establishment will be substituted for exposure. For risk assessment, the critical factor is the likelihood the GE organism will become established in a community, which is P(E). This conclusion does not mean that risk cannot occur without establishment. As discussed later, if a transgene causes local species extinctions, either because the population size is critical or because the transgene produces a Trojan gene effect, considerable harm might result. However, these are special cases that can be addressed as such. The likelihood of establishment is dependent on an organism’s fitness and ability to escape and disperse in diverse communities (Scientists’ Working Group on Biosafety, 1998), and the qualities of the receiving community.
Once a transgene is introduced into a community, whether by vertical or horizontal gene transfer, natural selection for fitness will determine the ultimate fate of the transgene if the population is large enough to withstand the initial perturbations (Muir and Howard, 2001). Fitness is quantified relative to that of other individuals in the population and is simply the genetic contribution by an individual’s descendants to future generations of a population (Ricklefs, 1990). Fitness in this context refers not only to its survival component, but also its reproductive component, that is, to all aspects of the organism’s phenotype that affect spread of the transgene. Muir and Howard, in modeling the potential spread of a transgene (2001; 2002a,b), reduced these aspects to six net fitness components: juvenile and adult viability, age at sexual maturity, female fecundity, male fertility, and mating success. The model is based on the assumption that natural selection acting through these components will determine the ultimate fate of the transgene.
The last component, mating success, often is overlooked because it generally is not a factor in artificial breeding programs; it often is, however, the strongest factor driving natural selection (Hoekstra et al., 2001). For example, increased adult size in most species of fish is positively correlated with mating success (as, for example, in many salmonid species: Jones, 1959; Schroder, 1982; Jarvi, 1990; Groot and Margolis, 1991). With Japanese medaka (Oryzias latipes), males 25 percent above average in size realized a 400 percent increase in mating success (Howard et al., 1998). Such increases in mating success could result in the spread of a transgene even if the transgene reduces survival rate (Muir and Howard, 1999).
From a population genetics perspective, if a GE organism is more fit than its wild relatives in the receiving population, the GE organism eventually will replace its relatives or become established in that community. If it is less fit, the engineered trait eventually will be removed from the receiving population. If the fitness of transgenic and nontransgenic individuals is similar, the likely