Supplementary material
The principle of fitness relates to the ability of a particular genome (that is, the ensemble of genes that happen to be found together in one individual) to make it possible for its owner to reproduce and leave fertile offspring, thus helping propagate the individual's genes in new generations. Thus, a combination of certain genes that boosts one's fertility and appeal to members of the other sex will very likely result in a high level of fitness, while combinations that do the opposite will lead to a a low level of fitness.
One worry about (for example) transgenic salmon is that individuals might escape aquaculture farms and "pollute" natural stocks with their genes. Despite assurances from the industry, I can practically guarantee that this will happen: I can't imagine how any system could be so efficient, foolproof and resistant to sabotage as to make sure that no such fish could ever find its way to the sea. But before we run to the hills screaming "zombie salmon apocalypse", let us reflect on what fate will befall the escapee fish: will it be fit to live in nature? Probably not, and that's one worry that environmentalists have: what if the transgenic salmon, ill-suited to life in the wild, leaves his "bad" genes in the wild population? Well, if the salmon in question is ill-suited to life in the wild, it will simply die. Dead fish don't reproduce, so its fitness would be close to zero; problem solved. What happens if it does reproduce before the end? Its offspring, few as they are in the first place, may also be less-suited to life in the wild; they, too, will have a lower fitness and will see their "bad" genes disappear from the gene pool in a few generations as they get diluted by the "good" genes from the other fish. Problem solved again.
What happens if the farmed fish have a greater fitness than the wild ones? It's currently not the case but it could happen since it would be more advantageous, economically speaking, to farm healthy and strong salmon rather than sickly and lazy runts. Well, in that case the escaped farmed fish with a higher fitness would indeed see their genes spread in the wild gene pool, making the species stronger. What was the problem again?
We would have a problem if, due to his stupidity, Man were to overfish salmon stocks to the point of near extinction... and that the few surviving individuals suddenly had to compete with a lot of farmed fish ill suited to life in the wild. But mankind couldn't be stupid enough to bring such important resources to the brink of extinction, could it?
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