A test of a simple model of population structure for D. melanogaster and D. simulans

StatisticPanmixia4Nm = 14Nm = 0.7
D. melanogaster
D. simulans
  • We consider a symmetric two-island model; all individuals are sampled from one deme. 4Nm (3Nm for X-linked loci) is the population migration parameter and corresponds to the number of migrants per deme per generation. Chud and B' are measures of linkage disequilibrium (see materials and methods). P(Chud) and P(B') are one-tailed. Each cell lists the probability U that the distributions of P values for the statistic are uniformly distributed, as they should be under the null model. For Chud and B', low U values correspond to excess linkage disequilibrium in the actual data relative to simulations.

  • a For 4Nm = 0.7, there is significantly too little linkage disequilibrium in the actual data as measured by B'. (Note that this cannot be inferred from considering the value 1-U since B' is discrete.)

  • b Simulations were not performed for D. simulans with this migration rate.

  • c Even though this test does not reveal this feature, the distribution of B' is unusual when 4Nm = 1 (values of B' are lower in actual data than in simulated data; i.e., there is significantly less linkage disequilibrium in actual data than expected).