Morphological ghosts of introgression in Darwin’s finch populations – pnas.org

Significance

Genomes contain signatures of past gene exchange between species. However, genomic data are not available for many organisms. For these, morphology may substitute for genes, as exemplified by Darwin’s finches on the Galápagos island of Floreana. In 1835, Darwin and companions collected seven specimens of a uniquely large form of Geospiza magnirostris that became extinct in the next few decades. A surviving population of Geospiza fortis shows evidence of hybridization in a pronounced skew in the distribution of beak size in the direction of the absent G. magnirostris. The genetic and morphological residuum of an extinct species in an extant one has implications for its future evolution, as well as for conservation programs of reintroduction in extinction-depleted communities.

Abstract

Many species of plants, animals, and microorganisms exchange genes well after the point of evolutionary divergence at which taxonomists recognize them as species. Genomes contain signatures of past gene exchange and, in some cases, they reveal a legacy of lineages that no longer exist. But genomic data are not available for many organisms, and particularly problematic for reconstructing and interpreting evolutionary history are communities that have been depleted by extinctions. For these, morphology may substitute for genes, as exemplified by the history of Darwin’s finches on the Galápagos islands of Floreana and San Cristóbal. Darwin and companions collected seven specimens of a uniquely large form of Geospiza magnirostris in 1835. The populations became extinct in the next few decades, partly due to destruction of Opuntia cactus by introduced goats, whereas Geospiza fortis has persisted to the present. We used measurements of large samples of G. fortis collected for museums in the period 1891 to 1906 to test for unusually large variances and skewed distributions of beak and body size resulting from introgression. We found strong evidence of hybridization on Floreana but not on San Cristóbal. The skew is in the direction of the absent G. magnirostris. We estimate introgression influenced 6% of the frequency distribution that was eroded by selection after G. magnirostris became extinct on these islands. The genetic residuum of an extinct species in an extant one has implications for its future evolution, as well as for a conservation program of reintroductions in extinction-depleted communities.

Footnotes

• Author contributions: P.R.G. and B.R.G. designed research; P.R.G. and B.R.G. performed research; P.R.G. analyzed data; and P.R.G. and B.R.G. wrote the paper.

• Reviewers: P.A., Uppsala University; and A.P.H., McGill University.

• The authors declare no competing interest.

• This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2107434118/-/DCSupplemental.

Data Availability

All study data are included in the article and supporting information.

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