Field pansies in the area around Paris are evolving to have smaller flowers and produce less nectar – physical traits that are tied to pollination – according to new research from a team at the University of Montpellier in France.
Although the researchers cannot definitively say what caused the pansies’ traits to evolve in this direction, dramatic declines in pollinator numbers — due to a combination of threats like habitat loss, climate change and pesticide use — are a well-documented global phenomenon, including in Western Europe.
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The study compared plants grown from older, dormant seeds to their modern descendants. Their findings shed light on the pansies’ relationship to pollinators and, potentially, the health of their local ecosystem.
“The evolution that we see shows smaller flowers, shows flowers that are less attractive [to pollinators] and that produce less nectar,” said Samson Acoca-Pidolle, a PhD student at the University of Montpellier and lead author of the paper. “And we see no changes in other traits, like the size of the leaves.”
These pansies can reproduce in two ways. First, with the help of pollinators like bumblebees, who dutifully distribute pollen from one flower to the next as they gather nectar. But they are also capable of self-fertilizing, also known as selfing, without the use of genetic material from a different plant. The population observed in the study has pursued selfing as a means of reproduction more frequently over the past few decades.
The absence of pollinators has been shown in previous studies to be a strong selective pressure — an environmental reality that nudges a species in a particular evolutionary direction.
Acoca-Pidolle said it’s possible that more selfing among this population reflects an increasingly precarious relationship between pollinators and these plants. Producing big flowers to attract insects and nectar for them to feed on is only worth the effort when they show up, he noted. When they don’t, it’s a waste of valuable energy.
“It’s like preparing yourself for a date, and nobody is coming,” Acoca-Pidolle said.
What the study found
To explore differences in the physical traits between these plants across generations, Acoca-Pidolle’s team grew four sets of field pansies from seeds that were plucked from the wild between 1990 and the early 2000s, plus another four sets from the seeds of their modern offspring. They also brought in pollinators to see if and how they engaged with the pansies.
By using wild seeds, their research offered some insight into the local conditions that influenced those plants over time.
The researchers found that bumblebees frequented the ancestral plants more often than the descendent ones with less pollinator-friendly qualities.
Nectar production also declined in the descendent population. Nectar serves as a kind of reward for pollinators in exchange for cross-fertilization, he explained, and plants have no other incentive to spend the energy needed to generate it.
Although it’s impossible to prove that a drop in pollinator numbers is responsible for the changes observed among these particular field pansies, their evolutionary arc matches those that have been proven among other plants in experimental settings, Acoca-Pidolle said.
What happens when insect populations decline?
Organisms evolve in response to the conditions and limitations of their environment. Nora Mitchell, an assistant professor of biology at the University of Wisconsin-Eau Claire who wasn’t involved with the study, describes the process as a kind of pendulum that can swing in different directions, noting that “evolution is always context dependent.”
Selfing is often considered a kind of evolutionary dead end because without sexual reproduction, there’s a dearth of genetic variation for selection to act on, she added. At the same time, though, it’s always better to produce offspring than not, no matter how that goal is achieved.
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“When you think about the evolution of any trait — including reproductive strategies — there is no perfect solution, right?” said Anahí Espíndola, an assistant professor in the department of entomology at the University of Maryland who also wasn’t involved with the research. “There is no end point in evolution because each solution or each strategy or each trait that evolves may be good now, but it may not be good tomorrow, so it will be changing all the time.”
But changes in one species can cause ripple effects across ecosystems, and it’s important to consider that interconnectedness when we think about the consequences of climate change and other human-driven threats, Mitchell said.
Acoca-Pidolle pointed out that plants are the base of the food chain. When plants produce less nectar, pollinators may have a harder time feeding themselves. And when pollinator numbers decline, he said, the animals that feed on them have fewer options, too.
Mitchell also noted that Acoca-Pidolle’s research is a good example of what can be missed when looking at the world with a human-centric point of view. We might not notice field pansies getting slightly smaller or producing less nectar over time, but pollinators surely would.
“The things that may not seem important from a human’s perspective are important to other organisms, and it’s important to think about those as well,” Mitchell said. “Because having a human’s eye view and having a bee’s eye view are different things.”
