What I would like to know is, who do the children of such unions end up marrying? One polygamous man siring so many children creates a population of half-siblings. Are they going to inbreed with each other or migrate to other communities for mates?
For the past almost 70 years, the most infamous communities for open polygamy (practiced by the fLDS) has been the twin communities of Colorado City and Hildale, located near the AZ-UT border...one community apiece on each side of the state line.
Per Wikipedia (see Fundamentalist Church of Jesus Christ of Latter-Day Saints): The Colorado City/Hildale area has the world's highest incidence of fumarase deficiency, an extremely rare genetic condition.[82] Geneticists attribute this to the prevalence of cousin marriage between descendants of two of the town's founders, Joseph Smith Jessop and John Yeates Barlow.[82][83][84][85] It causes encephalopathy, severe mental retardation, unusual facial features, brain malformation, and epileptic seizures.[86][87]
Footnotes 82-->87:
82.^ a b Szep, Jason (2007-06-14). "Polygamist community faces rare genetic disorder". Reuters. http://www.reuters.com/article/domesticNews/idUSN0727298120070614. 83.^ Dougherty, John (2005-12-29). "Forbidden Fruit". Phoenix New Times. http://www.phoenixnewtimes.com/2005-12-29/news/forbidden-fruit/. 84.^ Hollenhorst, John (2006-02-08). "Birth defect is plaguing children in FLDS towns". Deseret Morning News. http://deseretnews.com/dn/view/0,1249,635182923,00.html.
85.^ "Doctor: Birth defects increase in inbred polygamy community". Daily Herald. 2006-02-09. http://www.heraldextra.com/content/view/165069/.
86.^ Bayley JP, Launonen V, Tomlinson IP (2008). "The FH mutation database: an online database of fumarate hydratase mutations involved in the MCUL (HLRCC) tumor syndrome and congenital fumarase deficiency". BMC Med. Genet. 9 (1): 20. doi:10.1186/1471-2350-9-20. PMC 2322961. PMID 18366737. http://www.biomedcentral.com/1471-2350/9/20.
87.^ Kerrigan JF, Aleck KA, Tarby TJ, Bird CR, Heidenreich RA (2000). "Fumaric aciduria: clinical and imaging features". Ann. Neurol. 47 (5): 583–8. doi:10.1002/1531-8249(200005)47:5<583::AID-ANA5>3.0.CO;2-Y. PMID 10805328.