Effects of Immigration on the Evolution of Populations

Abstract

The response to selection for increased numbers of sternopleural chaetae in isolated lines of Drosophila was compared with that of lines receiving unselected immigrants each generation. Under 90% selection fertility declined in the isolated line, and it went to extinction in the eighth generation after gaining 6.47 chaetae. A line with limited immigration (1 immigrant per 4 selected natives) gained nearly 10 chaetae in 8 generations and did not show a noticeable loss in fertility. With 4 times as much immigration (1 immigrant per selected native) a third line gained only 1.17 chaetae during the same period. This line tended to reflect the random changes in chaetae number which occurred in the line used as a source of immigrants. When 3 similar lines, receiving the same number of immigrants, were subjected to 60% selection, only the isolated line increased its mean chaeta number. The results of these experiments gave further evidence that: (1) although immigration may result in a diluting or "swamping" effect, the introduced variation may be beneficial to the population and (2) the degree of isolation necessary for divergent evolution to occur has been overestimated.