Cacti (Cactaceae family) are a remarkably diverse group of plants.  Some are a few centimetres in size.  Moreover, others have tree-like habits such as the giant Pachycereus pringlei otherwise known as the elephant cactus.  This cactus can reach heights in excess of fifteen metres.  Their strange shapes and multiple spines might make you think that these angiosperms are truly ancient, perhaps amongst the first flowering plants to evolve.  However, the origins of the Cactaceae family might be relatively recent.  A new study by the Milner Centre for Evolution at the University of Bath has shed new light on cacti evolution and the mystery as to their remarkable diversity.

Cacti Evolution and Diversity

It had been thought that the dryness of the climate was the main driver in cacti evolution.  This new research published in “Nature Communications” challenges this view.  The study found that diurnal temperature range, sand content in the soil and seasonal changes are the biggest drivers of cacti evolution.

Cacti are popular house plants.  However, whilst they have a global distribution today, being found in homes all over the world, they are in fact native to the Americas.  The highest number of species are to be found in Mexico.

In a bid to better understand the diversity of the Cactaceae family and their evolution, researchers built the largest evolutionary tree for cacti.  The data tree was compiled using detailed DNA sequences.  Numerous biological variables were considered, including soil sand composition, plant size, geographical range, aridity and diurnal temperature range.  The diurnal temperature range measures the difference between the maximum and the minimum temperatures experienced over a day.  This database was then correlated using data on biodiversity and the perceived rates of the evolution of new species.

Cacti evolution outlined in a new scientific study. Picture credit: Everything Dinosaur.

Picture credit: Everything Dinosaur

 Machine Learning

The team used sophisticated machine learning techniques to model the interactions between several variables simultaneously.  Results were then validated using traditional evolutionary methods. Surprisingly, the team found that whilst the highest number of species was found in Mexico, the country also had the lowest evolution rates of new species (speciation).

Aridity was previously assumed to be the main driving factor of their evolution.  However, the new study points to the diurnal temperature range, sand content in the soil and seasonality as being more important drivers of cacti evolution.

Commenting on the results, first author Dr Jamie Thompson stated:

“Cacti are a really interesting, diverse family of plants that have evolved relatively recently in evolutionary time to live in a wide range of environments across the American continents. People think they are very hardy because they are able to live in such extreme climates, but in fact they are more at risk of extinction than other types of plants.”

The doctor carried out the research whilst at the Milner Centre for Evolution (University of Bath), however he has moved on to become a lecturer at the University of Reading.

He added:

“Because there is the highest biodiversity in Mexico, it was assumed this was because the conditions there were best for evolution of species. However, our study shows this isn’t the case – instead Mexico has the slowest rate of speciation but has lots of biodiversity because the extinction rates are slower. In other words, the Mexican climate is not necessarily better at producing new species but good at preserving existing ones.”

A small cactus. Around 1,750 species have been described. Picture credit: Everything Dinosaur.

Picture credit: Everything Dinosaur

When Did Cacti Evolve?

Team members are not aware of any definitive Cactaceae fossils having been found. It is difficult to establish their evolutionary origins. However, the geographical distribution of cacti can offer a clue. Wild cacti are found in South America and the southern parts of North America. This suggest that the cacti family evolved after the fragmentation of Gondwana and the splitting off of South America from the rest of the landmass. This suggest that the ancestors of today’s cacti evolved in the Early Cretaceous.

In contrast, molecular studies suggest that the Cactaceae are a more recent lineage of angiosperms. These studies suggest that the first cacti evolved during the late Eocene or early Oligocene Epochs. During this time, it is thought that the world became drier. The increased aridity led to a decline in forest habitats, and this may have helped fuel the evolution of plants adapted for water conservation.

Commenting on the research, co-author Dr Nick Priest (University of Bath) explained:

“Our work shows that cacti are not as successful as we thought, and are more vulnerable to climate change. This is concerning – if cacti can’t thrive in our changing climate, what chance do other plants have?”

The research team included scientists from the University of Reading, the Desert Botanical Garden (Phoenix, Arizona) and the Tecnológico Nacional de México (Mexico).

For a recent article highlighting the discovery of nine new species of ancient grapes: Nine New Species of Fossil Grape Identified in Study.

Everything Dinosaur acknowledges the assistance of a media release from the University of Bath in the compilation of this article.

The scientific paper: “Identifying the multiple drivers of cactus diversification” by Jamie B. Thompson, Tania Hernández-Hernández, Georgia Keeling, Marilyn Vásquez-Cruz and Nicholas K. Priest published in Nature Communications.

The Everything Dinosaur website: Prehistoric Animal Toys.