Genetic Deterioration in Threatened and Endangered Species


Introduction to Conservation Genetics. Frankham, R., J. D. Ballou, and D. A. Briscoe . 2002 . Cambridge University Press , New York . 638 pp. $130.00. (hardcover) ISBN 0-521-63014-2 . $50.00 (paperback). ISBN 0-521-63985-9.

In dealing with endangered species, we are concerned with habitat destruction, fragmentation, pollution, overexploitation, and introduced exotic species. One might ask, should we also worry about genetic deterioration? Such concerns must surely be far down the list. Indeed, until a few years ago genetic problems were just a theoretical possibility. But in their remarkable book on conservation genetics, Frankham, Ballou, and Briscoe document that genetic deterioration is a problem for many threatened and endangered species. Further, they provide background and examples for understanding how genetic deterioration occurs and how it can sometimes be overcome by judicious management.

The first third of the book is an excellent introduction to population and quantitative genetics. The use of equations is generally pragmatic and directed toward understanding relationships; derivations are minimized. Although they make a perfunctory bow to history, the authors focus on genetic deterioration and developing management tools; thus, historical contributions are merely incidental. Frankham et al. show appreciation for the insight that comes from simplified theoretical models and balance this by discussing the shortcomings such models typically have for direct application to real-life situations. Their advocacy of computer simulations for such purposes, with pointers to currently available software, is almost unique.

The second third of the book delves into more detail about the special applications of population and quantitative genetics to conservation. The authors observe that “[e]volutionary processes are changed when a species declines from a large size to become small and endangered” ( p. 218 ). They discuss those changes caused by inbreeding, demographic stochasticity, stochastic loss of suitable habitat, habitat fragmentation, and loss of genetic variation needed to cope with environmental change. How does each of these occur? How important are they in practice? The authors describe what is expected from theory and tabulate what is known from those threatened or endangered species that have been studied. Especially important is their discussion of minimally viable population size. If the authors are correct, and I believe they probably are, then the current World Conservation Union criteria for minimally viable populations are much too small. This needs careful study and a consensus from the relevant scientific community.

My generation learned the evidentiary basis for population genetics almost exclusively from Drosophila and for quantitative genetics from laboratory mice and agricultural animals. This book features a wonderful variety of examples, including condors (Gymnogyps californianus), elephant seals ( Mirounga angustirostris ), fish, wolves (Canis lupus), gazelles (Gazella spp.), pumas (Felis concolor), tamarins (Leontopithecus rosalia), butterflies, and a variety of plants. It is rare to see a book on population genetics where plants get nearly as much attention they do here. Some of these species have been studied in detail so that examples can be followed and used as illustrations through several topics. For example, dwarfism in condors is used to illustrate Hardy-Weinberg calculations, single-locus selection, and managed inbreeding avoidance. The hairy-nosed wombat ( Lasiorhinus latifrons ) is used to calculate loss of genetic diversity with small population size, to estimate effective population size, to survey with molecular methods, and to resolve questions about the genetic uniqueness of populations. The rich variety of examples described in this book cannot help but stimulate interest and bring enthusiasm to new generations of students.

In the final third of the book the authors take up separate issues that are of greater importance to conservation genetics than to population or quantitative genetics generally. These include resolving questions of genetic uniqueness and taxonomic status, molecular forensics to identify alleged hunting or collecting, and management of wild or captive populations. So much bluster and smoke has been generated by the question of what distinguishes species that I, at least, have usually been content to present the difficult issues to students but take no position myself. In conservation genetics that luxury is not allowed. It is absolutely critical to have clear and agreed-upon criteria for differentiating populations and assigning them an objective taxonomic status. The sensible review of ideas about species criteria presented here in table format is especially welcome. There are also nice tabular reviews of population bottlenecks, inbreeding depression, effective population size, and case studies for evaluating management options.

The overall message is one of serious concern, not despair. The goal is to engender appreciation for the problems of threatened and endangered species and to help development of practical management tools. It is encouraging to learn that there have been times when knowledge of conservation genetics did help solve problems. An example is the use of molecular markers to determine that some superficially distinct populations of pumas are not very distinct genetically so that introductions from other sites will not introduce harmful hybrid sterility, but will instead help reduce the frequency of homozygote recessive mutants and alleviate inbreeding depression generally.

Introduction to Conservation Genetics is an important book; it has authoritative reviews and lucid descriptions of population and quantitative genetics. It is beautifully put together and illustrated ( with line drawings by Karina McInness ). The authors aspired to write another Falconer's Quantitative Genetics, which inspired and taught several generations. They have accomplished that admirably.

Much in population genetics is contentious. The authors sometimes take stronger stands than seem warranted by current knowledge, but as new techniques become available and the evidentiary basis grows it will be possible to fortify some of the issues presented here with new examples and to revise positions on others. Overall, this book is a fine account of the field. Conservation genetics is important and growing more so. I anticipate that this book and its successors will be the standard text and reference for years to come.