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Contrary to what many people think, two species can hybridize naturally in the wild to give rise to a third species, a finding that fundamentally alters our understanding of the evolution of species
Ligers! Tigons! And bears! Oh my!
Not long ago, I told you about a fascinating songbird that was discovered to be the hybrid offspring of three different species (more here). This unique bird raised an intriguing question: can hybrids give rise to a perfectly valid species?
Hybrid speciation is quite rare in animals, but it does occur naturally. In this scenario, the resulting hybrid population is an independent new species that is reproductively isolated from both parental species. One such example is the Heliconius butterfly (ref). These brightly colored butterflies are widespread throughout South and Central America and are even found in parts of North America. They are remarkable for their astonishing diversity of wing patterns and for extensive mimicry within the group.
Amongst birds, probably the best-known examples of hybrid species are the Italian sparrow, Passer italiae, golden‐crowned manakin, Lepidothrix vilasboasi, and a recently identified but currently unnamed Galapagos finch that was estimated to have first popped up in the 1980s (ref).
Despite these examples, the idea that a new species could evolve through the hybridization of two other species goes against Darwin’s thinking (ref) because it violates the fundamental definition of a species: reproductive isolation. Additionally, because species are adapted to their own special niches, and because hybrid offspring show a more-or-less intermediate blend of their parents’ characters, hybrids should be less adapted to either parent’s niche, and therefore, less likely to survive. As a result, hybrids tend to disappear over time. For these reasons, hybrid speciation is an almost completely foreign concept in the animal kingdom.
However, in the very rare situation where a hybrid species is more adapted to a specific niche than are both of its parental species, the hybrid may replace one or both of its parent species, or it may establish its own place in a special niche that was previously unoccupied.
There are, of course, genetic constraints that work against hybrid speciation in animals. The most persistent hybrid species possess the same number of chromosomes as both parental species. This means both parental species must be closely related to avoid genetic and developmental problems associated with unpaired chromosomes in their hybrid offspring. This form of hybridization, known as homoploid hybrid speciation, is most common amongst animals. (The most famous mammalian hybrid, the mule, is the product of a horse and a donkey, which have different numbers of chromosomes, thus rendering their hybrids infertile.)
A most surprising discovery
“Actually, we were looking at the relationships among the the prions, of the monophyletic genus Pachyptila, with a distribution exclusive to the Antarctic and sub-Antarctic waters of the Southern Ocean,” said Juan Masello, a Principal Investigator in the Department of Animal Ecology & Systematics at Justus Liebig University. Dr. Masello specializes in the behavioral ecology, parasitology, and molecular ecology of wild populations of birds, particularly parrots and seabirds. Dr. Masello investigates population genetics under the mentorship of co-author, Yoshan Moodley, a professor of zoology at the University of Venda.
Pachyptila prions are a small group of closely-related seabirds that have the same ancestor (“monophyletic”). They are pigeon-sized, with white underparts, blue-grey upper parts, and a soot-colored “M” that extends across their back from one long slender wingtip to the other. They roam widely across the Antarctic Ocean and they all look remarkably similar to each other, especially when glimpsed as they zoom over rough, wind-tossed sub-Antarctic seas.
Dr. Masello, Professor Moodley and their collaborators were carefully examining these enigmatic seabirds to finally identify precisely how many Pachyptila species there are.
“Based on phylogenetics, between two and seven species were recognised, however this was controversial,” Dr. Masello and Professor Moodley explained in email.
But as Dr. Masello, Professor Moodley and their collaborators worked to identify how many species there are, they ran across something ... strange.
“We investigated these relationships in two previous studies [ref and ref] but noticed that something was different than expected in [Salvin’s medium-billed prion] salvini,” Dr. Masello said in email. So Dr. Masello, Professor Moodley and their collaborators investigated further.
Because each species of prion breeds only on one or on a few very specific sub-Antarctic islands and at their own distinct times of the year, Dr. Masello, Professor Moodley and their collaborators could map out exactly where and when each species’ nest burrows could be found.
They showed that the hybrid species, Salvin’s medium-billed prion, Pachyptila salvini (grey circle; Figure 1), only breeds on Marion Island (grey circle; Figure 1), so this species is maintained as genetically separate from both of its parental species, the broad-billed prion, Pachyptila vittata (orange circles; Figure 1) and the Antarctic prion, Pachyptila desolata (green circles; Figure 1).
Bird bills are specialized cutlery that evolved for particular diets
Not only can prions be identified by where and when they breed, but their bills are different, too. In fact, the length, width and heighth of bird bills are critically important for efficient foraging. Thus, Pachyptila prion species can be distinguished in-hand by carefully examining their bills: three of the six species in the genus have flattened bills with a fringe (lamellae) on the sides that act as strainers to retain small marine crustaceans and fishes, similar to how baleen whales feed. This structure is reflected in these birds’ common name, which comes from the Greek priōn (“saw”), a reference to the serrated edges of the birds’ saw-like bill. They also are commonly known as “whalebirds”.
When Dr. Masello, Professor Moodley and their collaborators measured the dimensions of the bills for all six of the Pachyptila species, they found no overlap between any of the species (Figure 2):
What do the genes tell us?
Dr. Masello, Professor Moodley and their collaborators sequenced and analyzed DNA from 425 individuals for five of the Pachyptila species, and for the closely-related blue petrel, Halobaena caerulea. Genetically, the mitochondrial DNA (mtDNA) groups Salvin’s medium-billed prion in with the Antarctic prion, yet nuclear microsatellite DNA places it with either the Antarctic prion or the broad-billed prion, depending on the method used to estimate genetic distance. A hybrid origin for Salvin’s medium-billed prion could explain these unusual results.
But what surprised Dr. Masello most about this study’s findings? That the combination of parental traits led to both increased fitness and reproductive isolation for Salvin’s medium-billed prion, he said.
This study shows that “hybridization between species is not necessarily the end of the line for evolution, and that sometimes, a new species can be formed this way,” Dr. Masello elaborated in email.
“It also draws our attention to the idea that this method of speciation might be more common than first thought, and especially in recently or rapidly evolving groups of species, where reproduction isolation has not fully developed.”
Source:
Juan F. Masello, Petra Quillfeldt, Edson Sandoval-Castellanos, Rachael Alderman, Luciano Calderón, Yves Cherel, Theresa L. Cole, Richard J. Cuthbert, Manuel Marin, Melanie Massaro, Joan Navarro, Richard A. Phillips, Peter G. Ryan, Lara D. Shepherd, Cristián G. Suazo, Henri Weimerskirch, and Yoshan Moodley (2019). Additive Traits Lead to Feeding Advantage and Reproductive Isolation, Promoting Homoploid Hybrid Speciation, Molecular Biology and Evolution, msz090 | doi:10.1093/molbev/msz090
Hybrid Speciation: When Two Species Become Three | @GrrlScientist
