Experiments provide evidence for behavioral evolution. Researchers are carrying out experiments on organisms with short life spans, looking for evidence of evolution in laboratory populations. Marla Sokolowski studied a polymorphism in a gene for foraging in Drosophila melanogaster. The gene is called for, and it has two alleles. One allele, for R , results in a “rover” phenotype in which the fly larva moves more than usual. The other allele, for S , results in a “sitter” phenotype in which the fly larva moves less than usual. Sokolowski reared Drosophila at high and low population densities for 74 generations. The for S allele increased in low-density populations, while for R increased in high-density populations. At low densities, short-distance foraging yielded sufficient food. At high densities, long-distance foraging helped the larvae to move beyond areas of food
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