string: 25 (Carlson and Seamons 2008). Although this is a modest value, it does provide ample opportunity for natural selection to operate. The other condition ‚Äö√Ñ√¨ a selective differential ‚Äö√Ñ√¨ is clearly met. Hydropower development has profoundly changed freshwater environmental conditions experienced by Snake River fall Chinook salmon, so ample reason exists to believe that selective pressures for age at smolting could have changed as well. Our analyses suggest that the possibility of rapid evolutionary change in juvenile migration tactic within the last 50‚Äö√Ñ√¨60 years. Credence is lent to this hypothesis by research that has concluded that evolutionary changes have been experienced by introduced Chinook salmon populations over a 90-year period in New Zealand. There, Unwin et al. (2000) documented a divergence in juvenile migratory timing associated with differences in water temperatures that had affected juvenile growth, an association similar to that observed in Snake River fall Chinook salmon.