LITMIR.BIZ

      This kind of imbalance, as extreme as it may seem, is fairly common. Genetic analyses that determine the mother and father of offspring frequently indicate that in many species relatively few individuals, especially among males, are responsible for a disproportionate share of a population’s reproduction (Parker and Waite 1997). Many apparently healthy adults do not leave any offspring. Such inequity is generally not a problem – it is the basis for natural and sexual selection – but it may lead to difficulties in populations that suddenly get small because of its effect on genetic diversity. Consider the endangered Española Island giant tortoise of the Galápagos (see Case Study 1.1, “Return of the Tortoises to Española”). These tortoises likely numbered in the thousands but after most were killed for food by early mariners the population plummeted to 15, consisting of 12 females and just three males, which fortunately were rescued, placed in captivity, and have produced more than 1200 offspring since 1950 that have been released back to the island where they now breed on their own (Gibbs et al. 2014). Sounds like a success? It is. But it has also been discovered that the unequal sex ratio of those 15 survivors along with the unequal reproductive activity among them had led to a genetic effective population size of just 5.7 tortoises, far smaller than the census size of 15 might suggest (Milinkovitch et al. 2004). What little genetic variation remains in this population is severely threatened by the genetic drift exerted during the very long and “tight” bottleneck this species endured for many decades. Carefully managed pairings of surviving tortoises are now being made to maximize “capture” of what little genetic diversity remains for the newer generations. The bottom line to remember is that the effective population size is often substantially less than the actual number of individuals in a population, often only 10–20% (Vucetich et al. 1997). Thus, if you want a “population” of bison with N_e = 100 (hopefully sufficient to retain 99.5% of its genetic variability through at least one generation; see Table 5.3), you actually need a census population of somewhere between 500 and 1000.

      Inbreeding

      Inbreeding refers to the mating between closely related individuals; such individuals are likely to share identical copies of some of their genes because they have ancestors in common. We measure inbreeding with the inbreeding coefficient, F, which is the probability that two copies of the same allele are identical by descent – in other words, derived from a common ancestor (Templeton and Read 1994). For example, in our bison example, if both MDH‐1 X alleles in the X/X homozygous buffalo calf were derived from its grandmother, those alleles would be considered identical by descent.

_{Among several methods to estimate F (Frankham et al. 2009), the simplest involves counting links in the pedigree chain: F = (½) ⁿ , where n is the number of individuals or links in the pedigree chain starting with one parent, going back to the common ancestor, and then going down the other branch to the other parent. Figure 5.12(a) shows the pedigree chain for the offspring (A) of a half‐sister (B) mating with her half‐brother (C) (i.e. B and C have the same mother, D, but different fathers). The inbreeding chain has three links – B, D, and C – and thus F is equal to (½)³ = 1/8 = 0.125. If B and C were full siblings (i.e. they had both the same mother D and the same father E) (Fig. 5.12b), then there would be two chains, one for each common ancestor (B, D, and C for the mother plus B, E, and C for the father). In this case the F values for each chain would be added: (½)³ + (½)³ = ¼ = 0.25.}