Genetic Variation in Toxicant-Stressed Populations: An Evaluation of the “Genetic Erosion” Hypothesis

The question whether environmental pollution affects genetic diversity in natural populations remains unanswered to date despite the fact that genetic variation is one of the three pillars of biodiversity recognized in the Rio convention of 1993. The loss of genetic diversity in populations subjected to anthropogenic stress can be designated as “genetic erosion” and may be considered as a factor of concern in risk assessment of toxic chemicals. Theoretically there are four different ways in which toxicants can affect genetic variation: (i) by increasing mutation rates, (ii) by directional selection on tolerant genotypes, (iii) by causing bottleneck events, and (iv) by altering migration. This paper reviews studies that have documented genetic change in animal populations exposed to environmental pollution. In these studies, genetic variation is measured in a variety of ways: heritability of quantitative characters, heterozygosity of allozyme loci, haplotype diversity in mitochondrial DNA, and variability in RAPD fingerprints. Studies on cadmium tolerance of Collembola living in metal-contaminated soil suggest that strong directional selection pressure may decrease genetic variability of traits immediately linked to tolerance. Allozyme studies in fish have documented a similar decrease of genetic variation in populations living in strongly acidified waters. A correlation between RAPD-PCR-based genetic similarity and site contamination has been documented in crayfish. Overall, there is significant support for the genetic erosion hypothesis, but the issue cannot be considered settled. In most studies insufficient attention is given to factors such as population size, bottlenecks and mutation, which may influence genetic variability in addition to the toxicant selection regime. At the moment, there does not seem to be a sound scientific basis for incorporating genetic diversity measurements into risk assessment, despite the variety of easily applicable molecular techniques available. It is often not known what kind of variation is measured by these techniques (neutral or selectable) and how the markers are inherited. Given the importance of the issue, as stressed by the Rio Convention, a concentrated research effort is necessary to better define the question and find a general approach to evaluate its importance in ecological risk assessment.