- Characiform fishes form one of the most diverse freshwater fish clades in the world. Comprising more than 2000 species and distributed primarily in South America and Africa, characiforms vary dramatically in their ecomorphology. However, the evolutionary processes responsible for the immense ecomorphological diversity remains unknown. Recently, a study postulated that the unparalleled ecomorphological diversification arose through an ancient adaptive radiation, as evidenced by the clear segregation of morphological traits, such as body shape, among different trophic and habitat groups. However, no formal macroevolutionary analyses have been conducted on the entire order of Characiformes and the mechanism responsible for the diversity remains unknown.
Here, I conduct a macroevolutionary analysis of body shape diversification to determine if Characiformes evolved through an adaptive radiation. I estimated the first time-calibrated molecular phylogeny for the order Characiformes, assembled the first ever geometric morphometric body shape dataset, and compiled an exhaustive trophic ecology database.
In my second chapter, I combined these datasets to test whether body shape adapted to shifts into different trophic guilds and to reconstruct body shape diversification in the Old and New World radiations. I found that body shape adaptation resulted in many non-repeated morphologies; lineages that shifted into the same trophic ecology evolved different morphotypes, except for convergent piscivores. Furthermore, we found that body shape diversification between the Old and New World radiations followed very different pathways, with the New World radiation occupying twice as much morphospace as their Old World counterparts. Both radiations exhibited higher morphological disparity than would be expected under Brownian motion, early in cladogenesis, matching expectations of an adaptive radiation.
In my third chapter, I tested whether evolutionary modularity increased body shape diversification in the order. I found that characiform body shape was comprised of three independent modules that diversified at different times and rates while under different selective regimes. I postulate that the high evolutionary modularity plausibly explains why many body shapes evolved only once, when lineages evolved similar trophic ecologies across the radiation. The generality of the relationship between evolutionary modularity and increased morphological disparity has not been well studied in vertebrate lineages. More studies need to look at the role that evolutionary modularity and integration can have in shaping deterministic and contingent patterns of evolution across broad and restricted radiations of vertebrates.
In my fourth chapter, I analyzed the rates of lineage and morphological diversification to determine whether Characiformes exhibited an early burst of speciation and morphological evolution as predicted by classical adaptive radiations. I found that the rate of speciation and evolution on the first morphological principal component were very high early in cladogenesis and quickly slowed down, following a pattern of adaptive radiation. However, the evolutionary simulations indicate that heavily pruning the tree overestimates the speciation rate early in cladogenesis, making the results consistent with a constant rates model. Higher taxon sampling is needed to fully understand whether the order exhibited the speciation patterns consistent with an adaptive radiation.
My dissertation presents the first ever time-calibrated molecular phylogeny of Characiformes, the largest geometric morphometric dataset of Characiformes, and the most densely sampled clade-wide analysis of modularity in fishes. I found that characiform body shape likely radiated early in cladogenesis giving rise to many of the distinct morphologies that define the order.