Question about evolution, genetic bottlenecks, and inbreeding

Hi Harry,Welcome to EvC The primary cause of inbreeding depression is the creation of individuals who are homozygous for deleterious alleles. But, conversely, it also concentrates the advantageous alleles. These two factors interact to produce an optimal level of inbreeding; usually around the 2nd or 3rd cousin range, but as close as 1st cousins in some species.What can happen in small populations (especially among plants) is that although inbreeding reduces the individual fitness of early generations, the overall process can weed out individuals heterozygous for deleterious alleles and thus increase the average fitness of the population.

Each of the ancestors A-H are equally far from the offspring at the end, therefor they're equally recent.

MRCA and Y-adam are different concepts, you're conflating and confusing them. In reality, the MRCA of all humans is much more recent than our Y-adam (which makes if you think about it, mitochondrial eve traces the female line only, Y-adam by the male line only, and the MRCA by both).

MRCA, mitochondrial Eve and y-chromosome Adam can all be individuals - however they can all also be populations.Taking it theoretically, if we look just as the MRCA, it's possible that it's a single individual, because they could have parented multiple offspring with different partners, or it could be that the MRCA was a couple who only had children together in which case both parents are equally recent ancestors.Secondly, on a more practical level, we can't find individuals from genetic data. Eve and Adam are both traced by unmodified genetic data passed by the maternal and paternal lines (respectively) so the only modifications in these lines are mutations, however we only get data when a modification occurs, so there is no way to distinguish between a small population that had the same markers and a single individual.On both grounds then, it is better to talk of these common ancestors on the basis of small populations than as single individuals.Oh, and another very important point: even if you consider y-chromosome Adam and mitochondrial Eve to represent individuals, this does not mean that they were the only individuals alive at that time, only that the very narrow piece of genetic material being tested hails from them.Edited by Mr Jack, : Another point.

The could be our MCRAs without either their Y-chromosome line or their mitochondria being represented in the modern population if, just for example, the had only a single son, and then that son had only daughters (this argument applies equally to a single MCRA).

Probably; although they might have an equally recent pair of ancestors and likely some other edge cases. But, the reasoning that leads to a MRCA for humans applies equally to humans and apes. However, it's not an interesting outcome, and it is not what people are talking about when they talk about the common ancestor of humans and apes, which is a species or population, not an individual.

Theoretically there will always be a multitude of individuals that all humans and all apes are descended from. One or more of those will be the most recent.