Chromosome evolution in flowering plants is often punctuated by
polyploidy, genome duplication events that fundamentally alter
DNA content, chromosome number, and gene dosage.
Polyploidy confers postzygotic reproductive isolation and is thought to drive ecological divergence and range expansion. The adaptive value of
polyploidy, however, remains uncertain; ecologists have traditionally relied on observational methods that cannot distinguish effects of
polyploidy per se from genic differences that accumulate after genome duplication. Here I use an experimental approach to test how
polyploidy mediates ecological divergence in Achillea borealis (Asteraceae), a widespread
tetraploid plant with localized hexaploid populations. In coastal California,
tetraploids and hexaploids occupy mesic grassland and xeric dune habitats, respectively. Using field transplant experiments with wild-collected plants, I show that hexaploids have a fivefold fitness advantage over
tetraploids in dune habitats. Parallel experiments with neohexaploids--first-generation mutants screened from a
tetraploid genetic background--reveal that a 70% fitness advantage is achieved via genome duplication per se. These results suggest that genome duplication transforms features of A. borealis in a manner that confers adaptation to a novel environment.