How do plant species arise?

What is it about?

In the study we sought to understand how common it is for plant species to evolve without being geographically isolated. One of the questions that motivated this work was: Could the relative immobility of plants increase their likelihood of speciating in the same place? While some animal species—like crater lake cichlid fishes or parasitic Indigobirds—have shown that it is possible for species to split without being separated by physical barriers (a process known as sympatric speciation), such empirical examples are rare. One exceptional case in plants, involving Howea palms on a remote island, sparked intense debate but has since been confirmed as a true instance of sympatric speciation. That discovery also raised new questions such as: How frequent is this phenomenon in plants more broadly? And does speciation always involve visible morphological differences? In this study, we chose to test these questions on palms and conifers, as these two groups span temperate and tropical latitudes, are morphologically diverse, and occur in numerous habitats. We started by gathering all available information on geographical ranges, phylogenetic relationships, and estimated age of species. In total, we obtained information for 740 species and analysed these data to determine the dominant geographic mode of speciation in plants. We then used a modelling approach to infer geographical speciation modes while accounting for post-speciation transitions between geographical isolation and overlap. We found that while speciation in plants generally requires geographical separation as in animals, the incidence of sympatry varies with time and across plant groups. A second question explored was whether speciation necessarily involves morphological divergence. Analysing morphological traits from 168 palm species, we expected to find that new species diverge in morphology. Instead, we found the contrary: sister species often converge on similar shapes and structures, regardless of whether they evolved in isolation or not.

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Photo by Maximus Beaumont on Unsplash

Photo by Maximus Beaumont on Unsplash

Why is it important?

How do new species emerge, and what keeps them distinct? These are among the most fundamental and fascinating questions in biology. While this separation is most commonly geographical, it can also occur due to environmental differences or sudden genetic changes. In vertebrates, geographic isolation is considered the primary route to speciation. A few rare exceptions exist, such as certain fish and bird species that have diverged without physical separation. However, until now, large-scale assessments of how often speciation without geographical barriers occurs in plants—across many species and habitats—have been lacking.

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