AboutZoos, Since 2008


Chal­leng­ing Dar­win: ‘Be dif­fer­ent or die’ does not drive evolution

pub­lished 23 Decem­ber 2013 | mod­i­fied 03 Novem­ber 2014

A new study has found that species liv­ing together are not forced to evolve dif­fer­ently to avoid com­pet­ing with each other, chal­leng­ing a the­ory that has held since Darwin’s Ori­gin of Species.

Ovenbirds phylogenetic treeBy focus­ing on oven­birds, one of the most diverse bird fam­i­lies in the world, the Oxford University-​led team con­ducted the most in-​depth analy­sis yet of the processes caus­ing species dif­fer­ences to evolve. They found that although bird species occur­ring together were con­sis­tently more dif­fer­ent than species liv­ing apart, this was sim­ply an arte­fact of species being old by the time they meet. In fact, once vari­a­tion in the age of species was accounted for, coex­ist­ing species were actu­ally more sim­i­lar than species evolv­ing sep­a­rately, oppo­site to Darwin’s view which remains widely-​held today.

A first glance at our data sug­gests the same pat­terns that Dar­win had expected. It is only when account­ing for the fact that species vary in age, and then com­par­ing between lin­eages of sim­i­lar age, that the pic­ture changes
Dr Nathalie Sed­don, co-​author, Depart­ment of Zool­ogy, Oxford University »

“It’s not so much a case of Dar­win being wrong, as there is no short­age of evi­dence for com­pe­ti­tion dri­ving diver­gent evo­lu­tion in some very young lin­eages,” said Dr Joe Tobias of Oxford University’s Depart­ment of Zool­ogy, who led the study. “But we found no evi­dence that this process explains dif­fer­ences across a much larger sam­ple of species.”

“The rea­son seems to be linked to the way new species orig­i­nate in ani­mals, which almost always requires a period of geo­graphic sep­a­ra­tion. By using genetic tech­niques to estab­lish the age of lin­eages, we found that most oven­bird species only meet their clos­est rel­a­tives sev­eral mil­lion years after they sep­a­rated from a com­mon ances­tor. This gives them plenty of time to develop dif­fer­ences by evolv­ing separately.”

The study, pub­lished online on 22 Decem­ber in Nature, com­pared the beaks, legs and songs of over 90% of oven­bird species. To tackle the huge chal­lenge of sequenc­ing genes and tak­ing mea­sure­ments, Oxford Uni­ver­sity sci­en­tists were joined by col­leagues at Lund Uni­ver­sity (Swe­den), Louisiana State Uni­ver­sity, Tulane Uni­ver­sity (New Orleans) and the Amer­i­can Museum of Nat­ural His­tory (New York).

Although species liv­ing together had beaks and legs that were no more dif­fer­ent than those of species liv­ing apart, the most sur­pris­ing dis­cov­ery was that they had songs that were more sim­i­lar. This chal­lenges some long­stand­ing ideas because the stan­dard view for the last cen­tury has been that bird species liv­ing together would need to evolve dif­fer­ent songs to avoid con­fu­sion.

“Look­ing at the big­ger pic­ture, ‘be dif­fer­ent or die’ doesn’t appear to explain evo­lu­tion,” said Dr Tobias. ‘Oven­bird species use a wide vari­ety of beaks, from long and hooked to short and straight, but these dif­fer­ences appear to evolve when liv­ing in iso­la­tion, sug­gest­ing that com­pe­ti­tion is not the major dri­ving force pro­duc­ing species dif­fer­ences. Instead, it seems to have the oppo­site effect in pro­mot­ing the evo­lu­tion of sim­i­lar songs.”

“The rea­sons for this are dif­fi­cult to explain and require fur­ther study, but they may have some­thing to do with the advan­tages of using the same ‘lan­guage’ in terms of sim­i­lar aggres­sive or ter­ri­to­r­ial sig­nals. For instance, indi­vid­u­als of two closely related species with sim­i­lar songs may ben­e­fit because they are able to defend ter­ri­to­ries and avoid harm­ful ter­ri­to­r­ial con­tests not only against rivals of their own species but those in other species coex­ist­ing in the same places and com­pet­ing for sim­i­lar resources.”

“The real nov­elty of this research is that it takes the evo­lu­tion­ary age of species into account,” said Dr Nathalie Sed­don of Oxford University’s Depart­ment of Zool­ogy, co-​author of the study. “A first glance at our data sug­gests the same pat­terns that Dar­win had expected. It is only when account­ing for the fact that species vary in age, and then com­par­ing between lin­eages of sim­i­lar age, that the pic­ture changes.”

“These insights are the result of a hugely col­lab­o­ra­tive ven­ture, and a good exam­ple of stand­ing on the shoul­ders of giants. It took almost a decade to com­pile genetic sequences and trait mea­sure­ments from 350 lin­eages of oven­bird. The song record­ings were made in over twenty coun­tries by numer­ous trop­i­cal ornithol­o­gists, includ­ing our­selves, and the museum mate­r­ial was based on spec­i­mens col­lected by hun­dreds of indi­vid­u­als stretch­ing back to the famous British nat­u­ral­ist Alfred Rus­sell Wal­lace in the 1860s.”

(Source: Uni­ver­sity of Oxford news release, 23.12.2013)

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