Phylogeography is the examination and interpretation of the relationships found between living things and their whereabouts. It is also used to study geological events and their resulting effect on the distribution of animals and plants. The term was first coined in 1987 and came about from the striking patterns discovered in the geographic distributions of the mitochondrial DNA lineages of a number of different species. The scientists began to discover that genealogy and geography seemed to be linked.
Phylogeography involves the analysis of the spatial distribution of haplotypes, reconstructing how these haplotypes are related to one another, detecting similarities in the geographic variation of related haplotypes and inferring historical events that may have induced the geographical spread. Past events that can be inferred include population expansion - increased birth rates, such as those that followed the introduction of farming techniques and invention of porridge; population bottlenecks - the reduction of variation in the gene pool for instance due to natural disasters or plagues; and migration - the movement from one place to another.
Coalescent Theory is a model used in Population Genetics. It is concerned with tracing markers shared by many people in the population to a single, common ancestor - or the most recent common ancestor (MRCA).
Coalescent theory involves running models of genetic drift backwards in time to investigate the lineage of ancestors. This works well for the Y Chromosome and mitochondrial DNA as they do not recombine with any other DNA.
Phylogeography and coalescent theory have made it apparent that historical population demography and genealogy are very much correlated.