James A. Bailey, October 30, 2001
Species are the most recognized and protected units of biodiversity. Yet, we tend to ignore the importance of genetic diversity that is fundamental to species survival, and to the continued evolution of new species. Almost certainly, unique genes have been lost from very many species that have experienced substantial declines with isolated populations and local extirpations. However, for most species of plants and animals, there have been few or no studies to document this trend.
Small, declining or isolated populations will suffer from two related genetic problems: inbreeding depression and loss of genetic variation. The former has implications for short-term survival. The latter may limit long-term persistence.
Inbreeding (reproduction by closely related individuals) occurs in small and isolated populations. Inbred individuals are homozygous at many gene locations, allowing expression of normally recessive, deleterious genes. Common results are infertility, low survival rates and compromised resistance to disease. These results have been clearly documented in zoo populations and have been observed in several wild populations. In a decreasing population, inbreeding depression of reproduction and survival may accelerate a decline toward extinction.
In a particularly well-studied population of greater prairie-chickens in Illinois, isolated and small populations showed declining hatchability of eggs, correlated with their loss of genetic diversity. As a last ditch management solution for inbreeding depression, novel genes were imported with animals from larger populations in other states. Declining hatchability of Illinois greater prairie-chicken eggs was reversed by this costly translocation of birds. However, if the habitat problems of Illinois prairie-chickens have not been solved, we should expect the cycle of decline and inbreeding to begin again.
How large must a breeding population be to avoid significant inbreeding depression? This will depend upon the demographics of the population (normal reproductive rates, longevity, age at first breeding, population fluctuations), and on its breeding system (monogamous, polygamous). In addition, some species seem to have a greater "genetic load" of deleterious genes, exacerbating the effects of inbreeding. Lacking a careful analysis, a broad estimate is that inbreeding negatively impacts large-mammal populations of less than 100-300 animals.
However, populations barely large enough to prevent significant inbreeding depression will gradually lose the genetic variation that is necessary for continued evolution and adaptation to changing environments. To maintain genetic variation over long periods, populations of one-to-several thousands may be necessary. Moreover, studies have shown that populations of widespread species often contain subpopulations with somewhat different genetic compositions. As a species declines and its geographic range shrinks, subpopulations with their unique genes are lost and the long-term persistence of the species is compromised.
The federal Endangered Species Act contains indefinite language allowing the Fish and Wildlife Service to conserve genetic diversity by listing and protecting unique subpopulations. Species may be listed as threatened or endangered if they are in trouble throughout only "a substantial portion" of the species range. Subspecies and distinct populations may also be listed. The scientific community has wrestled with the legal and biotic implications of defining "distinct populations." Proposed criteria include: substantial reproductive isolation; an "important component" of a species evolutionary legacy; unique genetic composition; occupation of distinctive habitat; and occurrence of unusual adaptations to an environment. With these criteria, the most prominent efforts to conserve genetic diversity have been the separate listings of seasonal "runs" of salmon species. Among other influences, the commercial values of these salmon have spurred decision-making and listing.
The New Mexico Wildlife Conservation Act is even less clear regarding protection of genetic resources. Species may be listed as threatened, but apparently not as endangered, if they are in trouble throughout only a substantial portion of the species range. Subspecies may be listed. The Wildlife Conservation Act does not recognize distinct populations. I know of no subpopulations that have been listed under the Act for the purpose of conserving genetic resources. Usually, species have declined substantially and probably lost genetic diversity before they were listed under the New Mexico law.
In todays political climate, it has been difficult enough to preserve species, let alone their genetic variations. For most species, little or nothing is known of their genetic variety. We seem to be content with our efforts to conserve small or modest populations of species in fragments of their once larger geographic ranges. Genetic diversity, the most basic component for long-term species survival, is surely being lost.
If natural selection is the paintbrush of evolution, genetic diversity is its palette. Given a skimpy palette with few shades and colors, the resulting picture will be limited, perhaps dull and monotonous. But with a rich and diverse palette, the paintbrush of evolution can create an infinitely diverse and interesting scene a continuum of combinations and variations molded into one harmonious landscape. It has done so before, but may never do so again. This is the legacy we are leaving to future generations.