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Why are high inbreeding coefficients more likely in small isolated populations?

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consanguinity (Left) Absence of inbreeding: horizontal lines connect mates, vertical lines connect parents with their child; (right) an inbreeding loop: horizontal bar at top of loop connects brother and sister, all other lines and bars as above

High inbreeding coefficients are more likely in small, isolated populations—often referred to as isolates—because the limited number of available mates forces individuals to select partners from within a restricted gene pool over many generations.1

In these groups, the probability that two alleles are identical and derived from the same ancestor increases significantly because the pool of potential mates is small, leading to a higher frequency of intrafamilial marriages. As members continue to mate with one another, the coefficient of inbreeding (F) accumulates, reflecting the increased likelihood that offspring will inherit identical alleles from a common forebear.1

This process is further influenced by genetic drift, a random statistical effect that occurs in small populations where chance events can substantially alter the gene pool. Because these populations lack the genetic diversity found in larger groups, they are more susceptible to the rapid loss of genetic variability and the expression of deleterious, recessive traits. Consequently, small isolated groups, such as certain religious colonies or endangered species, often exhibit higher levels of inbreeding as a direct result of their limited population size and restricted mating options.231