Exploring the macroevolutionary signature of asymmetric inheritance at speciation

Popular comparative phylogenetic models such as Brownian Motion,
Ornstein-Ulhenbeck, and their extensions, assume that, at speciation, a
trait value is inherited identically by two descendant species. This
assumption contrasts with models of speciation at a micro-evolutionary
scale where descendants’ phenotypic distributions are sub-samples of the
ancestral distribution. Different speciation mechanisms can lead to a
displacement of the ancestral phenotypic mean among descendants and an
asymmetric inheritance of the ancestral phenotypic variance. In contrast,
even macro-evolutionary models that account for intraspecific variance
assume symmetrically conserved inheritance of ancestral phenotypic
distribution at speciation. Here we develop an Asymmetric Brownian Motion
model (ABM) that relaxes the assumption of symmetric and conserved
inheritance of the ancestral distribution at the time of speciation. The
ABM jointly models the evolution of both intra- and inter-specific
phenotypic variation. It also infers the mode of phenotypic inheritance at
speciation, which can range from a symmetric and conserved inheritance,
where descendants inherit the ancestral distribution, to an asymmetric and
displaced inheritance, where descendants inherit divergent phenotypic
means and variances. To demonstrate this model, we analyze the evolution
of beak morphology in Darwin finches, finding evidence of displacement at
speciation. The ABM model helps to bridge micro- and macro-evolutionary
models of trait evolution by providing a more robust framework for testing
the effects of ecological speciation, character displacement, and niche
partitioning on trait evolution at the macro-evolutionary scale.