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Common Garden experiment  with  Yarrow (Achillea spp.)

    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.

    Yarrow 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.
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Figure 2002 by Griffiths et al.; all text material 2014 by Steven M. Carr