Selection on offspring size and contemporary evolution under ocean acidification

Ocean acidification may have deleterious effects on many species, but
anticipating long-term changes in the abundance of populations will
require an understanding of ocean acidification as an evolutionary force.
Here I show that ocean acidification alters natural selection on offspring
size and is likely to drive contemporary evolution. In a detailed study of
a coastal fish species (California Grunion), I demonstrate that larval
mortality is highly sensitive to ocean acidification and that mortality
rates are lower for larger larvae. However, these effects are countered by
tradeoffs between offspring size and number, suggesting that measurements
of maternal fitness are critical for quantifying selection through ocean
acidification. Measurements of selection and genetic variation were used
to project the evolution of larval size as seawater conditions changed
incrementally over many decades. Results for California Grunion suggest
that contemporary evolution may offset the projected decline in
reproductive success by about 50%.