Humans tend to be partilocal, meaning the sons tend to stay resident and the daughters tend to set up housekeeping elsewhere. So in very small local populations replacement of local mtDNA strain by means of immigrant new female breeders can occur in a single generation should there have been a consistant influx from populations of a different mtDNA type.
mtDNA contains no data whatsoever about ancestral contributions to chromosomal traits. In fact, surviving mtDNA lineages, especially in very small populations where variation cannot be supported, say nothing at all about proportionality even of mtDNA in female ancestry, since any maternal condition passed through a son on the maternal side is transparently discarded right along with all maternal mtDNA expressions on the paternal side.
mtDNA represents only surviving unbroken mother-daughter strings. There is no proportionality from recombination or recessive allele conservation possible, except perhaps in the extreme minority of cases which are something like several zeros to the right of the decimal before significant digits in any kind of probability and therefore irrelevant in general population dynamics. mtDNA is the ultimate minimum in ancestral representation. It shows only a small part of only a single ancestor per generation, and that is all it can show.
Dale
Has there been any simulation work on the possibility of Neanderthal contributions to AMHs? I think a properly laid out simulation could answer how long it would take for mtDNA to be almost invisible, and even provide insights into the general question of how much genetic contribution a small population can make to a much larger one.
