On the Galápagos island of Daphne Major, drought-driven selection (2003–2005) and continuing introgressive hybridization with the small ground finch Geospiza fuliginosa made the medium ground finch G. fortis almost morphologically indistinguishable from G. fuliginosa, prompting Grant & Grant (2016) to ask whether the process will end in permanent species fusion, reverse if the seed ecology reverts, or carry the admixed population onto a new evolutionary trajectory. Follow-up genomic time series confirmed directional erosion of species boundaries — G. fuliginosa ancestry in G. fortis rose about 14% between 1988 and 2012, and heterospecific ancestry in both G. fortis and G. scandens increased linearly over three decades, without slowing or reversing — but no study establishes the endpoint. The open core is to determine, by continued field and genomic monitoring, mechanistic modeling of introgression and mate choice, and cross-taxon comparison, which outcome the Daphne Major hybridizing complex is heading toward and under what ecological conditions fusion, stable admixture, reversal, or novel-lineage formation occurs.

Background

Darwin's ground finches (Geospiza) on the Galápagos archipelago have been studied continuously on the small (~0.34 km²) island of Daphne Major since 1973, yielding one of the world's most complete long-term records of evolution in the wild. Introgressive hybridization — interspecific mating followed by backcrossing, so that genes cross species boundaries — is a recurring feature of this system: it can erode differences between species and even reverse speciation, but it can also inject new genetic variation that fuels adaptation or founds new lineages. The resident medium ground finch G. fortis (~17 g) breeds occasionally with the resident cactus finch G. scandens (~21 g) and with the small ground finch G. fuliginosa (~12 g, smaller with a more pointed beak), which appears on Daphne Major only as a rare immigrant and has no independent breeding population there. Mating is largely assortative because finches sexually imprint on their father's song and choose mates by beak and body size, but occasional misimprinting on a heterospecific neighbor's song produces hybrids; strikingly, these hybrids and their backcrosses show little or no fitness loss. Earlier work (Grant et al. 2004) had documented 30 years of morphological convergence between G. fortis and G. scandens caused by bidirectional introgression. Grant & Grant (2016) then analyzed the third member of the complex: G. fuliginosa alleles enter G. fortis through repeated immigration and hybridization, a severe drought in 2003–2005 selectively killed large-beaked G. fortis, and afterwards G. fortis became almost indistinguishable from G. fuliginosa, with continued hybridization and beak-size decrease producing 'a potential morphological fusion of the two species under entirely natural conditions'. This made Daphne Major a natural analog of human-mediated hybrid-zone dynamics (e.g., urbanization erodes niche segregation in these finches elsewhere in Galápagos, as De León et al. 2018 showed for Santa Cruz). Subsequent genomic work sharpened the picture: G. fortis acts as a genetic conduit passing G. fuliginosa genes to G. scandens, which never hybridizes with G. fuliginosa directly (Grant & Grant 2020); whole-genome sequencing of 3,958 individuals showed G. fuliginosa ancestry in G. fortis increased ~14% and G. fortis self-ancestry declined ~20% over 1988–2012, with beak-size change associated with fuliginosa ancestry at large-effect loci including the major beak-size supergene (a chromosomal block of linked genes inherited as a unit) containing the HMGA2 gene region (Enbody et al. 2023). Reverse speciation and re-emergence after collapse have both been documented in other systems — e.g., species pairs of three-spined sticklebacks collapsed after disturbance to mating isolation and could re-emerge when isolation restored (Behm et al. 2010; Gilman & Behm 2011) — so neither branch of the Daphne dichotomy can be excluded by analogy. On the same island, a single immigrant large cactus finch (G. conirostris) founded the reproductively isolated 'Big Bird' lineage in 1981, demonstrating that hybridization can also generate a new lineage in this system (Lamichhaney et al. 2018), and Grant & Grant (2025) describe an independent apparent case of speciation reversal through convergence on Darwin Island. A 'solution' here is not a theorem but a resolved empirical and mechanistic determination of the trajectory and its endpoint conditions.

Problem Statement

Determine the evolutionary fate of the hybridizing Geospiza complex on Daphne Major that Grant & Grant (2016) posed: now that G. fortis has converged on the immigrant G. fuliginosa and continues to exchange genes with it (and with G. scandens), will the process culminate in permanent fusion of the species into a single admixed population, stabilize as a persistent admixture of distinguishable forms, reverse toward restored species distinctiveness if the seed ecology returns toward the pre-1983 state, or found a reproductively isolated novel lineage along a new trajectory, as the Big Bird lineage did on the same island? Concretely, an answer must (i) establish from continued longitudinal field and genomic monitoring of the Daphne Major population which of these four outcomes the system is on track toward and whether fusion is completing, stalling, or reversing, quantifying ancestry, hybridization frequency, assortative mating, and fitness of hybrids; (ii) identify the ecological and behavioral conditions (seed community, drought/El Niño regime, immigration rate, song-imprinting fidelity) that separate the fusion, stable-admixture, reversal, and novelty branches, giving conditions for every one of the four outcomes; and (iii) place the outcome comparatively, quantifying how often environmental perturbations across taxa catalyze speciation reversal versus hybrid speciation. The reversal branch is contingent on the environment actually changing back, which field observation cannot force.

