reveals about the potential for the generation of novel phenotypes. Introgressive hybridization increases the potential by increasing additive genetic variances, altering genetic covariances, and constructing new combinations of interacting genes (12, 15). As expected, we found increased morphological variances in both dihybrids and trihybrids. A previous study provided evidence of relaxed genetic covariances in dihybrids (17), as has also been found in fish (21, 70). Multivariate effects should be greater through hybridization of three species than of two species. We found no clear case of extreme phenotypes beyond the range of variation of both of the contributing parental groups (transgressive segregation) however, three trihybrids displayed allometric transgression, lying outside the region of body size variation but not outside the range of beak shape variation of the parental groups. Some dihybrids displayed similar patterns. The evolutionary potential of allometric transgression has been demonstrated with the origin of a new lineage on Daphne Major. The lineage was initiated by hybridization of an immigrant Geospiza conirostris with a resident G. fortis, resulting in the formation of a reproductively isolated population that displayed allometric transgression, that is, a deviation from the two species in the relationship between beak size and body size (27, 78). Transgressive morphology is implicated in reproductive isolation because mate choice is based in part on beak and body size (61, 62, 72) and the relationship between the two (79).
Triad hybridization is likely to be more common than is currently recognized by the few cases that have been documented. For example, we would expect it in species-rich adaptive radiations that have diversified relatively recently. Foremost among them are the hundreds of cichlid fish species in the Great Lakes of Africa, for which genome data are becoming rapidly available (78 ⇓ ⇓ ⇓ ⇓ ⇓ –84), and the many species of Heliconius butterflies that participate in mimicry rings (3, 26, 43, 85). Other possible groups are numerous species complexes that are similar morphologically and genetically and sometimes difficult to resolve taxonomically, such as Cottoid fish in Lake Baikal (86), Anastrepha flies (87, 88), ant-nest beetles (89), some groups of mosquitoes (44) and, among plants, Andean lupins (90) and the Hawaiian Silversword Alliance (91).
Since triad hybridization occurs in contemporary time, it must have occurred in the past. Together with hybridization of one species with two others separated in time or in space (a pair of dyads) (92, 93), triad hybridization may be responsible for polytomies in phylogenetic trees. Polytomies occur when branching cannot be resolved into bifurcations because an ancestral species and two descendant species are genetically so similar (94 ⇓ ⇓ –97). The power to measure divergence is likely to be reduced when the samples are affected by triad hybridization.
Hybridization on Daphne is relevant to a global outlook on the future. The frequency and success of hybridization on the island increased from 1983 onward as a result of a change in vegetation caused by abundant rain associated with an intense and prolonged El Niño event. This major perturbation is a natural analog of an anticipated unnatural, human-caused change in the global environment. Whether or not populations have sufficient genetic variation for adaptive responses is the subject of ongoing critical debate (98 ⇓ ⇓ –101). One way that variation is enhanced is through interspecific gene exchange (19, 102). Hybridization is believed to be increasing in frequency as a result of anthropogenically caused change to climate and to habitat that reduces population sizes and brings together previously separated species (102 ⇓ ⇓ ⇓ –106). Hybridizing species may therefore be disproportionately successful in coping with a changing environment in the future, as in the past (9, 18, 107). Fur seals on Macquarie Island (39) provide a prime example of what might come. The original population became extinct as a result of overexploitation in the 19th century. The island has been recolonized by two interbreeding species, Arctocephalus gazella and Arctocephalus tropicalis. A ménage à trois was created by a single male of a third species, Arctocephalus forsteri, and it led to the production of offspring with genes from all three species. It remains to be seen in this case and more generally if multispecies hybridization has significant consequences in terms of evolution and conservation in a world of increasing habitat destruction and climate change.
Some experts think that even today hybrid speciation may be far from rare. Under the most commonly accepted speciation model, called allopatric speciation, populations get geographically separated — by a change in a river’s course, say, or the formation of a mountain, or diverging migrations — and then adapt to distinct competitors and environments over long periods. If the groups ever meet again, they may no longer be similar enough to interbreed.
But very fast hybrid speciation events like the one the Grants saw on Daphne may often occur in bursts — only to end with the new species dying out before we have time to observe it. “Speciation is common. It’s happening all around us,” said James Mallet, an evolutionary biologist at Harvard University. “It’s just that usually we don’t recognize the divergent lineages that are appearing as separate species.”
He continued, “I believe that speciation is more of a continuum than people have been thinking.” At one extreme, species can be cleanly divided from one another, with no interbreeding and exchange of genes. But to greater or lesser degrees, hybridization could also allow genes to flow into a species from others, and the resulting hybrids could sometimes develop an identity of their own, even if only temporarily.
Mallet argues that hybrid species could very well be appearing all the time, only to collapse and disappear just as quickly, with the hybrids either going extinct or being absorbed back into a parent population. “To me, that shows the abundance of opportunities for speciation,” he said. Just because many hybrids go extinct — a fate that is still very likely to befall the Big Bird lineage, according to Mallet — doesn’t mean that hybridization is not a real source of new species in nature.
Instead, he sees speciation as an almost fractal process that can be observed in ecosystems over different timescales. “If you look at the macro level over millions of years,” Mallet said, “you’ll see a few species evolving and going extinct.” But at the micro level, on the order of dozens of years or less, species may be forming and dissolving all the time. Most biologists simply don’t have the long-term data to show it, he said.
What made it possible to identify such an event in Darwin’s finches was the Grants’ decades of careful fieldwork, followed by detailed genomics studies. The new paper illustrates “the value of continuous, long-term studies in nature,” Peter Grant said. Without that, “we would not have detected or been able to interpret the immigration of an individual of one species and interbreeding with a member of the resident species.”
Although biologists do not yet know how much animal hybrid speciation occurs outside the Galápagos, they are becoming increasingly aware of hybrids as a powerful agent of evolution. “I think this paper really increases that signal,” Edwards said. “Researchers like me are going to be looking for it much more regularly.”