Effects of selection and migration on geotactic and phototactic behaviour of Drosophila . I

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Abstract
Starting with a population of Drosophila pseudoobscura which was neutral to light and to gravity, we have obtained by directional selection photopositive and photonegative, as well as geopositive and geonegative descendants. The heritability of the phototactic and geotactic responses is, under the conditions of the experiments, rather low. The realized heritability over the first fifteen generations of selection is between 8 and 10% for phototaxis, and about 3% for geotaxis. Despite so low a heritability, these behavioural traits can be sharply modified by selection. Pairs of populations were made to exchange 20% of their members in each generation. One population of each pair was selected for phototaxis and the other for geotaxis. The experimental model of the process which is known as 'social mobility' in human populations was so contrived that the migrants taken from the donor populations were phenotypically contrasting with the individuals selected to perpetuate the donor populations themselves. The donor populations changed, as expected, in the directions in which they were being selected. The genetic changes caused in the recipient populations by the immigrants were more interesting. They went in the same directions in which the populations from which the immigrants came were changing. With characters of low heritability, phenotypic rejects coming from genetically improving populations may transmit the genetic improvements to the recipient populations.