A week ago it was St Valentine’s day, a day that has become synonymous with the sending of cards and gifts to one’s sweethearts. There are some similarities between these cards and reviewers’ reports: they are traditionally unsigned, and the recipient can expend considerable energy trying to identify the author. Valentine’s cards are, however, rarely as unflattering as the average reviewer’s report. Many contain variations on this botanically incomplete verse: Roses are red, violets are blue / honey is sweet, and so are you.

But all roses are not red. Conventional breeding, and genetic engineering of the p450 enzymes involved in the synthesis of flavonoids and anthocyanins, have made a kaleidoscope of colours available to modern rose lovers1. Even before the practices of horticulture made their mark, the wild rose ranged in colour from white, through pink, to deep red.

Nevertheless, roses remain rhetorically red, and there would have been a time when even an experienced botanist presented with a yellow rose would have found it profoundly disturbing, resulting in a substantial shift in their world view. Such an event would now be called a ‘black swan’: an occurrence so rare that it is assumed impossible until it happens, resulting in a reassessment of previous assumptions. As late as the 16th century, the phrase ‘as rare as a black swan’ was used in England to mean that something was unthinkable. All swans that had ever been seen in Europe had white plumage, so by definition swans were white. It was an existential shock when Dutch explorers, such as Willem de Vlamingh, visited Western Australia in the 1690s and encountered Cygnus atratus, unquestionably a swan species and unquestionably black.

Two hundred years ago, another ‘black swan’ was described by the English dental surgeon and zoologist Thomas Bell. This was a new type of iguana2 that, instead of having a “long, pointed, narrow muzzle” (like all other iguanas to that point), had “a short, obtusely truncated head, not so long as it is broad.” Bell rightly concluded that this reptile would have “some striking peculiarity in its food and general habits.” It was, in fact, a marine iguana, Amblyrhynchus cristatus, from the Galápagos Islands, rather than Mexico as Bell believed. Uniquely among iguanas, A. cristatus’s ancestors evolved an aquatic lifestyle involving diving in the ocean to forage for algae, which make up the bulk of its diet.

The geographic isolation of the Galápagos Islands, 900 km west of the coast of Ecuador, and their formation where an eastward-moving tectonic plate is passing over a mantle plume have produced a unique flora and fauna that continues to shape our understanding of evolution. On his return from voyaging on HMS Beagle in 1836, Charles Darwin would have told John Bell of watching his iguanas warming themselves on lava rocks before diving into the cold ocean; of the giant tortoises whose shell morphology varies dramatically from island to island, depending on the forms of vegetation available for them to eat; and of many other unfamiliar varieties of life.

Darwin was particularly influenced by the variation in morphology between populations of mockingbirds on different islands, each of which has evolved a unique beak shape that depends on their preferred food sources. Beak shape is also highly diverse among Darwin’s finches, a collection of almost 20 species endemic to the Galápagos. These are not true finches, but the absence of other small birds has allowed them to evolve into a diverse array of ecological niches. The finches on the uninhabited island of Daphne Major have been the subject of a 30-year study by a team led by Peter and Rosemary Grant. In that time, the researchers observed the amazingly rapid emergence of a new species following breeding of native females with a male bird blown in from another island3. In three generations, the offspring had become reproductively isolated, and in six they were a functional species.

For a plant biologist, the most dramatic ‘black swan’ of the Galápagos is the cactus Opuntia galapageia, a type of prickly pear. Opuntia grow as a series of flat fleshy paddles (cladodes), with flowers and new cladodes originating from the surfaces of mature paddles. They vary greatly in size — many are large shrubs, but on some islands of the Galápagos, O. galapageia forms trees. Older cladodes fuse and swell to form hollow, woody trunks, a defence against herbivory by giant tortoises and land iguanas.

Like the finches, O. galapageia provides a superb opportunity to study diversification and speciation. All O. galapageia on the islands appear to have arisen from a single colonization event, but whether the different taxa are different varieties, subspecies or true species is not clear. A recent genomic study O. galapageia has found that, despite the large phenotypic differences between cacti on different islands, there is little genomic diversification, as well as a complicated pattern of genetic exchange between island populations4. At least in this clade of plants, phenotypic change occurs in advance of genetic divergence.

Isolated and exotic locations such as Australia and the Galápagos are attractive locations for making surprising and informative discoveries, but they do not have a monopoly on wonder. We should embrace ‘black swans’ wherever we encounter them, and listen to the stories they have to tell.