Mind the outgroup and bare branches in total-evidence dating: a case study of Pimpliform Darwin Wasps (Hymenoptera, Ichneumonidae)

Taxon sampling is a central aspect of phylogenetic study design, but it
has received limited attention in the context of total-evidence dating, a
widely used dating approach that directly integrates molecular and
morphological information from extant and fossil taxa. We here assess the
impact of commonly employed outgroup sampling schemes and missing
morphological data in extant taxa on age estimates in a total-evidence
dating analysis under the uniform tree prior. Our study group is
Pimpliformes, a highly diverse, rapidly radiating group of parasitoid
wasps of the family Ichneumonidae. We analyze a data set comprising 201
extant and 79 fossil taxa, including the oldest fossils of the family from
the Early Cretaceous and the first unequivocal representatives of extant
subfamilies from the mid Paleogene. Based on newly compiled molecular data
from ten nuclear genes and a morphological matrix that includes 222
characters, we show that age estimates become both older and less precise
with the inclusion of more distant and more poorly sampled outgroups.
These outgroups not only lack morphological and temporal information, but
also sit on long terminal branches and considerably increase the
evolutionary rate heterogeneity. In addition, we discover an artefact that
might be detrimental for total-evidence dating: “bare-branch attraction”,
namely high attachment probabilities of certain fossils to terminal
branches for which morphological data are missing. Using computer
simulations, we confirm the generality of this phenomenon and show that
 a large phylogenetic distance to any of the extant taxa, rather
than just older age, increases the risk of a fossil being misplaced due to
bare-branch attraction. After restricting outgroup sampling and adding
morphological data for the previously attracting, bare branches, we
recover a Jurassic origin for Pimpliformes and Ichneumonidae. This first
age estimate for the group not only suggests an older origin than
previously thought, but also that diversification of the crown group
happened well before the Cretaceous-Paleogene boundary. Our case study
demonstrates that in order to obtain robust age estimates, total-evidence
dating studies need to be based on a thorough and balanced sampling of
both extant and fossil taxa, with the aim of minimizing evolutionary rate
heterogeneity and missing morphological information.