Date: Thu, 11 Jun 1998 12:45:37 -0400 To: email@example.com From: Christopher Thrash
Subject: Viable population (was Re: Distinguishing the Vilani) >Date: Wed, 10 Jun 1998 08:50:13 -0400 >From: Walter Smith >Subject: re: Distinguishing the Vilani > >ObTrav: How many people does it take to make a viable breeding >population on a colony world cut off by a long night? If your planet >is a bunch of people under the minimum, what do you do? Change >social codes on reproduction? Assuming that viability depends on genetic variability, the figure that matters is "Effective Population Number" (N): N = 4MF/(M+F) [Kimura and Ohta, "Effective Population Number", 1977?] where M and F equal the number of males and females, respectively. Large differences in numbers of males and females reduce N towards the lower number. Variation in average family size (number of offspring per female), overlapping generations, and fluctuations in the population from generation to generation also reduce N over time. The "inbreeding coefficient" per generation (f) is given by: f = 1/2N [Franklin, "Evolutionary Change in Small Populations", 1980] Animal breeders will accept f = 0.01, implying N = 50 for short-term viability. For long term viability, Franklin [ibid.] suggests N = 500. "Below this latter value, it is likely that genetic variance for complex traits is lost at a significantly faster rate than it is renewed by mutation." Animal breeders find that a population shows significant viability problems when the inbreeding reaches 0.1, and may collapse altogether when it reaches 0.5-0.6. The number of generations (t) to reach this extiction threshold also relates to N: t ~ 1.5N [Soule, "Thresholds for Survival: Maintaining Fitness and Evolutionary Potential", 1980] To increase the effective population number, while the actual (census) number remains constant, there are two basic strategies [Senner, "Inbreeding Depression and the Survival of Zoo Populations"]. First, by reducing the variation in family size as much as possible, the effective population can be doubled over the random case. The effective population can also be doubled over the random case by control over breeding partners, either through maximum avoidance of inbreeding or through deliberate half-sibling or first cousin breeding (these last improve N even over max avoidance of inbreeding, but only after ~16 generations). The two strategies overlap, so only one doubling is possible. Needless to say, the social impacts of either strategy on a human population are extreme. Note that a bottleneck produced by ruthless culling may have the effect of mitigating later inbreeding problems. Grey wolf populations in Minnesota seem to have experienced a severe bottleneck sometime in the past that eliminated most (if not all) undesirable recessive genes. In one experiment, breeders conducted brother-sister matings of grey wolves for four generations without producing *any* bad recessives [presentation by the Wolf Survival Group, Patuxent Wildlife Research Lab, 1986].