Reading last May's "Where is the Promise of Christ's Coming?" , the question came to me: how do we square the facts of closely-related "inbred" lines in genealogy with the Biblical fact that all of humanity descends from Adam via the eight of the Ark?First, the Bible does indicate to us that human lifespan has dramatically decreased from the multiple century lifespans before the flood to the sub-century typical lifespans after the flood, and even today. Whether this is due to genetic corruption from an early period of "in-breeding" or whether it is due to changes in the Earth's protection from solar radiation associated with the Great Flood (something I've heard AiG types suggest), there has been some significant change to human life.
We know from the condition of the Habsburgs that such close relations lead to deformities and even mental incapacity (Charles II) over the span of less than 500 years of that family's history. Has humanity overall been preserved from this biological fact which interbreeding seems to engender over long periods of time? Was this simply a curse of God on the Habsburgs?
How do the 8 saved in the ark translate to the ~7 billion of today with such varied racial characteristics? It seems contradictory.
While I understand what you mean by "racial characteristics," it needs to be recognized that categories like "race" are largely conventional. Typically, pronounced physical differences are associated with relatively isolated groups. This isolation does tend to reinforce particular physical characteristics that are less common outside the group.
An example that most people may think of is the Pygmy people group, while the blue skinned people would be another less well known example. In God's providence, neither of those groups ever got large enough to be referred to as a "race," but the basic principle is the same.
Thus, "in-breeding" (very loosely defined) of each group may help to explain the very distinctive appearance of Europeans as compared to sub-Saharan Africans, peoples of the Indian sub-continent, and far-east Asians (to take some examples).
The Bible explains that at Babel language confusion resulted in dispersion of people from the tower. Moreover, in the days of Peleg the world was divided. This dispersion and division tended to have the result of people groups becoming more segregated and distinctive. It's not totally clear how uniform in appearance the pre-Fall humans were, but even if they were all quite similar in appearance to one another, this post-fall dispersion and segregation would be expected to produce groups with distinctive looks after a number of generations.
We all derive from one pair of adults (Adam and Eve), and also from a maximum of 8 grandparents (Noah and his wife and the in-laws of their three sons), all of which are descendants of that original pair. This led to some measure of marrying one's close relatives for a number of generations. However, over time this necessity decreased. Thus, by the time of Moses, there were prohibitions on marrying one's half sister, although only about seven generations earlier Abraham had married his own half-sister, though clearly there was already stigma associated with marrying a full-blooded sister at that time.
Why then are we not all then deformed? First, as the relationships become more distant, inbreeding does not invariably produce and emphasize harmful, recessive mutations. Second, the "survival of the fittest" principle applied to a large extent in pre-socialistic societies. Those with physical and/or mental defects would tend to remove themselves from the gene pool in a variety of ways. Thirdly, it is entirely possible that human genetics were not as prone to mutation before the fall, and that they only gradually became prone to mutation after the fall. For example, if the mechanism that produces a short life is also the mechanism that leads to harmful mutations, and if that mechanism is solar radiation, then it seems possible that there was no significant genetic risk to mutation before the fall.
A lot of this is, of course, speculation. God does not necessarily give us the answers to all these questions. We have no way of knowing whether the Hapsburg family was specially cursed by God, or whether their condition had some other purpose in God's plan. Whatever the answer to that question, since the history of man before and after the flood does not require men to marry their first cousins or full blooded sisters for many successive generations, the risks associated with in-breeding today among a population of 7 billion derived from an original pair is small.