Evolutionary dynamics of ecological niche in three Rhinogobio fishes from the upper Yangtze River inferred from morphological traits

In the past decades, it has been debated whether ecological niche should be conserved among closely related species (phylogenetic niche conservatism, PNC) or largely divergent (traditional ecological niche theory and ecological speciation) and whether niche specialist and generalist might remain in equilibrium or niche generalist could not appear. In this study, we employed morphological traits to describe ecological niche and test whether different niche dimensions exhibit disparate evolutionary patterns. We conducted our analysis on three Rhinogobio fish species (R. typus, R. cylindricus, and R. ventralis) from the upper Yangtze River, China. Among the 32 measured morphological traits except body length, PCA extracted the first four principal components with their loading scores >1.000. To find the PNC among species, Mantel tests were conducted with the Euclidean distances calculated from the four principal components (representing different niche dimensions) against the pairwise distances calculated from mitochondrial cytochrome b sequence variations. The results showed that the second and the third niche dimension, both related to swimming ability and behavior, exhibited phylogenetic conservatism. Further comparison on niche breadth among these three species revealed that the fourth dimension of R. typus showed the greatest width, indicating that this dimension exhibited niche generalism. In conclusion, our results suggested that different niche dimensions could show different evolutionary dynamic patterns: they may exhibit PNC or not, and some dimensions may evolve generalism.