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Gene flow between moderately contaminated populations and heavily polluted popula-
tions can induce a reduction of the adaptation speed in the polluted systems (Groenendijk
et al. 1999). In a contaminated population, the identification of immigrants by genetic tools
can be particularly pertinent, allowing the exploration of the effects of contaminants, pri-
marily on residents and secondly on immigrants (Theodorakis 2001).
14.2.4 Selection
Selection preferentially retains in a particular environment the individuals displaying the
higher fitness. Selection can oppose genetic drift in extreme environments where selection
can maintain a particular polymorphism over thousands of generations, even for isolated
populations of a few dozens of individuals (Nevo 2001). The demonstration of the effects of
selection on molecular variants (i.e., the estimation of their values in terms of physiological
performance) is generally very difficult to carry out, DNA diversity being very complex
to analyze. Thus, selective pressure is sometimes more easily detected by measuring the
heritability of a phenotypic trait in a population, considering that the trait is integrating
the physiological effects of several genes (Hauser et al. 2003).
The selection modes induced by contamination on populations are very diversified and
complex (Van Straalen and Timmermans 2002). If a polymorphism is linked to a pattern of
resistance, one should detect an increase in the tolerant genotype frequency, possibly by
different modes:
• Directional selection, without a heterosis effect (i.e., without heterozygote superi-
ority), where a reduced genetic variability is observed over the population.
• Stabilizing selection, itness being highest at some intermediate phenotypes and
lower at the phenotypic extremes (decrease of genetic variability).
• Variable selection in space and time (alternately toxic and nontoxic periods); if
fitness is highest for the tolerant genotype in the toxic period but lower in the non-
toxic period, genetic variability will thus be maintained.
• Disruptive selection where itness is lowest at some intermediate phenotype and
is higher at the phenotypic extremes; genetic diversity will be also maintained.
14.3 Common Markers of Genetic Diversity in Ecotoxicology
Effects of contaminants on genetic diversity can be considered:
• Indirectly through the measure of biometric characters linked to itness with a
heritable component (quantitative genetics approach)
• Directly through the study of allozymes or DNA markers (population genetics
approach)
Biometric characters, potentially under control of several genes, may generate differen-
tial physiological performances and thus lead to population effects in the face of environ-
mental stress (Woods and Hoffman 2000). These biometric approaches are based on the
hypothesis that selective pressures determine the genetic characteristics of a population
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