"Are you applying this equally to both sympatric and allopatric modes of speciation? My thinking is that the difference between your speciation model and random drift model will be more apparent for the sympatric situation where natural selection may cause a divergence within a population while drift may not.
In case of allopatry both natural selection and/ or drift may cause divergence."
Yes, you are right. The first example shown would be
While the allele frequency of a population is an average over the whole population, this frequency would not occur at every point within the population. At any given point some allele would be more frequent, others less frequent.
If a sub-population becomes isolated, it's allele frequency will not change much, but the allele frequency of the parent population will. This is because some allele have now been relatively concentrated in the parent population (those alleles occurring less in the sub population), compared with others that have been relatively diluted in the parent population (those occurring more in the sub-population).
Thus, in this model, isolation would be represented as a shift in the position of the parent population in morphospace, with the sub-population orbiting close by.
I'll try and get some diagrams up shortly.