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The birds and the bees

some flowers are pollinated by bees
Since plants can't run off to look for a mate and reproduce, many have evolved elaborate mechanisms of pollination — often cheating or bribing animal pollinators into doing the work for them. The focal point of these efforts is the flower. Flowers attract pollinators by tempting them with the promise of a reward (usually nectar) and then use the opportunity to distribute or gather pollen (essentially plant sperm) via the pollinator.

However, different pollinators look for different things in a flower. Hummingbirds, for example, like bright red tubular flowers. Bees like flowers with a landing platform and a UV nectar guide (floral markings which reflect light that bees — but not humans — can see and which guide the bees to the nectar source). Flies like dark flowers that smell like rotting meat. These sets of characteristics that attract different pollinators are called 'pollination syndromes.'

some flowers are pollinated by birds some flowers are pollinated by bees some flowers are pollinated by flies
The flower on the left, a Heliconia, is pollinated by hummingbirds; the middle flower, northern catalpa, is pollinated by bees; and the flower on the right, California Dutchman's pipe, is pollinated by flies.

The preferences of different pollinators can cause flower form to evolve. Over many, many generations, natural selection may shape a flower into a form preferred by a particular pollinator. It works like this. Imagine two different-looking plant species that are both pollinated by the same species of bird. The bird happens to be most attracted to red, tubular flowers. Any individuals of the two species with flowers that happen to be a bit redder or have a slightly more tubular shape are more likely to be pollinated and to reproduce. The offspring inherit the general flower characteristics of the parents. Then the process repeats itself — any of those offspring that happen to have flowers that are even redder and even more tubular will have an advantage in terms of pollination. Over time, the two species will tend to evolve similarly shaped and colored flowers, even if their flowers looked very different at the beginning of the process.

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Flower characteristics can evolve relatively easily, first of all, because the selection pressure to be pollinated is strong (no pollination means no offspring!) and secondly because there is a lot of variation in flower form from which pollinators can choose. Because they change so much, characters that evolve easily are not good indicators of evolutionary relationships. Bright red, tubular flowers in two different species may not mean that the species are closely related. The two species might be distantly related but have evolved the same flower form because they are both pollinated by an animal attracted to bright red tubular flowers. This is an example of convergent evolution.

a dung beetle on a flower
A dung beetle attracted to a smelly flower.
Some ginger species have evolved to take advantage of a pollinator much less attractive than hummingbirds or butterflies: the dung beetle. Dung beetles eat, well...dung, which they collect, roll up into a ball, and store. However, that dung ball is at risk of being stolen by another dung beetle. One ginger lineage, the Lowiaceae, subverts this strange system for its own pollination purposes by producing a dark flower that smells strongly of processed dung. Thieving beetles go from flower to flower in search of ill-gotten goods — and in the process, pollinate the deceptive plants.



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Seeing past the petals

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Practicing parsimony


Photo of bee on flower provided by Joyce Gross; Heliconia flower (the one pollinated by birds) photo provided by Chelsea Specht; Catalpa flower (the one pollinated by bees) photo provided by Louis-M. Landry; Aristolochia flower (the one pollinated by flies) photo by Charles Webber © California Academy of Sciences; Dung beetle photo by Shoko Sakai, Kyoto University and © Botanical Society of America

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