Beach daisy underwent rapid evolution

A species of South African beach daisy has undergone substantial and unexpected changes since being introduced into Australia in the 1930s, a new study has found.

It is, the Australian and Danish researchers say, a “clear and unusual” example of rapid evolution in plants, and one which offers a little hope that some plant species may be able to adapt to the pressures of climate change.

But there is a downside, as well.

“This plant has massively changed in less than a century,” says co-author Angela Moles, from the University of New South Wales (UNSW) in Australia. 

“This means they are evolving to get better at growing in Australia, which means they may get more invasive as time goes on.”

The researchers undertook some dogged detective work to get their findings, which are published in the journal Proceedings of the Royal Society B.

In an earlier study, they had gathered seeds of the beach daisy (Arctotheca populifolia) from South Africa and Australia and generated genetic maps of each population by locating shared microsatellites – short sequences of DNA which are repeated in multiple locations in the genome of an organism.

Building on this work, they used genetics to figure out exactly which beach in South Africa the current Australian plants came from, drawing a direct genetic link between the original and introduced populations.

“It’s a key step that we got the parent population because that is extremely rare and enables us to make the most precise evolutionary comparisons,” says lead author Claire Brandenburger, also from UNSW.

After establishing the genetic link, the researchers grew the plants over two generations to observe how the South African ones would grow in comparison to the Australian ones, all in controlled conditions in a glasshouse at UNSW.

They found that the morphology of the Australian population in Australia is now strikingly different to that of the parent population.

In particular, while the leaves of South African plants develop distinctive lobes as they reach adulthood, those on the Australian plants retain the simple broad, rounded shape of youth. This is a phenomenon known as paedomorphosis – meaning “keeping its child-like form”.

“If a naturalist had seen these two plants 200 years ago, they would have probably described them as different species because they look so distinctive,” Brandenburger says.

“We’re not sure whether this is an adaptive change – that is, whether the shape changed to adapt to the landscape – or genetic drift, where by chance the gene that says ‘become a lobed adult plant’ might have been switched off in Australian plants.”

Surprisingly, the plants develop smaller, thicker leaves and a low, sprawling growth form in Australia. Usually, introduced plants follow what is known as the Evolution of Increased Competitive Ability (EICA) hypothesis.

“The EICA hypothesis offers an explanation for how introduced species can suddenly seem to be doing so well,” Brandenburger says. 

“When plants leave their original home, they leave a lot of enemies behind. When they arrive in a new environment, instead of wasting energy defending themselves, they now have extra resources which they can put into growth, reproduction and competition – usually resulting in bigger leaves and taller plants.”

However, the Australian beach daisy population was found to be more conservative, with a low and spreading growth form and smaller, thicker leaves similar to succulents – consistent with adaptation to a drier climate.

This is despite rainfall being two to three times higher in the Australian study sites, compared to the South African one.

The authors say the study is a starting point and they need to go further to see if the change in appearances is associated with any evolutionary advantages.

“We need more research to find out why this is happening,” notes Moles. “More evidence is coming out showing that when there is a selective pressure in place, plants and animals can change really, really fast.”

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