Migration and inbreeding: The importance of recipient population size for genetic management
Sewall Wright demonstrated 70 years ago that the number of migrants required to maintain specified levels of gene flow (i.e. avoid excessive inbreeding) is virtually independent of the size of the recipient population. According to conventional wisdom, this idea is valid provided population size exceeds ∼20. It is well known that this independence implicitly assumes that a population's effective size (Ne) is equal to its census size (N). However, it is not obvious whether independence between the required number of migrants (to avoid excessive inbreeding) and population size constitutes a reasonable assumption for real populations of conservation concern. Relying on empirical data, we demonstrate that for real populations, the assumption (i.e. Ne = N) is routinely violated to a degree such that the required number of migrants is strongly dependent on the size of the recipient population. Because a population's effective size (Ne) is typically much smaller than its census size (N), the number of migrants required to avoid inbreeding is actually dependent on N even when it is considerably greater than 20. For example, when Ne/N = 0.1, the number of migrants required to maintain the inbreeding coefficient (F) at 0.2 doubles (from 4 to 8) as N increases from 9 to 60. Similarly, when Ne/N = 0.05, the number of migrants required increases by 50% as N increases from 18 to 45, and increases again by 50% as N increases from 45 to 260. Thus, for populations much larger than 20, the required number of migrants increases asymptotically with N, and dramatically so when Ne/N≪1. Simple conventions regarding the requisite number of migrants may not apply to many populations of conservation concern. Genetic management should routinely rely on models that explicitly account for this and other recent considerations. Failure to do so may jeopardize the viability of populations that are sensitive to altered levels of inbreeding. 'Where m [the migration rate] is less than 1/2N there is a tendency toward chance fixation of one or the other allelomorph. Greater migration prevents such fixation. How little interchange would appear necessary to hold a large population together may be seen from the consideration that m = 1/2N means an interchange of only one individual every other [sic] generation, regardless of the size of the subgroup [emphasis ours].' - S. Wright (1931: 127-128).
Migration and inbreeding: The importance of recipient population size for genetic management.
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