The response of quantitative traits in translocated populations has
not been well explored. Evolutionary shifts in quantitative traits, such
as life history characteristics, are of particular interest because such
traits contribute to our perception of biodiversity. In a recent survey,
Stockwell (1995) examined life history variation of four populations of
western mosquitofish (Gambusia affinis) that were derived from a
common ancestral population 55-58 years earlier. Three life history traits
differed considerably among the four sites and appeared to be correlated
with local environmental conditions. To test whether these differences
were due to plasticity or rapid evolution, fish from the four sites were
maintained under common-garden conditions for two generations. After two
generations, no differences among populations were observed for one of
the three traits: offspring size. However, the other two (fat content and
size at maturity) varied significantly among populations. Thus, differences
observed in the field reflected phenotypic plasticity for offspring size
but rapid evolution apparently occurred for size at maturity and fat storage
for these recently established mosquitofish populations. These life history
differences may have arisen by genetic drift and/or natural selection.
These data, combined with data from a variety of other taxa, suggest that
translocations may alter the evolutionary trajectory of the targeted taxa.
Our findings provide additional impetus for the protection of species in
their native range.