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........

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