Polyploidy and ecological transfiguration in Achillea.

P olyploidy is a prominent chromosomal/genetical feature in most flowering plant groups (1, 2). Change in the number of chromosome sets (and thus gene dosage) is an important factor in the genesis of new species, in part because plants with higher ploidal levels often have ecological tolerances that differ from those of their progenitors. Plant evolutionists have been very interested in the origin of such differences, which may arise in two distinctive ways (3, 4). In the absence of hybridization, ecological divergence may be a simple by-product of change in ploidal level. Ecological divergence also may arise from natural selection acting on polyploid populations. It is impossible to distinguish between these alternatives from studies of two established entities that may have diverged in ploidal level thousands of years ago. In PNAS, Ramsey (5) demonstrates that a ploidal increase alone indeed may allow the invasion of a habitat that was previously unassailable by a longestablished species. This study also demonstrates that long-standing polyploids are better adapted to this new habitat than are newly emergent ones.