A MULTIFACTORIAL GENETIC INVESTIGATION OF SPECIATION THEORY USING DROSOPHILA MELANOGASTER

Abstract

to different environments for several years. MATERIALS AND METHODS Populations.-Four cage populations, designated i C, 2C, ID and 2D were studied. The populations with the prefix 1 originated from a common gene pool of 600 parents collected from a natural population in Cephalonia, Greece, while those with the prefix 2 originated similarly from a natural population in Gavros-Achaia, Greece. The 600 parents were derived from 100 isofemale lines collected in the summer of 1973. Populations were set up about 20 days after the capture of flies. Three virgin pairs 4-6 d old, were taken from each of the isofemale lines; the 300 virgin females and the 300 males were allowed to mate randomly in a cage (IB or 2B). Populations ic, ID and 2C, 2D were derived by replicating the IB or 2B populations , respectively. Consequently, we may regard the replicate populations of each series as possessing practically the same gene pool at the time of their origin. For more details see Alahiotis and Pele-canos (1978, 1979) and Alahiotis et al., (1976). Populations l C and 2C were kept at 18 ± 0.5 C for the first two years, and at 14 ± 0.5 C since 1975, and 43 ± 4% R.H. Conditions for populations ID and 2D were 25 ± 0.5 C and 90 ± 0.5% R.H. Flies were reared on cornmeal-sugar-agar medium (Alahiotis, 1976). The populations were maintained in plexiglas cages (41 x 41 x 16 em) containing 14 food vials (2.5 x 10 em). The population size was estimated to be 1,100 (average) flies (Ala-hiotis, 1976; Alahiotis et al., 1976). In sampling the cages several vials containing fresh food medium were placed in the cage. After 48 h the food vials (containing 730 1 To whom all correspondence should be addressed. Speciation is one of the more striking features of evolution. Two opposing theories of speciation are receiving increasing attention. According to the first theory, speciation is a by-product of the process of adaptation (e.g., Ayala et al., 1974). According to the second or "founder-flush speciation theory," speciation involves a genetic reorganization dependent upon a founder event followed by one or more cycles of exponential population growth and sudden contraction or crash. The second theory holds that adaptive differences are accumulated largely after the genetic reorganization has occurred (Carson, 1968, 1971, 1973, 1976). Hence, the genetic differentiation that accompanies spe-ciation may not shed light on speciation mechanisms. On the other hand, since be-havioral genetic variations are also of great importance in evolutionary processes (Dobzhansky, 1970; Parsons, 1973), we choose to base our investigation of specia-tion theory on critical multifactorial tests incorporating both allozyme and behav-ioral genetic variations. We experimentally manipulated temperature and relative humidity in several laboratory populations of Drosophila me-lanogaster. These populations (possessing a common gene pool at their origin) were established five years ago in our laboratory and were maintained isolated under different constant environments, a situation which may be considered analogous to that of ecologically marginal populations in nature. The present study was designed to measure reproductive isolation and genic differentiation between populations exposed