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Squawfish Population Viability Analysis --July 1993 Page 21 carry a high genetic load for balancing selection. That is, if there are alternative alleles A and B at a gene locus, with the AB genotype the fittest, it is quite possible to select against AA and BB recombinants without lowering population growth rates so long as this hard selection is done against the early life stages. Low heterozygosity is the dynamic result of a low genetic effective population size, denoted Ne. The current number of Colorado squawfish adults is low (less than 104 [Tyus 1991]). The species may, through a history of geologic change in its substrate, have experienced similar low population sizes in the past. Over long periods of time, Ne is based on the harmonic mean of population sizes (see below), which is most sensitive to the low values in the series. And there is one additional reason why Colorado squawfish could have had low H. The species could have a metapopulation structure (see the book Metapopulation Dynamics by Gilpin and Hanski [1991]). IT the Colorado squawfish occasionally were to go extinct in one or another drainage and is then recolonized by a few fish, this would periodically put local populations through genetic bottlenecks. Over long periods of time, this can squeeze out genetic variability. 2.5 Has H Changed in Value Recently? Relative changes in H levels are more important than absolute levels of H. For most species of concern to conservation biology (the small and the fragmented), the normal direction for a change in H is downward. The only situation where H would increase would be a situation where human modification of the landscape allowed two or more previously isolated population to mix. This is probably not the case for the Colorado squawfish (but see discussion below). The expected decline in H occurs for two reasons. Population sizes may become very small and genetic drift can cause H to fall (see the section on Ne below). Or, a strong selection pressure may act on the population, allowing only a small fraction of the adults to reproduce successively, thus passing the gene pool of the population through a kind of bottleneck or founder event. 2.6 Time Scale for Genetic Drift The time scale of genetic drift is the generation time. For the squawfish this is long, almost certainly greater than 10 years under pristine conditions