Parental plants grown from seed in a single greenhouse (a "common garden") at low elevation show a continuous range of heights (above, bottom graph). Cuttings from the same plants (which therefore have the same genotypes) grown at middle and high elevations show a range of heights: these heights are not predictable from the growth pattern at low elevation. For example, genotype #4 does best at medium elevation; genotype #1 does well at low and  high elevation but not medium elevations.

    Achilea shows a geographic cline (continuous gradient) of variation in height, depending on the altitude at which it grows. The experiments taken together indicate that height is a non-linear function of both genotype and environment.