Darwin’s finches on the Galápagos Islands are a classical example of adaptive radiation . A total of 13 species have been formed from a single ancestral species that colonized the Galápagos 2–3 million years ago. Each species occupies a distinct and unique feeding niche. For evolutionary biologists, they pose a challenge. What is the explanation for the evolution of such a variety of species in a relatively short space of time? What governs the rate of speciation and the directions of evolution? Answers to these questions are of general significance in the context of understanding why the world is so rich biologically.

Our research has been directed towards answering these questions. Speciation is the process by which two species form from one, and we would like to know how it happens. To find out, we have concentrated on members of the ground finch genus Geospiza because they are generally common and easy to capture for measurement, blood sampling for DNA analysis, and for marking uniquely so that after being released they can be identified and their behavior recorded. We have studied finches throughout the archipelago but concentrated mainly on the island of Daphne Major because it is small (0.34 km2) and has never been disturbed by human settlement or introductions of alien species. By following the fates of marked birds for more than 30 years, we have discovered several facts that are relevant to questions of species formation. Here, we provide a summary of the major findings. A longer treatment is given in a book (P. R. Grant and B. R. Grant 2008, How and Why Species Multiply. Princeton University Press, Princeton, NJ).

The first discovery is that evolution occurs when the environment changes. In 1977, a severe drought affected the whole archipelago. More than 80 % of the medium ground finches (G. fortis) on Daphne died, and mortality was size-selective: large birds, especially those with large beaks , survived much better than the rest. They survived by being able to crack open the large and hard woody fruits of Tribulus cistoides that remained in relative abundance after almost all of the small and soft seeds of many other plant species had been consumed. Natural selection gave rise to an evolutionary change in the next generation because beak size is heritable; offspring strongly resemble their parents in beak and body size traits owing to the transmission of genes from parents to offspring.

Second, competition with other species for food influences the direction of evolution. This discovery was made in 2005. In 1982–1983, an exceptionally long El Niño event enabled a population of the large ground finch (G. magnirostris) to become established on the island. The numbers gradually built up, reaching a peak just prior to a severe drought in 2003–2004 that was comparable in severity to the drought of 1977. Most finches died, but now, in contrast to 1977, large-beaked members of the medium ground finch population were outcompeted for a limited supply of Tribulus fruits by the large ground finch, and they died at a higher rate than birds with smaller beaks . Natural selection occurred, the average beak size fell, and because beak size is heritable, the next generation had small beaks . This is an example of the well-known evolutionary principle of character displacement , in which two species diverge morphologically as a result of competition between them. Thus, over a period of several decades, we have witnessed both selection and evolution oscillating between extremes of large and small beak sizes (Fig. 11.1).

Fig. 11.1
figure 1

Schematic diagram of the most extreme evolutionary changes in beak depth in response to natural selection; to large size in 1978 and to small size in 2005. The long-term average is indicated by an asterisk, and extreme directional changes are indicated by arrowheads

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The example of character displacement throws light on what happens during speciation when two populations, having diverged in separate locations such as islands, come together: they compete and diverge further, thereby increasing the chances of long-term coexistence. Finches on Daphne also hybridize, rarely but repeatedly. This is the third major discovery from our long-term research, and it adds further to an understanding of speciation . Introgressive hybridization has the potential to reveal the genetic factors that usually prevent newly formed species from exchanging genes. However, we discovered that in fact there are no genetic barriers to the exchange of genes between ground finches on Daphne. The medium ground finch hybridizes (rarely) with two other species, the small ground finch (G. fuliginosa), and the cactus finch (G. scandens), but not the much larger G. magnirostris. We found that hybrids survived just as well as the parental species but only when the food supply was suitable for birds of intermediate beak size, i.e., not dominated by large and hard seeds. Moreover, they had no difficulty in obtaining mates, producing fertile eggs, and fledging young.

Our observations on the breeding of finches revealed how mates are chosen and why hybridization occasionally occurs. Both, song and beak morphology are important cues used in mate choice. They differ between species and hence constitute important components of the behavioral barrier to interbreeding . Experiments performed by Robert Bowman on finches in captivity many years ago showed that sons learn their song from their fathers by imprinting on his song at an early age. Thus, the barrier to interbreeding is made up of genetic factors governing the development of species-specific beak sizes and shapes, and culturally transmitted factors governing song production and mate preferences. The normal process of imprinting is occasionally perturbed, for example, when a bird learns the song of another species instead of his father’s song. When that happens, hybridization can occur, and it may also lead to backcrossing of the hybrid to one of the parental species in the next generation.

The insight this provides into the process of speciation is that behavioral barriers to interbreeding arise first, at least in these species and probably many other species of birds. Only later do genetic factors that prevent interbreeding species from producing viable and fertile offspring arise by mutation . Hybridization provides an additional insight into when speciation might collapse and when it does not. Speciation may be reversed if there are no disadvantages to hybridization, for example, if the hybridizing species are ecologically similar, whereas it will be sustained if ecological differences between the newly formed species are large.

With regard to the other component of the barrier, beak size, molecular genetic work by Arhat Abzhanov and colleagues has begun to identify the genetic factors functioning in development. Variation in the timing and intensity of expression of two genes (Bmp4 and CaM) early in development creates differences in beak size and shape between species with deep beaks such as the large ground finch, and others with more pointed beaks such as the cactus finch. The genes produce signaling molecules at about day 6 of a 12-day period of embryonic development, and another set of molecules produced by three other genes have similar effects a little later.

In summary, modern molecular genetic research has now been combined with long-term ecological research to illuminate evolutionary processes that Charles Darwin could only guess at, a century and a half ago. The research will be broadly beneficial to evolutionary biology if it helps other biologists in Galápagos, continental Ecuador, and elsewhere to understand how their study organisms have evolved.