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December 04, 2009

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Feeding birds could change their evolution

Dec. 4, 2009
Courtesy Cell Press
and World Science staff

Feed­ing birds in win­ter is in­no­cent fun, but it can have pro­found—and rel­a­tively quick­—ef­fects on the ev­o­lu­tion­ary fu­ture of a spe­cies, re­search­ers say.

A re­port pub­lished on­line in the Dec. 3 is­sue of the re­search jour­nal Cur­rent Bi­ol­o­gy ar­gues that a popula­t­ion of birds known as black­caps has split in­to two “re­pro­duc­tively iso­lat­ed” groups in un­der 30 genera­t­ions, de­spite con­tin­u­ing to breed in the same forests. In oth­er words, the groups aren’t breed­ing with each oth­er, set­ting the stage for them to ev­olve dif­fer­ent­ly in the future.

Feed­ing birds in win­ter is in­no­cent fun, but it can none­the­less have pro­found—and rel­a­tively quick­—ef­fects on the ev­o­lu­tion­ary fu­ture of a spe­cies, re­search­ers say. Above, a male black­cap Syl­via atri­ca­pil­la. (Pho­to cour­tesy Mi­chael Apel; dis­tri­buted und­er the GNU Free Do­cu­ment­ation Li­cense)
The re­pro­duc­tive isola­t­ion be­tween these popula­t­ions is now stronger than that of oth­er black­caps that are al­ways sep­a­rat­ed from one anoth­er by dis­tances of 800 kilo­me­ters (500 miles) or more, the sci­en­tists said.

“Our study doc­u­ments the pro­found im­pact of hu­man ac­ti­vi­ties on the ev­o­lu­tion­ary tra­jec­to­ries of spe­cies,” said Mar­tin Schae­fer of the Un­ivers­ity of Frei­burg. “It shows that we are in­flu­enc­ing the fate not only of rare and en­dan­gered spe­cies, but al­so of the com­mon ones that sur­round our daily lives.”

The split that the re­search­ers ob­served fol­lowed the re­cent es­tab­lish­ment of a mi­gra­to­ry di­vide be­tween south­west- and north­west-mi­grat­ing black­cap (Syl­via atr­i­capilla) popula­t­ions in Cen­tral Eu­rope af­ter hu­mans be­gan of­fer­ing food to them in the win­ter. 

The two groups be­gan to fol­low dis­tinct mi­gra­to­ry routes—win­tering in Spain and the Un­ited King­dom—and faced dis­tinct pres­sures. Un­der these, the two groups have since be­come lo­cally adapted “e­co­types,” the re­search­ers ar­gued. Eco­types are sup­posed to rep­re­sent the in­i­tial step of dif­fer­entia­t­ion among popula­t­ions of the same spe­cies; if eco­types con­tin­ue down that path, they can ul­ti­mately be­come sep­a­rate spe­cies.

“The new north­west mi­gra­to­ry route is shorter, and those birds feed on food pro­vid­ed by hu­mans in­stead of fruits as the birds that mi­grate south­west do,” Schae­fer said. “As a con­se­quence, birds mi­grat­ing north­west have round­er wings, which pro­vide bet­ter ma­neu­ver­abil­ity but make them less suit­ed for long-dis­tance migra­t­ion.” They al­so have long­er, nar­rower bills that are less equipped for eat­ing large fruits like ol­ives dur­ing the win­ter.

Schae­fer said it is­n’t clear wheth­er the eco­types will ev­er be­come sep­a­rate spe­cies; in fact, he doubts they will be­cause the habits of hu­mans will tend to change over time. Even so, he said, the find­ings speak to a long-stand­ing de­bate about wheth­er ge­o­graph­ic separa­t­ion is nec­es­sary for specia­t­ion to oc­cur. In par­tic­u­lar, it had been con­ten­tious wheth­er se­lec­tion could act strongly and con­sist­ently enough to sep­a­rate a un­ited gene pool that is not di­vi­ded by phys­ical bar­riers.

“In highly mo­bile or­gan­isms such as birds, the con­sen­sus is that” this is “ex­tremely rare, mainly be­cause it is dif­fi­cult to en­vis­age how gene pools could be kept sep­a­rate” long enough, Schae­fer said. “Our re­sults now show that the in­i­tial steps of specia­t­ion can oc­cur very quickly in a highly mo­bile, mi­gra­to­ry bird,” and “it does­n’t have to take mil­lions of years.”

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Feeding birds in winter is innocent fun, but it can nonetheless have profound—and relatively quick—effects on the evolutionary future of a species, researchers say. A report published online in the Dec. 3 issue of the research journal Current Biology argues that a population of birds known as blackcaps has split into two “reproductively isolated” groups in under 30 generations, despite continuing to breed in the same forests. The reproductive isolation between these populations is now stronger than that of other blackcaps that are always separated from one another by distances of 800 kilometers or more, the scientists said. “Our study documents the profound impact of human activities on the evolutionary trajectories of species,” said Martin Schaefer of the University of Freiburg. “It shows that we are influencing the fate not only of rare and endangered species, but also of the common ones that surround our daily lives.” The split that the researchers observed followed the recent establishment of a migratory divide between southwest- and northwest-migrating blackcap (Sylvia atricapilla) populations in Central Europe after humans began offering food to them in the winter. The two groups began to follow distinct migratory routes—wintering in Spain and the United Kingdom—and faced distinct pressures. Under these, the two groups have since become locally adapted “ecotypes,” the researchers argued. Ecotypes are supposed to represent the initial step of differentiation among populations of the same species; if ecotypes continue down that path, they can ultimately become separate species. “The new northwest migratory route is shorter, and those birds feed on food provided by humans instead of fruits as the birds that migrate southwest do,” Schaefer said. “As a consequence, birds migrating northwest have rounder wings, which provide better maneuverability but make them less suited for long-distance migration.” They also have longer, narrower bills that are less equipped for eating large fruits like olives during the winter. Schaefer said it isn’t clear whether the ecotypes will ever become separate species; in fact, he doubts they will because the habits of humans will tend to change over time. Even so, he said, the findings speak to a long-standing debate about whether geographic separation is necessary for speciation to occur. In particular, it had been contentious whether selection could act strongly and consistently enough in sympatry to separate a united gene pool. “In highly mobile organisms such as birds, the consensus is that sympatric speciation is extremely rare, mainly because it is difficult to envisage how gene pools could be kept separate” long enough, Schaefer said. “Our results now show that the initial steps of speciation can occur very quickly in a highly mobile, migratory bird,” and “it doesn’t have to take millions of years.”