  We consider a model of "isolation with an initial period of migration" (IIM),
where an ancestral population instantaneously split into two descendant
populations which exchanged migrants symmetrically at a constant rate for a
period of time but which are now completely isolated from each other. A method
of Maximum Likelihood estimation of the parameters of the model is implemented,
for data consisting of the number of nucleotide differences between two DNA
sequences at each of a large number of independent loci, using the explicit
analytical expressions for the likelihood obtained in Wilkinson-Herbots (2012).
The method is demonstrated on a large set of DNA sequence data from two species
of Drosophila, as well as on simulated data. The method is extremely fast,
returning parameter estimates in less than 1 minute for a data set consisting
of the numbers of differences between pairs of sequences from 10,000s of loci,
or in a small fraction of a second if all loci are trimmed to the same
estimated mutation rate. It is also illustrated how the maximized likelihood
can be used to quickly distinguish between competing models describing
alternative evolutionary scenarios, either by comparing AIC scores or by means
of likelihood ratio tests. The present implementation is for a simple version
of the model, but various extensions are possible and are briefly discussed.
