Even global human populations without substantial degrees of isolation may eventually suffer the effects of inbreeding or sharing bad genes if one extrapolates from a recent study of Wooly Mammoths on Russia’s Wrangel Island.
http://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1006601
Apparently the ancient elephant-like creatures that were killed of at Wrangel Island Russia 3500 years ago compiled susbstantial genetic defects perhaps for many reasons such as bad diet, environmental conditions etc, and inbreeding. As all human populations interbreed and isolation from contaminated populations no longer exists as firewalls genetic decay may occur in the finite yet initially variegated pool as well. Natural isolation in moderation with adequate populations for sustainability seems good while no isolation even with large numbers may mass produce and share critical genetic errors. Obviously though possible human extinction criteria do not resemble closely those of wooly mamoths.
image credit-wiki commons
Jacque Cousteau anticipated humanity has fewer than 200 years remaining before its overuse of the environment and technological development (such as genetic engineering, biological war, nuclear, etc) led to mass extinction. The poor Wooly Mammoths gradually faded away as humanity consumed them as mobile fast food platforms.
https://books.google.com/books/about/The_Vanishing_Face_of_Gaia.html?id=pgaKp7wzNIQC
Garrison Gibson: books and blog 