Darwin's finches made the Galápagos a textbook case of evolution finding different answers to different problems. A group of plants on the same islands has now demonstrated the opposite trick: the same answer, reached repeatedly by different means.
Scalesia — the Galápagos giant daisies, ranging from low shrubs to trees — is the archipelago's largest endemic plant genus. All 15 recognised species emerged within roughly the past million years, and in that short window the group has spread from humid highland forest to hot, dry lowlands. Its most conspicuous variation is in the leaves, which run from large and unbroken to small and deeply lobed with serrated edges. Botanists have long suspected the lobed form is an adaptation to the islands' equatorial heat and drought, shedding warmth more efficiently while losing less water. How that adaptation was built genetically was unknown.
An international team sequenced 396 individual plants covering every species and combined the genomes with leaf measurements and gene-activity data from several developmental stages. Their paper in Nature Communications reports that deep lobing did not evolve once and then spread. It appeared independently, in separate branches of the family tree.
The same pathway, different switches
Each appearance ran through a different gene — but always one belonging to the same regulatory system, the pathway that establishes a leaf's upper and lower polarity as it develops. "Even more surprising was that each time this trait evolved, it did so through different genes," said Vanessa Bieker of the Royal Botanic Gardens, Kew, the paper's first author. Rather than a single master gene for leaf shape, evolution reached into a network of interacting genes and adjusted different components to arrive at the same outcome, under what the authors identify as diversifying selection.
The raw material came from the founders. The analysis traces the genus's unusual genetic richness to allopolyploidy — a doubling of the genome after two lineages merged — which left the first colonists with enough standing variation for selection to work with quickly, without waiting on new mutations.
"More than 150 years after Darwin's work on the Galápagos transformed our understanding of life on Earth, these islands continue to reveal new biology," said Michael D. Martin of the NTNU University Museum in Norway, part of a team that also included researchers from the University of California, Davis, the University of Copenhagen, the University of British Columbia and the Charles Darwin Foundation in the Galápagos.
The findings carry a practical edge. Scalesia forests are threatened, and the genomes reveal far more structure within species than expected, with populations long isolated from one another and apparently in the early stages of becoming species in their own right. On that reading, protecting the genus means protecting its populations individually, rather than treating each species as one interchangeable pool.