RESEARCH NEWS & VIEWS Initial experiments to address these ques- approach, rather than being the fog that pre- tions have failed to provide clear answers. vents us from understanding nervous-system Coen et al. show that song transitions are function, behavioural variability and complex- similar whether or not the singer is ultimately ity can be the searchlight that helps us to identify successful in mating. Yet pheromone-insensi- the computational problems that brains evolved tive males, who sing for normal durations but to solve. ■ have altered song patterning8, tend to be slower and less successful in convincing females to Bence P. Ölveczky is in the Department 50 Years Ago mate1,8. Whether these flies are handicapped of Organismic and Evolutionary Biology, in the courting game because of a defect in Harvard University, Cambridge, Recent investigations have shown how they vary their songs, or because of unre- Massachusetts 02138, USA. that the fluoride content of Greek lated effects, remains to be seen. But whether e-mail: [email protected] teeth from the cities of Athens and song patterning matters to females or not, we 1. Coen, P. et al. Nature 507, 233–237 (2014). Salonika was considerably high. This now know that its variability, and probably the 2. Hall, J. C. Behav. Genet. 8, 125–141 (1978). may explain, at least in part, the low variability of many other ‘fixed’ behaviours, 3. Stockinger, P., Kvitsiani, D., Rotkopf, S., Tirián, L. & prevalence of dental caries observed is not simply the consequence of noise in Dickson, B. J. Cell 121, 795–807 (2005). 6,7 4. Yu, J. Y., Kanai, M. I., Demir, E., Jefferis, G. S. X. E. & in Greece … With the exception nervous-system function . Rather, a sizeable Dickson, B. J. Curr. Biol. 20, 1602–1614 (2010). of sea salt, however, the fluoride fraction of that variability is likely to reflect 5. Spieth, H. T. Annu. Rev. Entomol. 19, 385–405 (1974). content of other foods commonly computations performed by reliable and pre- 6. Faisal, A. A., Selen, L. P. J. & Wolpert, D. M. Nature Rev. Neurosci. 9, 292–303 (2008). produced and consumed in Greece dictable brains on an ever-changing sensory 7. Destexhe, A. & Rudolph-Lilith, M. Neuronal Noise is not known … The analyses environment. (Springer, 2012). showed that the fluoride content of Importantly, this insight was made possible 8. Trott, A. R., Donelson, N. C., Griffith, L. C. & Ejima, A. PLoS ONE 7, e46025 (2012). olive oil from the Island of Crete by simulta­neously observing, at high temporal 9. Katz, Y., Tunstrøm, K., Ioannou, C. C., Huepe, C. was 0.36 p.p.m. and that from the resolution, the sensory environment and behav- & Couzin, I. D. Proc. Natl Acad. Sci. USA 108, area of Kalamai 0.63 p.p.m … ioural output of a genetically tractable organ- 18720–18725 (2011). 10. Censi, A., Straw, A. D., Sayaman, R. W., Murray, R. M. it appears that the inclusion of olive ism during a complex social inter­action. Such & Dickinson, M. H. PLoS Comput. Biol. 9, e1002891 oil in the daily Greek diet does not detailed analysis applied to natural behaviours (2013). make any significant contribution has the power, as Coen et al. aptly demonstrate, 11. Rideout, E. J., Dornan, A. J., Neville, M. C., Eadie, S. to the amount of ingested fluoride. to distil seemingly complex and unpredictable & Goodwin, S. F. Nature Neurosci. 13, 458–466 (2010). Thus, at present, sea salt remains an behavioural patterns into simple rules and sen- important source of dietary fluoride sorimotor transformations9,10. With such an This article was published online on 5 March 2014. in Greece for protection against dental caries. This may well be the case in other countries, such as EVOLUTIONARY BIOLOGY Taiwan, Ceylon and Lebanon, where because of local food customs the amount of sea salt consumed has been estimated to be considerable: Speciation undone about 16–20 g per person per day. From Nature 14 March 1964 Hybridization can cause two species to fuse into a single population. New observations suggest that two species of Darwin’s finches are hybridizing on a Galapagos island, and that a third one has disappeared through interbreeding. 100 Years Ago PETER R. GRANT & B. ROSEMARY GRANT Until Kleindorfer and colleagues’ report, Think of the Niagaras of speech three species of tree finch were known to occur pouring silently through the New he process of speciation, in which one together in the highlands of Floreana Island York telephone exchanges where species splits into two, is vulnerable to in the Galapagos (Fig. 1). They differ in body they are sorted out, given a new collapse in its early stages through inter- size and in the size and shape of the beak, but, direction, and delivered audibly Tbreeding and the exchange of genes, a process unlike many birds elsewhere, not in plumage. perhaps a thousand miles away. referred to as introgression. As explained The medium tree finch (Camarhynchus New York has 450,000 instruments by the evolutionary