Historical warming consistently decreased size, dispersal and speciation rate of fish

There is ongoing debate as to whether fish body size will decrease with
global warming and how these changes may impact dispersal ability and
speciation rate. Theory predicts that, under warmer temperatures, fish
grow to a smaller size, undergo a reduction in dispersal ability and
increase speciation rates. However, evaluations of such predictions are
hampered owing to the lack of empirical data spanning both wide temporal
and geographical scales. Here, using phylogenetic methods, we show that
smaller clupeiform fish (anchovies and herrings) occurred historically in
warmer waters, moved the shortest distances at low speed and displayed the
lowest speciation rates. Furthermore, fish moved faster and evolved
rapidly under higher rates of temperature change but these historical
rates are far lower than current warming rates. Our results predict a
future where smaller clupeiform fish that have reduced ability to move
will be more prevalent; this, in turn, may reduce future speciation.