References

  1. Peter R. Grant & B. Rosemary Grant (2016), 'Introgressive hybridization and natural selection in Darwin's finches', Biological Journal of the Linnean Society 117:812–822. DOI: 10.1111/bij.12702, https://doi.org/10.1111/bij.12702
  2. Peter R. Grant & B. Rosemary Grant (2018), 'Role of sexual imprinting in assortative mating and premating isolation in Darwin's finches', Proceedings of the National Academy of Sciences 115(46):E10879–E10887. DOI: 10.1073/pnas.1813662115, https://doi.org/10.1073/pnas.1813662115
  3. Peter R. Grant & B. Rosemary Grant (2019), 'Hybridization increases population variation during adaptive radiation', Proceedings of the National Academy of Sciences 116(46):23216–23224. DOI: 10.1073/pnas.1913534116, https://doi.org/10.1073/pnas.1913534116
  4. Peter R. Grant & B. Rosemary Grant (2020), 'Triad hybridization via a conduit species', Proceedings of the National Academy of Sciences 117:7888–7896. DOI: 10.1073/pnas.2000388117, https://doi.org/10.1073/pnas.2000388117 (PMC7148562)
  5. Sangeet Lamichhaney, Fan Han, Matthew T. Webster, Leif Andersson, B. Rosemary Grant & Peter R. Grant (2018), 'Rapid hybrid speciation in Darwin's finches', Science 359:224–228. DOI: 10.1126/science.aao4593, https://doi.org/10.1126/science.aao4593
  6. Erik D. Enbody, Ashley T. Sendell-Price, C. Grace Sprehn, Carl-Johan Rubin, Peter M. Visscher, B. Rosemary Grant, Peter R. Grant & Leif Andersson (2023), 'Community-wide genome sequencing reveals 30 years of Darwin's finch evolution', Science 381(6665):eadf6218. DOI: 10.1126/science.adf6218, https://doi.org/10.1126/science.adf6218
  7. Erik D. Enbody, Ashley T. Sendell-Price, C. Grace Sprehn, Carl-Johan Rubin, Peter M. Visscher, B. Rosemary Grant, Peter R. Grant & Leif Andersson (2022), 'Large effect loci have a prominent role in Darwin's finch evolution', bioRxiv preprint, DOI: 10.1101/2022.10.29.514326, https://doi.org/10.1101/2022.10.29.514326 (same Daphne Major 1988–2012 low-pass sequencing dataset as the Science version of record — 3,958 individuals sequenced, of which the Science version analyzes 3,955)
  8. Peter R. Grant, B. Rosemary Grant, Erik D. Enbody & Leif Andersson (2026), 'Fission, fusion, and the generation of novelty in Geospiza species on Daphne Major Island, Galápagos', Evolutionary Journal of the Linnean Society 5(1). DOI: 10.1093/evolinnean/kzag020, https://doi.org/10.1093/evolinnean/kzag020
  9. Peter R. Grant & B. Rosemary Grant (2025), 'Convergence of Darwin's finches through hybridization on a remote Galápagos Island', Evolutionary Journal of the Linnean Society 4(1). DOI: 10.1093/evolinnean/kzaf015, https://doi.org/10.1093/evolinnean/kzaf015
  10. Luis F. De León, Diana M. T. Sharpe, Kiyoko M. Gotanda, Joost A. M. Raeymaekers, Jaime A. Chaves, Andrew P. Hendry & Jeffrey Podos (2019), 'Urbanization erodes niche segregation in Darwin's finches', Evolutionary Applications 12(7):1329–1343. DOI: 10.1111/eva.12721, https://doi.org/10.1111/eva.12721 (published online December 2018)
  11. Peter R. Grant, B. Rosemary Grant, Jeffrey A. Markert, Lukas F. Keller & Karel Petren (2004), 'Convergent evolution of Darwin's finches caused by introgressive hybridization and selection', Evolution 58(7):1588–1599. DOI: 10.1111/j.0014-3820.2004.tb01738.x, https://doi.org/10.1111/j.0014-3820.2004.tb01738.x
  12. Jocelyn E. Behm, Anthony R. Ives & Janette W. Boughman (2010), 'Breakdown in Postmating Isolation and the Collapse of a Species Pair through Hybridization', American Naturalist 175(1):11–26. DOI: 10.1086/648559, https://doi.org/10.1086/648559
  13. Robert Tucker Gilman & Jocelyn E. Behm (2011), 'Hybridization, species collapse, and species reemergence after disturbance to premating mechanisms of reproductive isolation', Evolution 65(9):2592–2605. DOI: 10.1111/j.1558-5646.2011.01320.x, https://doi.org/10.1111/j.1558-5646.2011.01320.x