Evidence for continent-wide convergent evolution and stasis throughout 150 years of a biological invasion

The extent to which evolution can rescue a species from extinction, or
facilitate range expansion, depends critically on the rate, duration, and
geographical extent of the evolutionary response to natural selection.
Adaptive evolution can occur quickly, but the duration and geographical
extent of contemporary evolution in natural systems remains poorly
studied. This is particularly true for species with large geographical
ranges and for timescales that lie between ‘long-term’ field experiments
and the fossil record. Here, we introduce the Virtual Common Garden (VCG)
to investigate phenotypic evolution in natural history collections while
controlling for phenotypic plasticity in response to local growing
conditions. Reconstructing 150 years of evolution in Lythrum
salicaria (purple loosestrife) as it invaded North America, we
analyze phenology measurements of 3,429 herbarium records, reconstruct
growing conditions from more than 12 million local temperature records,
and validate predictions across three common gardens spanning 10 degrees
of latitude. We find that phenology clines have evolved along parallel
climatic gradients, repeatedly throughout the range, during the first
century of evolution. Thereafter, the rate of microevolution stalls,
recapitulating macroevolutionary stasis observed in the fossil record. Our
study demonstrates that preserved specimens are a critical resource for
investigating limits to evolution in natural populations. Our results show
how natural selection and trade-offs measured in field studies predict
adaptive divergence observable in herbarium specimens over 15 decades at a
continental scale.