Darwin's Finches, An Iconic Adaptive Radiation

Abstract

Darwin's finches are a prime example of an adaptive radiation. Eighteen species have diversified ecologically and morphologically from a common ancestor that arrived in the Galápagos archipelago approximately 2 million years ago (Ma). Speciation occurred when populations on different islands diverged, principally in body size and beak characteristics, and continued when populations came together on a single island. Field observations show that natural selection increases the ecological differences during food scarcity caused by droughts. Genomic studies have identified two important transcription factor genes that show genetic variation associated with beak morphology. Species typically do not breed with each other, instead, they recognise and respond to members of their own species by song and appearance. Nevertheless, they do occasionally hybridise and exchange genes because the hybrids are viable and fertile. Hybridisation provides an alternative route to the formation of a new species.

Key Concepts

  • Adaptive radiation is the rapid ecological diversification of species from a common ancestor.
  • Natural selection is the differential survival or reproduction of individuals due to differences in their phenotypic traits, and it leads to evolutionary change in the next generation if the traits are heritable.
  • Beaks are tools for gathering and processing foods and are key functional traits in the ecological diversification of Darwin's finches.
  • Divergence of beaks of coexisting species, known as character displacement, occurs when two species compete for a limited food supply and evolve away from each other in traits that reduce competition.
  • Sexually reproducing species are diagnosably distinct groups that rarely if ever breed with other groups of organisms.
  • Speciation occurs by the formation of barriers to gene exchange that may function either before mating occurs or later after fertilisation of the eggs has taken place.
  • A premating barrier to gene exchange in Darwin's finches is formed by a sexual imprinting process whereby species‐specific song and morphology are learned early in life and used when adults choose a mate.
  • The barrier may break down by accidental disruption of the learning process and then young finches may learn the song of another species and breed with that other species later in life.
  • Hybrids may be viable and fertile, which results in gene flow among closely related species.
  • Introgressive hybridisation is another route to speciation and occurs when the hybrids are sufficiently different from the parental species in song and appearance that they do not breed with them but only breed with members of their own (hybrid) group.

Keywords: phylogeny; adaptive radiation; speciation; hybridisation; Galápagos; ecology; beaks; genetics; song

Figure 1. (a) Map of Galápagos. (b) Four finch species on Daphne Major Island: medium ground finch G. fortis (upper left), small ground finch G. fuliginosa (lower left), large ground finch G. magnirostris (upper right) and cactus finch G. scandens (lower right). Source: Grant PR, Grant BR (2014) 40 Years of Evolution: Darwin's Finches on Daphne Major Island. Princeton NJ: Princeton University Press.
Figure 2. Phylogeny, from Lamichhaney et al. . (a) Nodes of the tree are dated in thousands of years with confidence intervals where appropriate. (b) Network tree on the basis of all autosomal sites. Source: Lamichhaney S, Berglund J, Almén MS et al. Evolution of Darwin's finches and their beaks revealed by genome sequencing. Nature 518: 371–375.
Figure 3. The speciation cycle. From Grant PR, Grant BR (2014) 40 Years of Evolution: Darwin's Finches on Daphne Major Island. Princeton NJ: Princeton University Press.
Figure 4. Two important genes affecting (a) beak shape and (b) beak size, identified by comparing genomes of species with contrasting phenotypes. Lamichhaney S, Berglund J, Almén MS et al. Evolution of Darwin's finches and their beaks revealed by genome sequencing. Nature 518: 371‐375. Lamichhaney S, Han F, Berglund J et al. (2016) A beak size locus in Darwin's finches facilitated character displacement during a drought. Science 352: 470–474.
Figure 5. (a) Natural selection on beak size in the droughts of 1977, 1985 and 2004, (b) Morphological separation of four species of ground finches. Both are from Daphne Major Island. Lamichhaney S, Han F, Webster MT et al. (2018) Rapid hybrid speciation in Darwin's finches. Science 359: 224–228.
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Further Reading

Coyne JA and Orr AR (2004) Speciation. Sinauer: Sunderland, Mass.

Ellegren H (2013) The evolutionary genomics of birds. Annual Review of Ecology, Evolution, and Systematics 44: 239–259.

Grant BR and Grant PR (1989) Evolutionary Dynamics of a Natural Population: The Large Cactus Finch of the Galápagos. University of Chicago Press: Chicago, IL.

Han F, Lamichhaney S, Webster MT, et al. (2017) Gene flow, ancient polymorphism, and ecological adaptation shape the genomic landscape of divergence among Darwin's finches. Genome Research 27: 1004–1015.

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Grant, Peter R, Grant, B Rosemary, Enbody, Erik D, Andersson, Leif, and Lamichhaney, Sangeet(Dec 2020) Darwin's Finches, An Iconic Adaptive Radiation. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0029107]