Fragmentation of animal and plant populations typically leads to genetic erosion and increased probability of extirpation. Although these effects can usually be reversed by re-establishing gene flow between population fragments, managers sometimes fail to do so due to fears of outbreeding depression (OD). Rapid development of OD is due primarily to adaptive differentiation from selection or fixation of chromosomal variants. Fixed chromosomal variants can be detected empirically. We used an extended form of the breeders' equation to predict the probability of OD due to adaptive differentiation between recently isolated population fragments as a function of intensity of selection, genetic diversity, effective population sizes, and generations of isolation. Empirical data indicated that populations in similar environments had not developed OD even after thousands of generations of isolation. To predict the probability of OD, we developed a decision tree that was based on the four variables from the breeders' equation, taxonomic status, and gene flow within the last 500 years. The predicted probability of OD in crosses between two populations is elevated when the populations have at least one of the following characteristics: are distinct species, have fixed chromosomal differences, exchanged no genes in the last 500 years, or inhabit different environments. Conversely, the predicted probability of OD in crosses between two populations of the same species is low for populations with the same karyotype, isolated for <500 years, and that occupy similar environments. In the former case, we recommend crossing be avoided or tried on a limited, experimental basis. In the latter case, crossing can be carried out with low probability of OD. We used crosses with known results to test the decision tree and found that it correctly identified cases where OD occurred. Current concerns about OD in recently fragmented populations are almost certainly excessive.
The concepts of "founder equivalent" and "founder genome equivalent" are introduced to facilitate analysis of the founding stocks of captive or other populations for which pedigrees are available. The founder equivalents of a population are the number of equally contributing founders that would be expected to produce the same genetic diversity as in the population under study. Unequal genetic contributions by founders decrease the founder equivalents, portend greater inbreeding in future generations than would be necessary, and reflect a greater loss of the genetic diversity initially present in the founders. The number of founder genome equivalents of a population is that number of equally contributing founders with no random loss of founder alleles in descendants that would be expected to produce the same genetic diversity as in the population under study. The number of founder genome equivalents is approximately that number of wild-caught animals that would be needed to obtain the same amount of genetic diversity as is in the descendant captive population. Founder equivalents and founder genome equivalents allow comparison of the genetic merits of adding new wild-caught stock vs. further equalizing founder representations in a captive population.
Population Viability Analysis (PVA) is the estimation of extinction probabilities by analyses that incorporate identifiable threats to population survival into models of the extinction process. Extrinsic forces, such as habitat loss, over-harvesting, and competition or predation by introduced species, often lead to population decline. Although the traditional methods of wildlife ecology can reveal such deterministic trends, random fluctuations that increase as populations become smaller can lead to extinction even of populations that have, on average, positive population growth when below carrying capacity. Computer simulation modelling provides a tool for exploring the viability of populations subjected to many complex, interacting deterministic and random processes. One such simulation model, VORTEX, has been used extensively by the Captive Breeding Specialist Group (Species Survival Commission, IUCN), by wildlife agencies, and by university classes. The algorithms, structure, assumptions and applications of VORTEX are described in this paper.VORTEX models population processes as discrete, sequential events, with probabilistic outcomes. VORTEX simulates birth and death processes and the transmission of genes through the generations by generating random numbers to determine whether each animal lives or dies, to determine the number of progeny produced by each female each year, and to determine which of the two alleles at a genetic locus are transmitted from each parent to each offspring. Fecundity is assumed to be independent of age after an animal reaches reproductive age. Mortality rates are specified for each pre-reproductive age-sex class and for reproductive-age animals. Inbreeding depression is modelled as a decrease in viability in inbred animals.The user has the option of modelling density dependence in reproductive rates. As a simple model of density dependence in survival, a carrying capacity is imposed by a probabilistic truncation of each age class if the population size exceeds the specified carrying capacity. VORTEX can model linear trends in the carrying capacity. VORTEX models environmental variation by sampling birth rates, death rates, and the carrying capacity from binomial or normal distributions. Catastrophes are modelled as sporadic random events that reduce survival and reproduction for one year. VORTEX also allows the user to supplement or harvest the population, and multiple subpopulations can be tracked, with user-specified migration among the units.VORTEX outputs summary statistics on population growth rates, the probability of population extinction, the time to extinction, and the mean size and genetic variation in extant populations.VORTEX necessarily makes many assumptions. The model it incorporates is most applicable to species with low fecundity and long lifespans, such as mammals, birds and reptiles. It integrates the interacting effects of many of the deterministic and stochastic processes that have an impact on the viability of small populations, providing opportunity for more ...
