Experimental isolation of positively and negatively phototactic phenotypes from a natural population of Daphnia magna Strauss: a contribution to the genetics of vertical migration

The authors isolated and cloned strongly positively and strongly negatively phototactic individuals from a single natural population of Daphnia magna using an experimental column of 2 m high and an irradiance of 2000 W. It is argued that the positively phototactic behaviour is genetic, recessive, that the individuals that showed the behaviour were heterozygous, and thus the genetic system probably polygenic. Populations should maintain such genotypes in heterozygous form and in low numbers, in spite of selection against them, because they are their guarantee against extinction, should the pressure of natural selection ever become reversed.