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Flower Evolution on Display in the Backyard

 Humans find flowers beautiful and even inspirational, yet the natural evolution of flowers is driven only by the success of plant sexual reproduction.  Except for some groups such as grasses and pines which are wind pollinated, flowers have specialized to attract animal pollinators such as birds, bats and insects.  But flowers are so much more than simply bright colors or bizarre shapes. They illustrate the marvelous variation that evolution has produced to accomplish the goal of plants to maximize the chances of passing on their genes to their progeny.  Thus when you look at a flower, ask yourself why it looks this way.  When contemplating the enormous variation in flowers, I am often struck by the trend for more specialized forms to group flowers together.  A quick look around my yard provides some examples of how evolution may have acted to select one of the most advanced flower types as illustrated by the aster family (the composites).  

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Consider the common daisy which is so familiar, yet how many people realize that one individual "flower" is in fact made up of many individual flowers of two different types- the ray and disc flowers.  Each petal is actually a sterile ray flower and the central disc is made up of numerous small fertile flowers.  You might indeed wonder how such an amazing thing could possibly have developed.  The insect shown on the daisy photo is also a marvel of evolution- it is a day-flying moth that mimics a wasp to minimize predation.

 

If you could go back in time and look at a simple flower of the most primitive form, it might be similar to a single large flower such as the tulip poplar, a type of magnolia.  Each flower attracts pollinators by  offering a reward of nectar and/or pollen in exchange for cross fertilizing with other flowers.  Magnolias tend to be pollinated by beetles since there were no bees around when they originated. But some birds are quite attracted to their nectar.  For such a primitive plant, the tulip tree is doing very well since it is widely distributed from southern Canada to central Florida, is common in forests, and is both an ecologically and economically important timber tree.

 

Yet many plants have grouped small flowers into compact clusters such as the red twig dogwood, in which all of the flowers are equal in size. In the competition for pollinators among plants, this type of inflorescence might be at a disadvantage in comparison with more showy, single flowers.  Yet it could be much more efficient at production of seeds.  One way to provide better advertising is shown by some viburnums which have an outer ring of large sterile showy flowers surrounding the inner group of small fertile flowers.  We grow a large viburnum shrub called Mariesii which is resistant to deer damage, has showy spring flower clusters attractive to insects, produces berries attractive to birds, and is often used as a site for bird nests.  Can you see how the transition from large single flowers  (magnolia), to groups of small equal-sized flowers (dogwood), to a ring of large sterile showy flowers around a core of small fertile flowers (Viburnum plicatum Mariesii) illustrates how asters might have evolved?  

 

Isn't it wonderful how nature can be appreciated on many levels?  A flower of any type is a marvel of bio-engineering and we can be in awe of the colors, scents, and bizarre forms presented to the eye. But isn't it even more amazing if you look for patterns that reveal how evolution has operated to generate such astounding variety in the natural world?

 

Bill Dunson 

Englewood, FL, and Galax, VA 
wdunson@comcast.net 

http://www.galaxgazette.com/blogs 
http://lemonbayconservancy.org/dunson_archives.htm 
http://lemonbayconservancy.org/wildflower.htm