from: http://dailynews.yahoo.com/h/nm/19990912/sc/britain_worm_3.html
"The chemical the scientists used caused mutations which affected the worms' ability to reproduce sexually, but the results showed the vast majority of the mutations decreased reproductive fitness by only a tiny amount.
``We drew the conclusion that most of the harmful mutations had very, very tiny effects indeed, and only about three or four percent had large effects,'' Keightley said.
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So most of the 'harmful mutations' had only very tiny effects.