Section 9 — Population Genetics 



exists a correlation between stronger evolution- 

 ary activity in the original area and the frequency 

 of dispersal. Intracontinental dispersal has been 

 studied in the American tropics. In addition to 

 long distance dispersal, followed by allopatric 

 evolution, slow penetration into new areas has 

 occurred followed generally by marginal dis- 

 ruptive evolution. Explosive or sympatric 

 disruptive evolution has also occurred after 

 slow penetration or dispersal into a new area. 

 In general, there seems to exist a correlation 

 between larger evolutionary steps (genus forma- 

 tion) with restricted diversity (fewer species) 

 and between smaller evolutionary steps and 

 high degree of diversification. 



9.26. Deviations from Panmixia as a Consequence 

 of Sex-determination in the Marine Copepod, 

 Tisbe reticulata. Bruno Battaglia (Padua, 

 Italy). 



Experiments have been carried out with the 

 purpose of testing if, between the various pheno- 

 types of the polymorphic Copepod Tisbe 

 reticulata matings occur at random. Observa- 

 tions concern the following forms: maculata 

 (f/m r/m) 5 violacea (V v V v ) and violacea-macu- 

 lata ( V v V m ), controlled by two of the alleles of 

 the V series. 



Results show that, at least under certain condi- 

 tions, the observed frequencies of some types of 

 crosses differ from the frequencies expected on 

 the assumption of panmixia. These deviations 

 are mainly due to an excess of backcrosses 

 whereas all crosses of other types are fewer 

 than expected. This situation seems to be condi- 

 tioned by the selective matings $ V v V m x £ 

 ym f/m and (j) I/v I/m c ^ l/v i/v. 



The hypothesis is made that the heterozygous 

 females are sexually more active than the homo- 

 zygous ones, and the homozygous males more 

 active than the heterozygous ones. This view is 

 supported by other observations on the different 

 fecundities of the three genotypes. 



The results of other experiments seem to in- 

 dicate that the deviations from panmixia depend 

 to a certain extent upon the degree of relation- 

 ship between the animals which mate. 



The problem of the possible bearing of the 

 above situation on the evolution of natural, 

 as well as laboratory populations is being in- 

 vestigated. 



The data will be published in full, possibly in 

 Evolution. 



9.27. Selection Experiments after Inbreeding in 

 Habrobracon. R. E. Scossiroli and R. C. 

 von Borstel (Pavia, Italy, and Oak Ridge, 

 U.S.A.). 



In previous experiments, long-winged sub- 

 strains had been selected from the small wing, 

 white eye stock of the parasitic wasp Habro- 

 bracon after six generations of inbreeding 

 (expected homozygosity = 0.97( J ). It was hypo- 

 thesized that selection for longer wings in the 

 inbred strains was made possible by either (1) 

 maintenance of genetic variance by persistence 

 of heterozygosity, or (2) a high spontaneous 

 mutation rate for quantitative characters. 

 Both long- and short-winged substrains were 

 selected from the same strain of Habrobracon, 

 using a family method and a selection pressure 

 of 1.47 s, that is, only those were selected that 

 were beyond 1.47 standard deviations from the 

 mean. Selection under identical conditions was 

 repeated after three generations of inbreeding by 

 back-crossing to the same haploid male of 

 daughters from subsequent generations. It was 

 again possible to select long- and short-winged 

 substrains (expected homozygosity = 0.94). On 

 the other hand, attempts to select these substrains 

 from three replicates after 12 generations were 

 unsuccessful (expected homozygosity = 0.99 +). 

 This shows that by prolonged inbreeding, homo- 

 zygosity can be obtained for characters affecting 

 wing length. Therefore, a high spontaneous 

 mutation rate does not contribute excessively 

 to the genetic heterogeneity for characters 

 affecting wing length. 



1. Scossiroli and von Borstel, Atti Asso- 

 ciazione Genet ica Italiana, 7, 191, 1962. 



9.28. Competition Research on Unisexual Poly- 

 morphism in Megaselia scalaris. Robert 

 Springer (Vienna, Austria). 



The newly discovered type of alternative sex 

 determination in the Phoride Megaselia scalaris 

 brings about a unisexual polymorphism in the 

 natural population. It has to be assumed that 

 three differing types of males co-exist in nature. 

 In these three types always one of the three 

 non-homologous chromosomes bears the sex 

 realizator. Competition research on these three 

 sex-determining types of males were carried 

 out with artificial populations. Present results 

 showed that the third chromosome always has a 

 strong selective advance. In natural populations, 

 however, the first chromosome mostly seems to 

 be sex-determinating. Further results can be 

 expected in due course. 



152 