biologist Theodosius pauper) is present only on Floreana, whereas — twice the number of those Dobzhansky1, “Introgressive hybridization the small tree finch (Camarhynchus parvulus) in London. Los Angeles has a may, then, be a passing stage in the process and large tree finch (Camarhynchus psittacula) telephone to every four inhabitants of species formation. On the other hand, the also occur together on several other islands. … Our whole social structure has adaptive value of hybrids may be as high as The pattern of distribution and size differences been reorganised. We have been that of their parent; introgressive hybridiza- led evolutionary biologist David Lack to sug- brought together in a single parlour tion may lead to obliteration of the differences gest3 that speciation had occurred on Floreana for conversation and to conduct between the incipient species and their fusion through the invasion of large tree finches from affairs, because the American into a single variable one, thus undoing the Isabela Island, followed by evolutionary reduc- Telephone and Telegraph company result of the previous divergent development.” tion in average size. The resulting medium tree spends annually for research … a Writing in American Naturalist, Kleindorfer finches did not interbreed with the large tree sum greater than the total income et al.2 offer a possible example of this process, finches that arrived later, apparently from of many universities. in a study suggesting that one population of Santa Cruz Island. From Nature 12 March 1914 Darwin’s finches has become extinct through Kleindorfer and colleagues now report that interbreeding with another. this pattern no longer exists: the large tree finch 178 | NATURE | VOL 507 | 13 MARCH 2014 © 2014 Macmillan Publishers Limited. All rights reserved NEWS & VIEWS RESEARCH has disappeared from Floreana! By comparing the morphological features of present-day Flo- a Small nch b Medium nch c Large nch reana finches (studied in 2005 and 2010) with historical data, and conducting a genetic study of current populations using DNA-sequence markers (microsatellites), the authors show that there are currently only two distinct popula- A–C: P. R. GRANT & B. A–C: P. tions on the island, corresponding to the small and medium tree finches. The analyses also revealed that individuals that do not fit into either population show intermediate char- acteristics, suggesting that they are hybrids. Consistent with the hypothesis of ongo- Santiago ing hybridization on the island, the authors Fernandina observed females of the morphologically larger group (the medium tree finch) pairing with Santa males of the smaller group, and they identified Cruz San Cristóbal 15% of yearling males in 2010 as hybrids. Isabela The authors suggest that hybridization may 2 have been responsible for the disappearance of the large tree finch from Floreana, and that 1 it may now be causing the remaining two spe- Floreana Española cies to fuse into one: speciation in reverse4. What has brought this about? The most likely Figure 1 | Invasion, evolution and loss. Three species of Darwin’s finches, the small tree finch answer is anthropogenic change to the habi- Camarhynchus parvulus (a), the medium tree finch Camarhynchus pauper (b) and the large tree finch tat. A human settlement was established on Camarhynchus psittacula (c), have been known to inhabit the Galapagos island of Floreana. The medium the island just before Darwin’s visit in 1835. finch occurs nowhere else in the archipelago, and its morphological distinctiveness was interpreted by evolutionary biologist David Lack to be the result of invasion of a small form of C. psittacula from The natural vegetation subsequently became Isabela (1), followed by an evolutionary reduction in size and change in beak shape3,5. Later, a larger form rapidly degraded, and by the end of the nine- of C. psittacula invaded from Santa Cruz (2), and remained unchanged. However, Kleindorfer et al.2 now teenth century two species of finch and a spe- report that this large species is no longer found on Floreana. cies of mockingbird had become extinct5. The large tree finch was rare: only 4 male wail from the ghost of an interbreeding past. their extinction through interbreeding10,11. and 13 female specimens were collected for Although there is some uncertainty about Rapid radiations of fishes11,12 and finches5 are museums between 1852 and 1906. The birds hybrid identification in this study, the disap- especially at risk because their morphologi- may have experienced difficulty in finding pearance of a species through hybridization is cal evolution is not accompanied by strong mates of their own species, hybridized with certainly plausible. On the small uninhabited barriers to gene exchange. Uniquely valuable medium ground finches and become absorbed island of Daphne Major, two species of ground in showing how speciation is done7,13, such into the population2,5.