This question of why some branches of the tree of life explode into thousands of species, while others remain small, has shaped evolutionary biology since Charles Darwin.
My colleague and I have published a new study of cactus flowers which may help explain the conundrum.
For more than a century, scientists have seen flowers that are specialised to a particular pollinator or environment as drivers of the evolution of new diversity. Our new research challenges that idea, which could change how scientists think about the forces that create biodiversity across the plant world.
The cactus family, exceptionally diverse and among the most threatened plant groups worldwide, offers a striking example of how some evolutionary lineages thrive while others struggle.
Cacti are icons of slow growth. A towering saguaro may take a decade to reach an inch tall and the psychedelic peyote takes decades to mature. Yet the cactus family is one of the fastest-evolving plant groups on Earth. Over the past 20 to 35 million years, around 1,850 cacti species have come into existence. Although this sounds slow, in geological time it is the blink of an eye. By comparison, about a quarter of the 415 other flowering plant families have five or fewer species. These plant families never branched rapidly like cacti did.
Deserts are often imagined as unchanging and unforgiving landscapes, yet they can be arenas of rapid evolutionary innovation.
Scientists have linked the large number of cactus species with pollinator specialisation, where cactus flowers adapt to particular pollinators, such as bees, moths or hummingbirds. Another idea attributes the evolutionary success of cacti to the expansion of deserts over the last 30 million years, as much of the Americas became drier and more open.
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Cacti seemed to fit this idea perfectly. Their flowers vary from small, understated blooms to large, night-opening blossoms. Some are pollinated by bees, others by hummingbirds, moths or bats.
Cactus flowers are fleeting and beautiful, often lasting only days, and are eagerly anticipated by devoted “plant parents”. Shorter flowers are typically linked to bee pollination, while longer, tubular forms have evolved repeatedly for bats, hummingbirds and moths.
Morgan Newnham/Unsplash
However, my 2024 study which sampled many more species than previous studies, found that neither aridity nor pollination – the two main hypotheses for cactus diversity – was a strong explanation. This challenged a long-standing idea dating back to Darwin, who suggested that specialised flowers could promote the formation of new plant species.
My colleagues and I recently published the Cactus Ecological Database (CactEcoDB), which provides trait data and family trees for cacti, to help researchers understand their origins and future. When we analysed this data in a recent article in the journal Biology Letters, we found an unexpected pattern. We compiled flower length data for more than 750 cacti species, revealing an extraordinary range, from two millimetre blooms to flowers the size of a large dinner plate. This variation reflects adaptation to very different pollinators.
When we analysed the cactus family tree, we found that the speed at which flower size evolves drives the formation of new species, across both recent and deep evolutionary timescales. Natural selection does not seem to favour any particular flower size. Nevertheless it caused repeated bursts of rapid change across the cactus evolutionary tree towards different sizes.
What this means is simple but powerful. It is not the presence of a particular flower type or pollinator that drives cactus evolution. It is the speed at which the evolution of flower types occurs, regardless of the outcome. Species with smaller and larger flowers can quickly split into new species, as long as they changed quickly throughout their evolution.
Why this matters
This insight has implications for conservation. Our study suggests that a plant’s capacity for evolutionary change, important for surviving periods of environmental change and extinctions – like the one Earth is currently experiencing – matters more than any specific adaptation.
Protecting biodiversity is not just about saving the species we see today, but also about preserving the evolutionary potential that allows new species to arise. Some species may seem stable or unremarkable now, yet hold great future potential.
Nearly a third of cactus species are threatened with extinction. This is among the highest proportions for any plant group and we risk losing entire evolutionary lineages of cacti, not just species.
Protecting cacti, and nature more widely, means protecting an ongoing evolutionary process, one that allows life to thrive in some of the harshest environments on Earth.
Jamie Thompson receives funding from The Leverhulme Trust.
