Population genetics is the study of the distribution and change in frequency of alleles within populations, and as such it sits firmly within the field of evolutionary biology.
The main processes of evolution are natural selection, genetic drift, gene flow, mutation, and genetic recombination and they form an integral part of the theory that underpins population genetics.
Studies in this branch of biology examine such phenomena as adaptation, speciation, population subdivision, and population structure.
Population stratification refers to differences in allele frequencies between cases and controls due to systematic differences in ancestry rather than association of genes with disease.
It would be caused by systematic differences in the ancestry of cases and controls.
Diploid genome refers to a genome that contains a balanced set of chromosomes derived equally from maternal and paternal sources.
Coalescent theory is a retrospective stochastic model of population genetics that relates genetic diversity in a sample to demographic history of the population from which it was taken.
That is, it is a model of the effect of genetic drift, viewed backwards in time, on the genealogy of antecedents.