The bolt of natural selection


“Yet mark’d I where the bolt of Cupid fell:
It fell upon a little western flower,
Before milk-white, now purple with love’s wound”

Hepatica unfurls its advertisements in both white and purple. This is true of North American species and those in Asia. Why the variation? Natural selection or genetic drift eliminates variation in a population unless some force keeps these purgative pressures at bay. In the case of Hepatica, the costs and benefits of white and purple flowers have not been measured in the wild (readers, correct me if I’m wrong here, please!).  Midwestern populations are more purple than their eastern cousins.

Could pollinators or the light environment be creating different selection pressures? Many populations contain a mix of colors (white is the most common here in Sewanee, but purples are scattered among them in significant numbers), so selection does not seem to favor just one color in each location.

In spring beauty flowers (Claytonia), flowers come in red, white, and intermediate. Natural selection acts on flower color in this species complex ways. Redder flowers are more attractive to pollinators, but whiter flowers have more flavonols, chemicals that act as herbivore deterrents. So, white flowers receive fewer pollinators but red flowers get more damage from slugs. Damage from herbivory changes from year to year and from place to place. So two opposing forces — pollination and herbivory — vary in space and time, resulting in a mix of flower colors. Perhaps a similar turbulent, Puckish convergence of forces also acts on Hepatica?

8 thoughts on “The bolt of natural selection

  1. Michael Asbell

    This is fascinating. For some time now I’ve been wondering why all the earliest wildflowers are white? Harbinger-of-Spring, Spring Beauty, Toothwort, Bloodroot, Hepatica — they’re all white, or mostly so. In my experience, Yellow Trout Lily, Celandine Poppy, and Violets, though early, usually bloom at least a week or more after the white flowered plants are in full swing. Now I’m wondering if it could be the increased flavonols you mentioned acting as a deterrent against the hungry herbivores of early spring. Fascinating stuff.

    1. David George Haskell Post author

      Yes, early ones almost all white. Light environment is important, but herbivory is also v important. For almost all species, we have no thorough study of costs and benefits. An area for future study!

  2. Joanne Kornoelje

    I’m wondering about non- wildflowers -like my favorite announcer of spring, the crocus. White and purple dominate in color. Can we apply the same concepts?

      1. Joanne Kornoelje

        Makes me wonder if the plants are making us give us what they want. If that makes any sense. Plants are controlling us… ?

  3. mickiebond

    Here in the Sonoran Desert the predominant flower color is YELLOW….. right now our desert screams yellow all around…… this is the same whether wind or insect pollinated. Clues?



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