Migratory lineages rapidly evolve larger body sizes than non-migratory relatives in ray-finned fishes

Migratory animals respond to environmental heterogeneity by predictably
moving long distances in their lifetime. Migration has evolved repeatedly
in animals, and many adaptations are found across the tree of life that
increase migration efficiency. Life history theory predicts that migratory
species should evolve a larger body size than non-migratory species and
some empirical studies have shown this pattern. A recent study analyzed
the evolution of body size between diadromous and non-diadromous shads,
herrings, anchovies and allies, finding that species evolved larger body
sizes when adapting to a diadromous lifestyle.  It remains
unknown whether different fish clades adapt to migration similarly.
 We used an adaptive landscape framework to explore body size
evolution for over 4500 migratory and non-migratory species of ray-finned
fishes. By fitting models of macroevolution, we show that migratory
species are evolving towards a body size that is larger than non-migratory
species. Furthermore, we find that migratory lineages evolve towards their
optimal body size more rapidly than non-migratory lineages, indicating
body size is a key adaption for migratory fishes. Our results show, for
the first time, that the largest vertebrate radiation on the planet
exhibited strong evolutionary determinism when adapting to a migratory
lifestyle.