Assessing Asymmetrical Rates in Multivariate Phylogenetic Trait Evolution: An Extension of Statistical Models for Heterogeneous Rate Estimation

Abstract

Understanding the rate of evolution provides insight into how rapidly species have historically evolved. We investigate the often-overlooked concept of asymmetry in evolutionary rates. We observe the variation in the rates at which different traits within the same organism, or the same traits across different organisms, evolve. Influenced by factors such as environmental pressures and genetic constraints, this asymmetry might lead to inconsistent rates of biological changes. To capture these diverse rates, we propose three advanced statistical models, transcending the traditionally employed Brownian motion model. These models—the phylogenetic multivariate Ornstein–Uhlenbeck model, the early burst model, and the mixed model—were applied to body length, forelimbs, and head length in salamanders. The results from our substantial dataset show these models’ effectiveness in highlighting the asymmetrical patterns of trait evolution, enhancing our understanding of the complex dynamics in species evolution. Therefore, our study underscores the importance of considering asymmetry when studying evolutionary rates.