Why Flowering Plants Are So Diverse

Both flowing plants and animal pollinators are incredibly diverse. One reason for this diversity is the close relationship they have with one another. Over many years, some flowers and pollinators have influenced each other's evolution. Today, when flower-pollinator pairs have traits that work very well together, it's a sign this kind of evolution may have happened.

butterfly on butterfly bush

Flower-pollinator pairs often have sets of traits that favor one another. A well-matched pair tells us the two species may have influenced each other’s traits over time.

Co-Evolution of Flowering Plants and Pollinators

Flowers and pollinators gradually adapt to one another over many generations.

When the traits of one species evolve in response to the traits of another species, it's called co-evolution.

Flowers and their pollinators affect one another's reproductive success. It's what makes them likely to co-evolve. The traits flowers have determine the traits that will give pollinators an advatage, and vice versa. Over many generations, favorable trait combinations become more common. Eventually, flower-pollinator pairs are very good at helping one another reproduce.

Co-evolution occurs over time, when several conditions are in place.

  • Traits must vary in a population.
  • Traits need to be heritable, so they can be passed from parents to offspring.
  • Some version of the trait must give a reproductive advantage.
  • Finally, two (or more) species must closely interact. This is what make co-evolution different from other kinds of natural selection.

Co-evolution is often due to a mutualistic relationship, like that between flowers and pollinators. But it can happen from other interactions as well. Other examples of co-evolution occur between predators and prey, parasites and hosts, and even viruses and the organisms they infect.

Explore the many traits that cause flowers and pollinators to favor one another in the Flower Traits Slideshow.

For more on variability, heritability, and reproductive advantage, visit the video Recipe for Natural Selection.

Is Co-Evolution Real?

graph of fly proboscis length

Average fly proboscis (mouthpart) length and flower depth. Each dot is a different location. Modified from Anderson and Johnson 2008.

Co-evolution is occurring right now. But the process is too slow for us to watch. How do we know it actually happens? If two species have corresponding traits, it's a good starting point. But to truly demonstrate co-evolution, researchers need more evidence.

As one example, long-tongued flies, Prosoeca ganglbaueri, and figwort flowers, Zaluzianskya microsiphon, live together in several places in South Africa. The flies are the flowers' main pollinator. They feed from the flowers' deep nectar tubes. Researchers studied the relationship of fly mouthparts and flower nectar tubes. They measured the length of each trait at distinct geographical locations. From one location to another, the mouthpart and nectar tube lengths vary. But, within a single location, the two traits are well-matched. This was evidence that flies and flowers that live together influence each other's traits.

The researchers also showed there is an advantage to matching traits. They mixed flies and flowers from different locations. Some fly tongues were too long and they fed from flowers without pollinating them. As a result, those flowers made fewer seeds. Those plants were less likely to reproduce since the traits didn't match.

Pollinator Shifts

Not all corresponding traits between flowers and pollinators are due to co-evolution. Pollinator shifts also have a big influence on flower traits.

A pollinator shift is when a different pollinator becomes the main visitor to a flower species. It occurs when a new pollinator moves into the flower's neighborhood. Or if a new flower moves into the pollinator's neighborhood. It also happens when a flower species has a new trait by chance, which makes it attractive to a different type of pollinator.

Visitation by the new pollinator causes the flower population to adapt. In the short-term, pollinator shifts are often a bit more one-sided compared to co-evolution. Traits in the flower population quickly change to ensure pollination from the new pollinator. But over a longer time, the two species will influence each other.

The frequency of traits in a flower population can change after a new pollinator moves to the neighborhood.

Flower-Pollinator Interactions Lead to New Species

monkeyflowers and pollinators

Pollinators influenced divergence of these monkeyflower species. Each species is so specialized to a specific pollination system that they are reproductively isolated and rarely cross in nature.

Over time, both co-evolution and pollinator shifts lead to reproductive isolation. The part of the population with new traits no longer interbreeds with the original population. As more time passes, the newly-isolated population continues to change. Eventually, the original species diverges into two.

Related Content

What kinds of reproductive barriers lead to speciation? And, when do the two groups become separate species? Find out in Reproductive Barriers and What is a Species?

Enhanced Biodiversity

Over millions of years, the interactions of flowers and pollinators led to much of the biodiversity we see today. Traits that might otherwise seem quite mysterious can often be explained by these relationships.

various flowers

Flower-pollinator relationship increase biodiversity. The relationships are likely responsible for many of the unique floral traits shown above.



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Ramsey, J., Bradshaw Jr, H. D., & Schemske, D. W. (2003). Components of reproductive isolation between the monkey flowers Mimul lewisii and M. cardinalis (Phrymaceae). Evolution, 57(7), 1520-1534.

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Van der Niet, T., Peakall, R., & Johnson, S. D. (2014). Pollinator-driven ecological Speciation in plants: new evidence and future perspectives. Annals of Botany, 113(2), 199-212.

Whittall, J. B., & Hodges, S. A. (2007). Pollinator shifts drive increasingly long nectar spurs in columbine flowers. Nature, 44(7145), 706.

Photo Credits

Columbine photo by Elena Kramer.