Effects of small population size and reduced genetic variation on the viability of wild animal populations remain controversial. During a 35-year study of a remnant population of greater prairie chickens, population size decreased from 2000 individuals in 1962 to fewer than 50 by 1994. Concurrently, both fitness, as measured by fertility and hatching rates of eggs, and genetic diversity declined significantly. Conservation measures initiated in 1992 with translocations of birds from large, genetically diverse populations restored egg viability. Thus, sufficient genetic resources appear to be critical for maintaining populations of greater prairie chickens.

Conservation genetics studies of populations bottlenecks are commonly framed under the detrimental paradigm of inbreeding depression. This conceptual paradigm presupposes a direct and unambiguous relationship between population size, genetic diversity, fitness, and extinction. Here, I review a series of studies that emphasize the role of chance, selection, and history in determining the genetic consequences of population bottlenecks. The variable responses of bottlenecks to fitness, phenotypic variation, and heritable variation emphasize the necessity to explore the relationship between molecular genetic diversity, fitness, adaptive genetic diversity, and extinction beyond the detrimental paradigm of inbreeding depression. Implications for conservation and management are presented as guidelines and testable predictions regarding the potential effects of bottlenecks on population viability and extinction.

Most, if not all, of the "classic," often-cited examples illustrating the genetic effects of a population bottleneck are open to alternative explanations due to the lack of adequate control populations, that is, low levels of genetic variability are often assumed to be the result of a past population bottleneck without having any prebottleneck measures. Here we provide the first clear case history where both prebottleneck and postbottleneck measures of genetic variability have been collected from a natural system. Analysis of DNA from museum specimens of the greater prairie chicken Tympanuchus cupido from central Illinois revealed the loss of specific alleles (known to have been present earlier in this century) following a demographic contraction. Lost alleles included common ones present in all other populations sampled and others unique to the Illinois population.

Translocations are becoming increasingly popular as appropriate management strategies for the genetic restoration of endangered species and populations. Although a few studies have shown that the introduction of novel alleles has reversed the detrimental effects of inbreeding over the short-term (i.e., genetic rescue), it is not clear how effective such translocations are for both maintaining neutral variation that may be adaptive in the future (i.e., genetic restoration) and increasing population viability over the long-term. In addition, scientists have expressed concerns regarding the potential genetic swamping of locally adapted populations, which may eliminate significant components of genetic diversity through the replacement of the target population by the source individuals used for translocations. Here we show that bird translocations into a wild population of greater prairie-chickens (Tympanuchus cupido pinnatus) in southeastern Illinois were effective in both removing detrimental variation associated with inbreeding depression as well as restoring neutral genetic variation to historical levels. Furthermore, we found that although translocations resulted in immediate increases in fitness, the demographic recovery and long-term viability of the population appears to be limited by the availability of suitable habitat. Our results demonstrate that although translocations can be effective management tools for the genetic restoration of wild populations on the verge of extinction, their long-term viability may not be guaranteed unless the initial conditions that led to most species declines (e.g., habitat loss) are reversed.

Although the theoretical relationship between population size, fitness, and genetic variation is well established, only a few studies have provided direct evidence that ties a decline in both genetic variation and fitness to a demographic bottleneck for a natural system. We report on a genetic comparison of four populations of the Greater Prairie Chicken ( Tympanuchus cupido ) with different demographic histories. Specifically, we compared a population from Illinois that has suffered an extreme demographic contraction and an associated decline in population fitness (measured in terms of hatchability rates) with populations from Kansas, Nebraska, and Minnesota with no known history of bottlenecks or associated declines in fitness. Using the polymerase chain reaction, we amplified six microsatellite loci from which levels of heterozygosity, allelic diversity, and geographic differentiation ( F ST and R ST ) of the studied populations were estimated. Results of this analysis showed that the Illinois Prairie Chicken had the lowest estimate of mean heterozygosity per locus and approximately two-thirds the allelic diversity, sharing 95-100% of all their alleles with each of the other populations. This finding suggests that the Illinois Prairie Chicken originally had higher levels of genetic diversity that were subsequently lost through an extreme demographic contraction. To our knowledge this is the first example of loss of genetic diversity being associated with a decrease in population fitness as a result of a known demographic bottleneck in a wild bird species.Evaluación Genética de un Cuello de Botella Demográfico en la Gran Gallineta de la Pradera Resumen: Aunque la relación teórica entre el tamaño poblacional, la aptitud biológica y la variabilidad genética ha sido bien establecida, son pocos lo astudios que han proporcionado evidencia directa que asocie una disminución de la variación genética y de la aptitud biológica con un cuello de botella demografico en sistemas naturales. Reportamos una comparación genética de cuatro poblaciones de la gran gallineta de la pradera ( Tympanucus cupido ) que tuvieron historias demográficas diferentes. Específicamente comparamos una población de Illinois, la cual ha sufrido una contracción demográfica extrema asociada con decrementos en la aptitud biológica poblacional (medida en términos de tasas de eclosión), con poblaciones de Kansas, Nebraska y Minnesota, las cuales no presentan una historia de cuello de botella poblacional ni decrementos en la aptitud bioloógica. Mediante el uso de la reacción en cadena de la polimerasa (PCR) amplificamos seis marcadores de microsatélites con los cuales estimamos niveles de heterocigósis, diversidad alélica y diferenciación geográfica ( F ST y R ST ) en las poblaciones estudiadas. La gran gallineta de la pradera en Illinois presenta el menor valor de heterocigosidad promedio por locus y aproximademente 2/3 de la diversidad alélica, compartiendo 95-100% de sus alelos con cada una de las otras poblaciones. Estos resultados sugieren que la p...