Data and reproducible code for Honor et al: Direct and indirect fitness effects of competition limit evolution of allelopathy in an invading plant

Upon introduction to new continents, invading species encounter novel
communities of consumers, pathogens, and competitors. Both phenotypic
plasticity and rapid evolution can facilitate adaptation across these
heterogenous communities, facilitating further invasion. However, the rate
and extent of adaptive evolution on contemporary timescales can be
constrained by phenotypic plasticity and limits imposed by genetic
co-variation for traits under selection. We measured phenotypic plasticity
and quantified genetic co-variation for growth, competition, and fitness
among 23 naturally inbred seed families of Alliaria petiolata
(garlic mustard) collected across its invasive range in eastern North
America. After growing a self-pollinated generation in a uniform common
garden to reduce maternal effects, we reared second-generation plants in a
two-year greenhouse and field experiment with naïve soil from an uninvaded
habitat. We measured selection gradients and lifetime fitness when reared
alone, with an intraspecific competitor, and under interspecific
competition with naïve Acer saccharum (sugar maple) saplings. Total
glucosinolate production was strongly correlated with the production of
chlorophyll a (Chl a) (R2 = 0.45) such that first principal
component (PC1) accounted for 84% of variation in these two traits.
Furthermore, PC1 exhibited high plasticity across growing environments (p
< 0.001) with limited broad-sense heritability (H2 = 2.91; p =
0.08). In contrast, investment in glucosinolate production relative to Chl
a (PC2) was significantly heritable (H2 =16.91, p < 0.001) with
minimal plasticity across treatments. Causal analysis revealed that
plastic variation for higher Chl a + glucosinolate production (PC1) had an
indirect positive effect on A. petiolata fitness via a direct, negative
effect on A. saccharum performance. In contrast, heritable variation for
higher glucosinolate investment (PC2) had a direct, positive effect on A.
saccharum performance and an indirect negative effect on A. petiolata
fitness.  Applying causal inference, we find that evolution of
allelopathy in A. petiolata has been constrained by (i) a lack of genetic
variation, (ii) selection against glucosinolate investment under
interspecific competition, and (iii) phenotypic plasticity. These factors
limit adaptive evolution but maintain fitness during population growth as
plants switch from interspecific to intraspecific competition during
invasion.