A pair of decades-long studies of birds moving into other birds’ neighborhoods show that immigration can have a quirkier effect than predicted by the usual textbook view.
Evolutionary biologists have often talked about two forces working against each other in shaping the mix of genes in a local population. Natural selection adapts a population to its particular home. Yet immigration, bringing in genes from outsiders, can overwhelm the effects of that fine-tuning and make diverse populations more alike.
Now, two unusually long studies of the birds called great tits (Parus major) reveal more-subtle effects of immigration, creating differences in populations living only a few kilometers apart. Both reports appear in the Jan. 6 Nature.
These findings indicate that “we should think more carefully about our populations,” comments David Coltman of the University of Alberta in Edmonton, who studies evolutionary adaptations in mammals.
The immigration effects emerged in the great tit work only because the researchers had so much data, explains Dany Garant of the University of Oxford in England. He and his colleagues used records from 1965 to 2000 of great tits in Wytham Woods, near Oxford.
Their work in these woods had found that birds in the northern zone had bigger territories and larger nestlings than birds in the east did. By looking at bird histories, the researchers found that newcomers hadn’t dispersed randomly within the woods. The biggest tits arriving from the outside tended to settle in the north, whereas the smallest immigrants moved into the east. Birds dispersing from the center of the woodland showed the same pattern.
The researchers also calculated that larger size provides a survival advantage to the nestlings, wherever they hatch. Therefore, the nestling size gap between the north and east woods comes mainly from immigration rather than from some local fine-tuning by natural selection.
On the Netherlands’ northeastern island of Vlieland, researchers Erik Postma and Arie J. van Noordwijk of the Netherlands Institute of Ecology in Heteren drew on
21 years of data. The researchers already knew that great tits in the western part of the island lay on average a little over 1 more egg per clutch than do tits a few kilometers east. Since western-hatched birds that moved east still produced relatively large clutches, the researchers concluded that genetics influences clutch size.
The researchers found that a lineage including immigrants from off the island predisposed birds to lay large clutches. In the small western population, outsiders moving in account for 43 percent of the new breeding population each year, but they make up only 13 percent of the larger eastern population. Therefore, the researchers concluded that the greater impact of immigration swamps natural selection against the immigrant genes on the western part of the island but not on the eastern part.
Andrew Hendry of McGill University in Montreal says that these are among the very few studies that quantify both flow of genes from immigrants and natural selection in free-living animals. The results emphasize “a diversifying role for gene flow.”