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Daisy Chains Unshackle Evolution’s Speed Limit

The beach daisy growing on the east coast of Australia.

The beach daisy growing on the east coast of Australia. Credit: Claire Brandenburger

By Claire Brandenburger & Angela Moles

The rapid evolution of an introduced beach daisy reveals how much we underestimate the ability of plants to adapt to climate change.

When Darwin first described evolution, it was thought to occur on timescales ranging from thousands to millions of years. But in the past 50 years it has been shown that evolution can occur much faster than that in a wide variety of plants and animals.

Many of these studies have been based on introduced species, which offer an excellent recipe for rapid evolution because they encounter different environmental conditions and interact with a whole new suite of plants, microbes and animals. Many of these introduced species thrive in their new ranges and become unwanted weeds, causing environmental and economic problems.

These weeds are also very interesting. The introduction of thousands of species to new environments has provided a giant experiment that has helped us learn a lot about evolution, ecology, genetics and more. My research has revealed what weeds can teach us about rapid evolution.

Is Rapid Evolution Common?

To answer this question, in 2011 our group used collections of dried, pressed plants as a kind of time machine to ask: “Is rapid evolution common in introduced plant species?” (http://tinyurl.com/rapid-evol). The study looked at plants that had been introduced to NSW over the past 150 years and measured how the plants had changed over time. By tracking the changes in these dried plants, the study found that rapid evolution is common in introduced plants. More than 70% of the introduced plants in the study showed substantial changes in things like plant height or leaf size since arriving in Australia only 150 years ago!

The only problem is, by just considering dried plants it’s hard to say whether the changes were due to evolution. The plants could have been changing in response to an environmental gradient, climate change over time or changes in local land use. The only way to find out for sure would be to study living plants.

This is where our research comes in.

The Beach Daisy Experiment

The South African beach daisy (Arctotheca populifolia) arrived in Australia in the 1930s. Looking at dried collections you can see that the beach daisy has become longer over time since arriving in Australia. Our prediction is that if we actually grow plants from South Africa and Australia together under the same conditions (called a common-garden experiment) then if the plants are undergoing rapid evolution then we would expect to see these differences in plant length – and other traits too.

In previous common-garden experiments using introduced plants, people grew plants from across the home range side-by-side with plants from the new range. However, sampling across the whole home range brings a large amount of unnecessary information into the experiment that could make it harder to detect if evolution is occurring.

Instead, we analysed small sections of repeated DNA (called microsatellites) from plants in both ranges to locate the actual parent population of the introduced daisy. This powerful genetic approach gave us the extra precision we needed to help us detect whether evolution is actually occurring in the beach daisy.

For our experiment we collected seeds from the parent population in Arniston, South Africa, and from four introduced populations on the east coast of Australia, ranging from Myall Lakes to the Victorian border. To make sure that all of the seeds we used for our experiments came from equally cared-for mother plants, we first germinated our beach-collected seeds and grew the plants through one generation. We then used the next generation of seeds to plant more than 400 experimental plants in the glasshouse at UNSW Sydney.


The South African plant (left) and Australian plant (right), showing some of the major evolutionary changes. Credit: Claire Brandenburger

What Evolutionary Changes Did We Expect to See?

First, we considered some of the environmental differences between where the plants grow in South African and Australia. In both countries the study populations are found at similar latitudes and experience similar temperatures, but rainfall in the Australian range is two to three times higher than it is where the South African parent population grows. We therefore predicted that the Australian plants would be bigger than the South African plants and have leaves that were larger, thinner and less dense.

Second, we considered some of the biological differences between the two ranges. It has been suggested that when plants are introduced to a new range they can escape their specialist herbivores – a process called enemy release. This allows introduced plants to use less of their resources to defend themselves and put more resources into growth and reproduction – which can make them bigger and better competitors. If the beach daisy did escape from its enemies when it arrived in Australia all those years ago, then we would once again predict the evolution of bigger plants with leaves that were larger, thinner and less dense.

Rapid Reshaping

Our most surprising finding was the striking change in leaf shape that has occurred in the Australian plants. Although both sets of plants start with young leaves that are spoon-shaped, the South African plants then develop wavy adult leaves with lobes around the edges. Remarkably, the Australian plants have totally lost the wavy adult form and just keep the spoon-shaped leaves throughout their lives.

The process by which a plant or animal keeps a young characteristic into adulthood is called paedomorphosis. A well-known example of paedomorphosis is the axolotl – a salamander that keeps its gills as an adult, unlike other salamander species that only have gills when they are young.

We also discovered that the leaves of the Australian plants have evolved to become smaller, thicker and less dense since they were first introduced. These changes could have evolved to help the plant store water in its leaves, or the changes might be a response to windier conditions or less nutrients in Australia. Working out why the leaves have changed so much since arriving in Australia is now one of the directions of our future research.

In addition, we found that the Australian plants have evolved a sprawling body form compared with the South African plants, which stand up straighter. The Australian plants are also longer and have shifted their body weight above ground. This suggests that they have cheaper construction costs and a faster growth rate – strategies that may have evolved to help them survive increased burial by beach sand.

It’s remarkable that so many changes have evolved in this plant in less than 90 years since arriving in Australia. However, we made a total of 24 predictions about how the beach daisy could have changed since arriving from South Africa in the 1930s, and only five of these were correct. This shows that rapid evolution happens, but not always in the direction that we expect.

Why Should We Care?

The good news is that if plants can evolve more rapidly than we appreciated, then native plants might be better able to adapt to future scenarios under climate change. In general, it is predicted that less mobile species (like plants) will suffer the effects of climate change more than mobile species (like animals) that could find favourable areas to inhabit. But with species like the beach daisy revealing the potential for rapid evolution, we hope that at least some species of plants will be able to evolve and adapt in their current locations.

The bad news is that if introduced plants are evolving so rapidly then they could become even more successful and more invasive as time goes on. Already more than 13,000 plant species have become naturalised in new areas all over the world. The better they become at fitting into their new environments, the harder they will be for us to manage.

Rapid Evolution of Introduced Species

The South African beach daisy is one of the many species that form part of the giant experiment that we have unintentionally set up by moving plants and animals all over the world. It’s an unwanted weed that would not be here if humans hadn’t interfered. It’s growing and spreading along our coast, affecting our native ecosystems.

But what if our research shows that this introduced plant is evolving so many changes so rapidly that instead of being an unwanted weed it may actually be on its way to becoming a unique new species: the Australian beach daisy! Would weed managers pack away the poison and put up a national park instead? Would the public change its mind about introduced species? Would you?


Claire Brandenburger is a PhD candidate with Professor Angela Moles in the Evolution & Ecology Research Centre at UNSW Sydney. The research described here has been published in Proceedings of the Royal Society B (http://tinyurl.com/daisy-evolution).