Mt. Kinnison et al., Migratory costs and the evolution of egg size and number in introduced andindigenous salmon populations, EVOLUTION, 55(8), 2001, pp. 1656-1667
The trade-off between reproductive investment and migration should be an im
portant factor shaping the evolution of life-history traits among populatio
ns following their radiation into habitats with different migratory costs a
nd benefits. An experimentally induced difference in migratory rigor for fa
milies of chinook salmon (Oncorhynchus tshawytscha). of approximately 86 km
and 413 in elevation, exacted a cost to somatic energy reserves (similar t
o 17% reduction in metabolizable mass) and ovarian investment (13.7% reduct
ion in ovarian mass). This cost was associated with a reduction in egg size
and paralleled the phenotypic pattern of divergence between two introduced
New Zealand populations of common origin, presently breeding at sites with
different migration distances, The genetic pattern of divergence of these
same populations, detected under common rearing, was consistent with compen
sation for migratory costs (the population that migrates farther invested m
ore in ovarian mass), but egg number more than egg size was associated with
this evolution. These evolutionary patterns are consistent with what is kn
own of the inheritance of these traits and with trade-offs and constraints
favoring initial evolution in offspring number over offspring size. Analysi
s of egg number-size patterns of other Pacific salmon populations in their
native range supported the hypothesis that migration strongly influences pa
tterns of reproductive allocation, favoring a higher ratio of egg number to
egg size with greater migration distance.