What did Charles Darwin write to follow up On the Origin of Species? A treatise on the pollination of orchids. Darwin had marveled at the diversity of these plants, and he saw the myriad ways in which the flowers and pollinating insects have adapted to each other as an extended case study in natural selection. “The contrivances for insect fertilization in Orchids are multiform & truly wonderful & beautiful,” he wrote to a horticulturist who had sent him specimens.
Evolutionary biologists never lost their fascination with orchids. With more than 25,000 species, they’re the biggest group within the plant kingdom, comprising roughly 8% of all vascular plant species. Biologists have proposed various explanations for this extraordinary diversity, but it has been impossible to nail down their relative importance.
Now, a new family tree of the orchids is a major step in that direction. “For the first time, they have a well-supported phylogeny for all main branches,” says orchidologist Barbara Gravendeel of the Naturalis Biodiversity Center in Leiden, the Netherlands, who was not involved in the work. “It’s a great study,” Gravendeel says, and it shows how orchids owe their diversity to a series of innovations—above all, the ability to grow in tropical mountains—that individually or jointly touched off explosions of new species.
Previous orchid phylogenies had compared just a few genes found in chloroplasts, the organelles that turn water and CO2 into sugars. (Chloroplast genes evolve slowly, which helps reveal distant evolutionary relationships.) By using new gene sequencing methods, a team led by Thomas Givnish, a plant ecologist at the University of Wisconsin, Madison, created a phylogeny of orchids with an unprecedented 75 chloroplast genes from 39 species, representing almost all major groups of orchids, as well as from 96 distant relatives among all flowering plants. Using the estimated ages of fossils of 17 flowering plants, including the few known orchid specimens, they were able to date the main branches in the phylogeny and calculate the rate at which new species appeared.
The team’s new evolutionary timeline begins 112 million years ago, when the first orchids appeared. About 90 million years ago, the major living lineages started to split from each other. Then, sometime before 64 million years ago, a key innovation occurred: Orchids developed a way to lump their pollen into sticky balls, called pollinia, so that pollinators would not lose any grains before reaching other orchids. As flowers evolved intricate structures to attach pollinia—some orchids stick them smack between the eyes of their favorite insect species, for instance—reproductive barriers likely formed, giving birth to new species. Indeed, in lineages that have pollinia, the speciation rate was 5.1% higher than in lineages that don’t, Givnish and colleagues report this week in the Proceedings of the Royal Society B.
Next, some orchids evolved an aerial lifestyle. By 35 million years ago, many had become epiphytes, plants that cling to trees. This shift opened up many new areas to colonize and new environmental conditions; the rate of net diversification increased 8.8% for epiphytes. To make up for having their roots exposed, some lineages adopted a kind of water-saving photosynthesis called crassulacean acid metabolism that likely helped them survive only on fog and rain; it increased their diversification rate by a remarkable 20.3%.
The biggest boost, however, came in lineages that moved into tropical mountains such as the Andes and the New Guinea highlands, where they found many new opportunities for diversification. The rate of speciation among these cloud forest dwellers rose 24.9% compared with lineages that stayed in the lowlands. Givnish says it’s difficult to disentangle the effect of mountain-climbing from tree-climbing, because the vast majority of mountain species are also epiphytes.
Scientists say these traits are all plausible drivers of diversification. But many puzzle at one result. One-third of all orchids deceive pollinators by luring them with structures or scents that resemble food, nesting sites, or even mates. Yet this trickery, another important evolutionary novelty, apparently didn’t speed up the evolution of new species. “It was a big surprise,” says Givnish, who is stumped. Gravendeel thinks that this deceit probably did accelerate speciation, but that researchers have either misidentified or missed anatomical details in many tropical orchids.
Other experts say even more robust family trees will come from studying DNA in the cell nucleus, in part because it contains far more genes than chloroplasts do. “The only way to get a real story, the closest we can get, is to sequence nuclear genomes from orchids,” says Victor Albert, a plant geneticist at the State University of New York at Buffalo. This past November researchers published the first orchid genome, for the tropical epiphyte Phalaenopsis equestris. More genomes will follow, and they are bound to shed more light on the evolution of traits that so delighted Darwin.