A computer simuiation program was used to examine interacting effects of genetic drift, mutation, immigration from outsidepopulations, directional and balancing selection, and population subdivision on the loss of genetic variability from small, managed populations. Stochastic ewnts were simulated with a pseudo-random number generator, and the genetic variation (expected hetmzygosity) witbin and between populations was monitored in 25 populations for 100 generations.Genetic dnyt was the overriding factor conholing the loss of genetic variation Mutation has no noticeable effect on populations of the size typically managed in zoos and nature preserves Immigration from a large source population can stri2ingly slow, halt, or even reverse the loss of genetic variation, even with only one or a few migrants per generation. Unless selection is stronger than commonly observed in natural populations, it is inefficient in countering drift when population sizes are on the order of 100 or f m . Subdivided populations rapidly lose variability from within each subpopulation but retain variation m s s the subpopulations better than does a panmictic population These results suggest that population managers should be concerned with the variation-depleting effects of genetic drift, perhaps almost to the exclusion of consideration of selection and mutation Drift can be countered by the introduction of vety occasional immigrants or, less effectively, by division of the managedpopulation into smaller breedinggroups that interchange enough migrants to prevent unacceptably deleterious inbreeding within each subpopulation Paper submitted 9/8/86; reukied munusmpt accepted 3/31/87. Resumen: A travks de un programa de simulacion por computadora se exarninaron 10s efectos interactivos de la deriva genic@ las mutaciones, la inmigracion de poblaciones externas, la seleccion balanceada y direccional, y la subdivision depoblaciones pequerias sujetas a manejo, debido a la phdida de variabilidad genetica Se simularon eventos estoccisticos con un generador de numeros pseudo-azarosos y se estudio la variacion gendtica inha e interpoblacional (heterocigosis esperada) en 25 poblaciones durante 100 generacionesLa deriva ghica fue el factorpredominante que controlo lap&-dida de variacion genetica Las mutaciones no tuvieron un efecto notable en poblaciones del tamano ti@ico manejado en zoologicos y areas protegidas. La inmigraci6n proveniente de ohas poblaciones m h grandes puede asombrosamente disminuir, detener 6 invertir la phdida de variacion genetic@ aun con la influencia de solo uno o pocos migrantes por generacion. Cuando el tamano de las pobiaciones es dei orden de 100 individuos 6 menos, no es necesario evaluar la deriva ghic@ a menos que la seleccion sea mcis fuerte que la comunmente observadu enpoblaciones naturales. Las poblaciones divididaspierden rapidamente su variabiiidad intra-subpoblacional, per0 retienen una mayor variacion intersubpoblacional que las poblaciones panmiticas. Los resultados sugieren que 10s manejadores depoblaciones deb...
The biological diversity of the planet is being rapidly depleted due to the direct and indirect consequences of human activity. As the size of animal and plant populations decrease and fragmentation increases, loss of genetic diversity reduces their ability to adapt to changes in the environment, with inbreeding and reduced fitness inevitable consequences for many species. Many small isolated populations are going extinct unnecessarily. In many cases, such populations can be genetically rescued by gene flow into them from another population within the species, but this is very rarely done. This novel and authoritative book addresses the issues involved in genetic management of fragmented animal and plant populations, including inbreeding depression, loss of genetic diversity and elevated extinction risk in small isolated populations, augmentation of gene flow, genetic rescue, causes of outbreeding depression and predicting its occurrence, desirability and implementation of genetic translocations to cope with climate change, and defining and diagnosing species for conservation purposes.
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