Cacti evolve slowly in the pot but split into new species faster than almost any plant

They sit on windowsills for years, barely growing. They survive on neglect. The cactus is the botanical symbol of patience, the plant you buy when you want something that will still be alive when you forget to water it for three months.

But this reputation for slowness is misleading. The cactus family is, in evolutionary terms, one of the fastest-diversifying plant groups on Earth. Roughly 1,850 species have appeared in the past 20 to 35 million years, spreading across the Americas from tropical forests to the driest deserts. And a new study from the University of Reading has identified what's driving that rapid speciation - and it's not what biologists expected.

Darwin's orchid assumption, tested on cacti

The conventional explanation for why some plant groups diversify faster than others goes back to Charles Darwin's work on orchids. Darwin argued that specialized flower forms drove the creation of new species. The logic was straightforward: a flower adapted to a specific pollinator becomes reproductively isolated from plants with different flower shapes, and that isolation eventually leads to speciation. Big, specialized flowers meant more species.

This idea has shaped plant evolutionary biology for more than 150 years. And it works well for orchids, which have some of the most elaborate pollination systems in nature. But Jamie Thompson, lead author of the new study and a researcher at the University of Reading, wanted to test whether it held for cacti - a group with enormous variation in flower size but a very different ecological context.

Thompson and colleagues studied flower length data for more than 750 cactus species, covering a 185-fold range in size from flowers just 2 millimeters across to blooms stretching 37 centimeters. The dataset, compiled from a new open-access database called CactEcoDB, represents seven years of work assembling cactus traits, habitats, and evolutionary relationships, published this month in Nature Scientific Data.

Size didn't matter - speed did

The results were clear, and they contradicted the Darwinian expectation. Flower length had almost no relationship with how fast a species split into new ones. Having a large, elaborate flower didn't make a cactus lineage more likely to diversify. Neither did having a tiny one. Size, it turned out, was essentially irrelevant to speciation rate.

What did matter was the rate of flower shape evolution - how quickly a lineage's flowers were changing form, regardless of what size they happened to be. Species whose flowers were evolving most rapidly were also the most likely to branch into new species. This correlation held across both recent evolutionary splits and deep divergences in the cactus family tree, suggesting it's not an artifact of timescale but a fundamental pattern.

The distinction is subtle but important. It's not the destination (big or small flowers) that predicts diversification - it's the journey (how fast flowers are changing). A cactus with a moderately sized flower that's evolving rapidly is more likely to give rise to new species than a cactus with a spectacularly large flower that has been stable for millions of years.

Thompson put it directly: they expected cacti with longer, more specialized flowers to be the ones creating the most new species. Instead, flower size made almost no difference. What matters is how quickly flowers change shape. Cacti whose flowers evolve rapidly are far more likely to split into new species than those whose flowers stay the same, however elaborate they are.

Deserts as engines of evolutionary change

The finding reframes how we think about desert ecosystems. Deserts are often characterized as harsh, static environments where life hangs on by a thread. But if cacti - among the most iconic desert organisms - are diversifying faster than most plant families, then deserts are actually sites of intense evolutionary activity. The harshness of the environment may even contribute to the pace of change, creating strong selective pressures that drive rapid morphological evolution.

The cactus family's spread across the Americas over tens of millions of years has produced species adapted to an enormous range of conditions. Tiny ground-hugging cacti in Patagonian scrubland. Towering columnar saguaros in the Sonoran Desert. Epiphytic jungle cacti that grow on tree branches in Brazilian rainforests. The diversity is staggering, and the new study suggests that rapid flower evolution has been a key engine behind it throughout the family's history.

This doesn't mean flower evolution is the only factor. Geographic isolation, pollinator availability, soil chemistry, and climatic shifts all play roles in speciation. But the strength and consistency of the correlation between evolutionary rate and diversification rate across the cactus family suggests that the speed of morphological change is a particularly important driver in this group.

Conservation implications for a threatened family

Nearly a third of all cactus species are threatened with extinction. Habitat destruction, illegal collection for the horticultural trade, and climate change all pose serious risks. The study's findings add a new dimension to how conservationists might prioritize protection efforts.

Thompson argued that evolutionary pace should become part of conservation planning. If the ability to evolve rapidly has generated cactus diversity over millions of years, then lineages with high evolutionary rates may have greater potential to adapt to changing conditions - though he was careful to note that being able to evolve rapidly does not guarantee resilience, especially when the planet is changing faster than most cacti can keep up.

The practical suggestion is that rather than searching for a single trait that predicts which cacti are most at risk, conservationists may need to assess how fast a species is evolving. Rapidly evolving lineages might be more adaptable; slowly evolving ones might be more vulnerable to environmental change. This is a hypothesis, not a proven strategy, but it offers a quantitative framework that could complement traditional assessments based on population size and habitat loss.

The CactEcoDB database opens new research avenues

The study was made possible by the CactEcoDB database, a collaborative effort involving ten co-authors from three continents, including six from the University of Reading. The database compiles cactus traits, habitat data, and evolutionary relationships into a shared open-access resource - the first comprehensive tool of its kind for cactus research.

Before CactEcoDB, researchers studying cactus biodiversity had to assemble their own datasets from scattered sources, a process that created duplication, inconsistency, and gaps. The new database provides a standardized platform that allows researchers worldwide to study cactus biodiversity, conservation priorities, and future vulnerability under climate change.

What remains uncertain

The study establishes a strong correlation between evolutionary rate and speciation rate, but it doesn't fully explain the mechanism. Why does rapid flower evolution lead to more species? One possibility is that rapidly changing flowers create reproductive barriers more quickly - if two populations evolve different flower shapes, their pollinators may no longer cross-pollinate them, leading to genetic isolation. But the study doesn't test this mechanism directly.

The dataset also focuses on flower length as the primary morphological measure. Flowers vary in many other ways - color, scent, nectar composition, opening time - that could independently affect pollinator interactions and speciation rates. Whether the pattern holds for these other traits is an open question.

And the conservation implications, while intriguing, are speculative. Evolutionary rate is a population-level property measured over geological timescales. Whether it translates to adaptive capacity over the decades relevant to current conservation challenges is unknown. A species that evolved rapidly over millions of years may still lack the genetic variation or generation time to respond to a 2-degree Celsius warming within a century.

Still, the core finding stands: in cacti, it's not how big your flowers are that matters. It's how fast they're changing. That result challenges one of the oldest ideas in plant evolutionary biology and suggests that the slow-growing cactus on your desk belongs to one of the most dynamic plant families on the planet.