Evolution News - Rapid Evolution in Foxgloves

Common foxglove, Digitalis purpurea, pollinated by bumblebees

Study finds rapid evolution in foxgloves pollinated by hummingbirds - British Ecological Society

Researchers from the University of Sussex, Universidad de Los Andes (Colombia) and Universidad de Costa Rica have shown how a change of environment and the presence of new pollinator species can induce rapid evolutionary change in flowers.

The common foxglove, Digitalis purpurea, is a native plant of Europe where it is normally pollinated by bumblebees. The markings on the inside of the corolla tube and the length of the tube are adaptations to facilitate these pollinators. However, in Costa Rica and Columbia, there were two independent introduction events about 200 years ago. These populations are pollinated by different bumblebee species and also hummingbirds, which are absent from Europe.

The researchers have shown that in these introduced populations, the corolla tube is significantly longer; a clear adaptation to new pollinator species. The British Ecological Society News release, explains:

The researchers compared foxgloves in the UK, which are pollinated by bumblebees, with foxgloves introduced in two independent events to Costa Rica and Colombia around 200 years ago, which are pollinated by different species of bumblebees and also hummingbirds. They found the base of the cone structure of the flowers, called the proximal corolla tube, was 13-26% larger in populations in the Americas.

We found foxglove populations in Costa Rica and Colombia now have flowers with longer tubes at the base, when compared to native populations. There is also substantial natural selection on this floral characteristic in the naturalised populations.

Long corollas are a common feature in many hummingbird-pollinated plants, likely because this improves the precision of pollen transfer during the pollination interaction. It is also possible that long corolla tubes exclude other pollinators that are less effective.

We counted pollen grains deposited in flowers and found that after a single visit [hummingbirds] can bring in more pollen than a bumblebee.

Our research shows how rapid evolutionary change in a new environment can be an important force behind the extraordinary diversity of flowers.

Dr Maria Clara Castellanos, Co-author
School of Life Sciences,
University of Sussex
Brighton, Susses, UK

Foxgloves have long, narrow proximal corolla tubes. This part of the flower holds the nectar and by being this shape, they restrict floral visitors to those with long mouthparts such as long-tongued bumblebees.

“We found foxglove populations in Costa Rica and Colombia now have flowers with longer tubes at the base, when compared to native populations. There is also substantial natural selection on this floral characteristic in the naturalised populations.” said Dr Maria Clara Castellanos at the University of Sussex and one of the authors of the study.

“Long corollas are a common feature in many hummingbird-pollinated plants, likely because this improves the precision of pollen transfer during the pollination interaction. It is also possible that long corolla tubes exclude other pollinators that are less effective.”

Because foxgloves are biennial (meaning each generation takes two years) these changes have occurred in around 85 generations, indicating a rapid evolutionary change.

In the study the researchers also confirmed that hummingbirds are effective foxglove pollinators. “We counted pollen grains deposited in flowers and found that after a single visit they can bring in more pollen than a bumblebee.” said Dr Castellanos.

The study also confirms how invasions can be used to understand evolution of floral structures.

Longitudinal section of Digitalis purpurea flower with part of the corolla and one stamen removed. The floral nectaries are located at the base of the ovary, within the constricted proximal part of the corolla tube

Source: Makin, C.R., et all (2021)

The researchers say that scenarios like this are likely to happen often as humans influence the range of plants and pollinators.

Foxgloves are now naturalised in many areas of the world. They were introduced to Colombia and Costa Rica in the 19th Century, most likely by English architects and engineers. In these new tropical environments, foxgloves grow at high altitudes above 2,200 meters where temperatures are broadly similar to those in their native European range. Because there are no seasons, populations flower at different times of the year.

In the study, the researchers looked at both native UK foxglove populations and populations in mountainous areas in Colombia and Costa Rica. They compared the shape of the flowers and the reproductive success of the plants. They also recorded the pollinators in each location and how effective each pollinator was at transferring pollen.

Although this correlation doesn't prove causation, the finding is consistent with changes caused by natural selection imposed by hummingbirds. To investigate this further, the team now plan to carry out controlled experiments in which selective pollinators can be excludes. They also plan to investigate genomic differences between the American populations and European native populations.

The team's research was published, open access, in the Journal of Ecology, two days ago:

Abstract

1. Changes in the pollinator assemblage visiting a plant can have consequences for reproductive success and floral evolution. We studied a recent plant trans‐continental range expansion to test whether the acquisition of new pollinator functional groups can lead to rapid adaptive evolution of flowers.
2. In Digitalis purpurea, we compared flower visitors, floral traits and natural selection between native European populations and those in two Neotropical regions, naturalised after independent introductions. Bumblebees are the main pollinators in native populations while both bumblebees and hummingbirds are important visitors in the new range. We confirmed that the birds are effective pollinators and deposit more pollen grains on stigmas than bumblebees.
3. We found convergent changes in the two new regions towards larger proximal corolla tubes, a floral trait that restricts access to nectar to visitors with long mouthparts. There was a strong positive linear selection for this trait in the introduced populations, particularly on the length of the proximal corolla tube, consistent with the addition of hummingbirds as pollinators.
4. Synthesis. The addition of new pollinators is likely to happen often as humans influence the ranges of plants and pollinators but it is also a common feature in the long‐term evolution of the angiosperms. We show how novel selection followed by very rapid evolutionary change can be an important force behind the extraordinary diversity of flowers.

Mackin, CR, Peña, JF, Blanco, MA, Balfour, NJ, Castellanos, MC.
Rapid evolution of a floral trait following acquisition of novel pollinators.
J Ecol. 2021; 00: 1– 13. doi: 10.1111/1365‐2745.13636

Copyright: © 2021 The authors. published by John Wiley & Sons Ltd on behalf of British Ecological Society.
Open access
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

As the risk of becoming repetitive, I invite Creationists to explain why this finding should be dismissed as an example of rapid evolution in as few as 85 generations, how accommodation of a new, more efficient, pollinator species should be regarded as 'devolution' (© 2019 Michael J. Behe/Discovery Institute) and/or how it shows evidence that the TOE is a theory in crisis, about to be overthrown by the magic-based notion of Intelligent [sic] Design.

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