Recent warming reduces the reproductive advantage of large size and contributes to evolutionary downsizing in nature

Body size is a key functional trait that is predicted to decline under
warming. Warming is known to cause size declines via phenotypic
plasticity, but evolutionary responses of body size to warming are poorly
understood. To test for warming-induced evolutionary responses of body
size and growth rates, we used populations of mosquitofish ( Gambusia
affinis ) recently established (less than 100 years) from a common source
across a strong thermal gradient (19–33°C) created by geothermal springs.
Each spring is remarkably stable in temperature and is virtually closed to
gene flow from other thermal environments. Field surveys show that with
increasing site temperature, body size distributions become smaller and
the reproductive advantage of larger body size decreases. After common
rearing to reveal recently evolved trait differences, warmer-source
populations expressed slowed juvenile growth rates and increased
reproductive effort at small sizes. These results are consistent with an
adaptive basis of the plastic temperature–size rule, and they suggest that
temperature itself can drive the evolution of countergradient variation in
growth rates. The rapid evolution of reduced juvenile growth rates and
greater reproduction at a small size should contribute to substantial body
downsizing in populations, with implications for population dynamics and
for ecosystems in a warming world.