The birds and the bees
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.
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.
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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
Understanding Evolution © 2021 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California