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4 R. FRANK HAM Thus, we predict that genetic adaptation to captivity will be positively related to intensity of selection, genetic diver- sity, effective population size and number of generations. Immigration from the wild into captive populations reduces genetic adaptation to captivity (Frankham & LoebeI1992), but is rarely feasible for rare and threatened species. There is empirical support for each of the above predictions. First, the extent of genetic adaptation to captivity in D. melanogaster has been shown to increase with generations until a limit was reached after about 80 generations (Gilligan et al. 2003). Related evidence of increased adaptation with generations has been reported for Drosophila serrata and Drosophila birchii (Ayala 1965a, b), fish (Allendorf & Luikart 2006) and plants (Allard 1988). Second, the response to artificial selection has been shown in numerous studies to be related to selection dif- ferential (Falconer & Mackay 1996). In the specific context of genetic adaptation to captivity, rate of increase in fitness per generation was higher with extreme than modest crowding (Table 1). Further, rate of adaptation was appro- ximately halved by equalization of family sizes which halves the opportunity for selection compared to variable family sizes (Table 1; Frankham et al. 2000). Third, the extent of genetic adaptation to captivity in D. melanogaster was shown to be related to genetic diversity by Reed et al. (2003). An outbred base population showed about twice as much adaptation in fitness as a population with an inbreeding coefficient of 0.59. Further, Ayala (1965a, b) found that the rates of adaptation in D. serrata and D. birchii were faster in mixed populations than in single strains with less genetic diversity. Fourth, response to artificial selection over the medium- to long-term term increases with effective population size, based upon studies in Drosophila, mice (Mus musculus), Tribolium and maize (Zea mays) (reviewed by Weber & Diggins 1990). Table 1 Relationship between intensity of selection and rate of genetic adaptation to captivity for reproductive fitness per generation across several studies in Drosophila. Selection intensity is positively related to crowding and negatively related to equalization of family sizes (EFS) vs. variable family sizes (VFS) Environment Response/ generation Reference Crowding (eight generations) Extreme crowding 12.5% Frankham & Loebel (1992) Modest crowding 9.0% Gilligan et aI. (2003) Low crowding (1 pair/vial) + CuS04* (25 generations) VFS 0,7% Frankham et aI. (2000) EFS 0.35% Frankham et aI. (2000) *Studies with CuS04 added to the medium at a concentration that reduced progeny production by about 50%. Means for minimizing the deleterious consequences of genetic adaptation to captivity Given the substantial potential for reduction in repro- ductive fitness for captive populations reintroduced to the wild, it is important to devise management to minimize its impacts. From equation 1, we predict that genetic adaptation can be minimized by (Frankham & LoebeI1992): (i) mini- mizing generations in captivity; (ii) minimizing selection; (iii) minimizing genetic diversity, and (iv) minimizing effective population size. In addition, immigration from the wild to captivity reduces genetic adaptation (Frankham & Loebe11992; Frankham 1995; Araki et al. 2007). I will examine each of these, as they are not always feasible. Minimizing genetic diversity at the species level conflicts with the need to minimize inbreeding and the desire to conserve evolutionary potential, and is only dis- cussed in the context of minimizing Ne using fragmentation. Minimizing generations in captivity The most effective means for minimizing genetic adaptation to captivity is to minimize the number of generations in captivity. Returning species to the wild as rapidly as possible is an obvious solution, but this is rarely feasible due to direct and indirect human impacts, except for the case of supplementation of harvested fish species (Araki et al. 2007). In plants, minimizing generations in captivity is widely practiced using storage of dormant seeds. The Millennium Seed Bank project, conceived, developed and managed by the Royal Botanic Gardens, Kew, England, in collaboration with 18 countries, held seeds of over 14000 species of plants in cold storage, as of early 2006. Seed banking also has a major role in the program of the Center for Plant Conservation in the USA. Seed storage cannot be used for plant species lacking seed dormancy, as what occurs in many tropical species. Minimizing generations in captivity can be achieved by cryopreservation (Johnston & Lacy 1995). It is a feasible option for plants where the technology appears to transfer well across species (Touchell & Dixon 1993). However, cryopreservation only works for a small minority of animal species, mostly those closely related to domestic species (Frankham et al. 2002). The technology has to be developed largely anew for each animal species. Further, a living population must still be maintained for production of living individuals from frozen embryos or semen. In theory, the number of generations in captivity can be reduced by breeding from older animals. However, this is impractical as animals whose reproduction is delayed may not breed at all, or they may reproduce poorly (Frankham et al. 2002). Overall, reducing generations in captivity is not feasible for most animal species. @ 2007 The Author Journal compilation @ 2007 Blackwell Publishing Ltd