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The devel­op­ment of a new species of Darwin’s finches in just two generations

pub­lished 26 Novem­ber 2017 | mod­i­fied 26 Novem­ber 2017

Darwin’s finches in the Galá­pa­gos arch­i­pel­ago pro­vide an iconic model for the evo­lu­tion of bio­di­ver­sity on earth due to nat­ural selec­tion. A team of sci­en­tists from Prince­ton Uni­ver­sity and Upp­sala Uni­ver­sity now reports that they have observed the ori­gin of a new species. A new lin­eage was formed by the hybridiza­tion of two dif­fer­ent species of Darwin’s finches. The study is pub­lished on 23 Novem­ber in the jour­nal Sci­ence.

Direct obser­va­tion of the ori­gin of a new puta­tive species occurred dur­ing field work car­ried out by Rose­mary and Peter Grant, Prince­ton Uni­ver­sity, on the small island of Daphne Major of the Galá­pa­gos arch­i­pel­ago for 40 con­sec­u­tive years. In 1981 they observed an immi­grant male that sang an unusual song and dif­fered in size from all res­i­dent species on the island. The male bred with a res­i­dent medium ground finch female and thereby ini­ti­ated a new lin­eage which they named the Big Bird lin­eage. Already in 2009 the Grants pub­lished about their obser­va­tions in PNAS. They fol­lowed the new lin­eage for 6 gen­er­a­tions over 30 years. DNA sequence data now reveal that the immi­grant male was a large cac­tus finch. Remark­ably, it must have flown to Daphne from Española Island, which is more than 100 km to the southeast.

A crit­i­cal step in spe­ci­a­tion is the estab­lish­ment of repro­duc­tive iso­la­tion. It is usu­ally assumed that this process takes a very long time but in the Big Bird lin­eage it hap­pened in just two gen­er­a­tions, say Rose­mary and Peter Grant.”

Charles Dar­win would have been excited to read this paper.

Leif Ander­s­son, co-​author, Upp­sala Uni­ver­sity, the Swedish Uni­ver­sity of Agri­cul­tural Sci­ences and Texas A&M University.

One impor­tant rea­son for this is the unique song of the immi­grant male, since sons learn the song of their father and females mate with males that sing like their fathers, con­tinue Peter and Rose­mary Grant. A sec­ond rea­son is the new lin­eage dif­fered from the res­i­dent species in beak mor­phol­ogy, which is also a major cue for mate choice.”

All 18 species of Darwin’s finches have been derived from a sin­gle ances­tral species that col­o­nized the Galá­pa­gos 12 mil­lion years ago. They have diver­si­fied into dif­fer­ent species, and changes in beak mor­phol­ogy in par­tic­u­lar have allowed dif­fer­ent species to uti­lize dif­fer­ent food sources on the Galá­pa­gos. Thus, another crit­i­cal require­ment for spe­ci­a­tion to occur through hybridiza­tion is the new lin­eage must be eco­log­i­cally com­pet­i­tive, and this has been the case for the Big Bird lineage.

Big Birds lineage evolution GalapagosSchematic illus­tra­tion of the evo­lu­tion of the Big Bird lin­eage on the Daphne Major island in the Galá­pa­gos arch­i­pel­ago. Pho­tos © K. Thalia Grant for G. conirostris and Peter R. Grant for the remain­der. Repro­duced with per­mis­sion from K. Thalia Grant, and Prince­ton Uni­ver­sity Press, which first pub­lished the remain­ing images in 40 Years of Evo­lu­tion (P. R. Grant & B. R. Grant, 2014).
Cre­ative Com­mons license: CC Attri­bu­tion, no derivatives

It is very strik­ing that when we com­pare the size and shape of the Big Bird beaks with the beak mor­pholo­gies of the other three species inhab­it­ing Daphne Major island the Big Birds occupy their own niche in the beak mor­phol­ogy space”, explains Sangeet Lamich­haney, cur­rently post-​doctoral fel­low at Har­vard Uni­ver­sity. “Thus, the com­bi­na­tion of gene vari­ants con­tributed from the two inter­breed­ing species in com­bi­na­tion with nat­ural selec­tion led to the evo­lu­tion of a beak mor­phol­ogy that was com­pet­i­tive and unique.”

A clas­si­cal def­i­n­i­tion is that good species respect species bound­aries and can­not pro­duce fully fer­tile prog­eny if hybridiza­tion hap­pens, as is the case for the horse and the don­key for exam­ple. How­ever, in recent years it has become clear that some closely related species, which nor­mally avoid breed­ing with each other, exchange genes by hybridiza­tion sur­pris­ingly often. The authors of this study have pre­vi­ously reported that there has been a con­sid­er­able amount of gene flow going on among species of Darwin’s finches for thou­sands of years.

The inter­est­ing aspect of this study is that a hybridiza­tion between two dis­tinct species led to the devel­op­ment of a new lin­eage that after only two gen­er­a­tions behaved as any other species of Darwin’s finches. If a nat­u­ral­ist had come to Daphne Major island with­out know­ing that this lin­eage arose very recently it would have been rec­og­nized as one of the four species on the island. This clearly demon­strates the value of long-​running field stud­ies”, says Leif Ander­s­son at Upp­sala Uni­ver­sity, the Swedish Uni­ver­sity of Agri­cul­tural Sci­ences and Texas A&M University.

It is very likely that new lin­eages like the Big Birds have orig­i­nated many times dur­ing the evo­lu­tion of Darwin’s finches. The major­ity of these have gone extinct but some may have led to the evo­lu­tion of con­tem­po­rary species. We have no idea about the long-​term sur­vival of the Big Bird lin­eage but it has the poten­tial to become a suc­cess, and it pro­vides a beau­ti­ful exam­ple of one way in which spe­ci­a­tion occurs. Charles Dar­win would have been excited to read this paper”, ends Leif Andersson.

(Source: Upp­sala Uni­ver­sity press release, 23.11.2017)

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