Changes in fecundity of Drosophila melanogaster and D. simulans in response to selection for competitive ability.

1. Equal numbers of the sibling species Drosophila melanogaster and D. simulans were allowed to compete in population cages. Two sets of 5 cages each-A & B-took 20 and 26 weeks respectively until D. simulans comprised less than 15% of the population for 2 successive generations. 2. At the beginning of each cage set, the fecundity of stock females of each species in the presence of con-& heterospecific females, con- and heterospecific larvae, distilled water and a water soluble extract from larvainfested media were determined. Fecundities of daughters of the last generation to be in competition were tested under the same conditions. 3. In all conditions, D. melanogaster stock females had a higher fecundity than D. simulans females at the beginning of each cage set. After competition, the fecundity of both species changes (generally increased) although that of D. simulans increased to a greater degree. 4. In test of the water-soluble extract of previously used media, females of both species showed decreased fecundity. This indicates that females can determine whether a food source has been used by larvae even when no larvae are immediately observable.