Divergent processes drive parallel evolution in marine and freshwater fishes

Evolutionary comparisons between major environmental divides, such as
between marine and freshwater systems, can reveal the fundamental
processes governing diversification dynamics. Although processes may
differ due to the different scales of their biogeographic barriers,
freshwater and marine environments nevertheless offer similar
opportunities for diversification in benthic, demersal, and pelagic
habitats. Here, we compare the evolutionary patterns and processes shaping
teleost diversity in each of these three habitats and between marine and
freshwater systems. Using specimens from the National Museum of Natural
History, we developed a data set of linear measurements capturing body
shape in 2266 freshwater and 3344 marine teleost species. With a novel
comparative approach, we contrast the primary axis of morphological
diversification in each habitat with the major axis defined by
phylogenetic signal. By comparing angles between these axes, we find that
fish in corresponding habitats have more similar primary axes of
morphological diversity than would be expected by chance, but that
different historical processes underlie these parallel patterns in
freshwater and marine environments. Marine diversification is more
strongly aligned with phylogenetic signal and shows a trend toward
lineages occupying separate regions of morphospace. In contrast,
ecological signal appears to be a strong driver of diversification in
freshwater lineages through repeated morphological evolution in densely
packed regions of morphospace. In spite of these divergent histories, our
findings reveal that habitat has driven convergent patterns of
evolutionary diversification on a global scale.