I've noticed that it becomes more difficult to answer questions if they include statements wrapped in them, especially if those statements are incorrect.

Do royals only marry other royalty? You may have noticed Camilla lacked any title before marrying Prince Charles (although she is the granddaughter of a baron). Kate Middleton was born a middle-class commoner.

I hope someone with more knowledge than I may elaborate on the relative gene pool of the world's nobility, but I expect they acted like any dwindling gene pool since the beginning of time: expand the "acceptable" marriage list.

The gene pool is larger than you would think. There were (and are) large numbers of princes and petty kings in Germany who, despite controlling only very small land holdings (or, in this day and age, none at all) had blood lines to make them perfectly acceptable marriage material. Think of the spouses of the Dutch royal family, or think of queen Victoria's husband prince Albert. Even the eighteenth century Russian empress Catherine the Great was born as a low ranking German noblewoman.

So it's never really been a problem. Except for the house of Habsburg in Spain, which was famously inbred, but then they were the exception rather than the rule.

The gene pool of all royals is large enough to maintain a low inbreeding coefficient. The only case that I have read about where it was a problem was with the Spanish Habsburgs. This is due to the fact that it became extremely common for them to have marriages between cousins, uncles, nieces, etc. rather than marrying royals from other families. It is hypothesized that the Habsburg inbreeding had a major role in bringing down the dynasty.

The last Habsburg King in Spain was Charles II, who had many physical and mental impairments, likely due to inbreeding. During his reign, the kingdom declined rapidly until he died at age 38 without a direct heir and led to the War of Spanish Succession.

A good read on the inbreeding hypothesis is The Role of Inbreeding in the Extinction of a European Royal Dynasty.

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