Natural interspecific hybridization in animals has two opposite macroevolutionary faces: it can homogenize gene pools and collapse or extinguish species, or it can inject genetic variation that selection sorts into new genomic combinations and even new species. Which effect dominates across animals remains a fundamental, explicitly restated open debate. The source works frame the problem as two linked components: quantifying how often distinct animal taxa exchange genes in nature and the demographic and phylogenetic limits of that exchange, and determining the net speciation outcome. The core remains open: incidence has been quantified only for individual clades (Heliconiina butterflies, birds, salamanders, swordtails, cichlids, lemurs, corals), promote-side and erode-side case studies accumulate on both sides, but no comparative synthesis across animal groups settles the net balance, and correlation-versus-causation remains unresolved even within clades.

Background

Under the biological species concept, species are groups of actually or potentially interbreeding populations reproductively isolated from other such groups. Interspecific hybridization is mating between individuals of distinct species; when hybrids backcross to a parental species, alleles cross the species boundary, a process called introgression. The classical view, following Mayr, treats hybridization as rare leakage across species boundaries that erodes diversity: genetic swamping, in which the gene pool of a rare species is overwhelmed by repeated backcrossing into the common one, and outbreeding depression, the reduced fitness of crosses between divergent lineages, can drive rare species to extinction ('extinction by hybridization'; Rhymer and Simberloff 1996), and increased interbreeding can reverse completed speciation, collapsing recently formed species back into one. Documented erosions include the collapse of a benthic-limnetic threespine stickleback species pair after habitat change (Taylor et al. 2006), and formal theory of speciation reversal and species collapse under environmental change (Seehausen et al. 2008; Gilman and Behm 2011; Todesco et al. 2016).

Against this, systematic surveys show that natural hybridization is far from negligible. Mallet (2005) estimated from hybrid records that at least 25% of plant species and about 10% of animal species, mostly young species, are involved in hybridization and potential introgression, and that species can remain incompletely isolated for millions of years. Mallet, Beltrán, Neukirchen and Linares (2007) performed the first clade-wide quantitative survey for a sexual animal radiation, the Heliconiina butterflies: 26-29% of named species hybridize in nature, yet hybrids are rare per individual (usually below 0.05% of individuals in sympatry, about 10% in one parapatric contact zone), the number of hybrids declines roughly exponentially with genetic distance, and gene exchange can persist for more than 3 million years after divergence - the species boundary behaves as a continuum, not a wall. Genome-era reviews confirm that limited introgression is taxonomically widespread (Taylor and Larson 2019), but quantitative incidence estimates remain unstable: even in birds, the best-documented class, species-level estimates range from about 10% to almost 20% depending on the taxonomic checklist, many hybrid records are unreliable, and the first individual-level estimate (about 0.064% hybrid individuals in the United States from citizen-science data) is acknowledged to be a rough underestimate (Ottenburghs 2023).

A parallel literature documents the generative face. Seehausen (2004) argued theoretically that hybridization can fuel adaptive radiation by supplying standing genetic variation. Empirically, ancient hybridization between two cichlid lineages fueled the hyperdiverse Lake Victoria radiation (Meier et al. 2017), and hybridization coinciding with ecological opportunity explains the rapid Lake Mweru cichlid radiation (Meier et al. 2019), leading to the 'combinatorial speciation' view that radiations arise by sorting and recombining ancestral standing variation (Marques, Meier and Seehausen 2019). In salamanders, a 469-species phylogenetic analysis found that lineages currently engaged in hybridization have significantly higher net diversification rates, driven by simultaneously increased speciation and decreased extinction (Patton et al. 2020). Complete genomes of all swordtail fishes show that hybridization often preceded speciation across the genus (Du et al. 2024), ancient hybridization fueled diversification in Acropora corals (Wu et al. 2024), and introgression repeatedly shaped the Heliconius radiation (Edelman et al. 2019; Singhal et al. 2021 for an avian radiation). In Madagascar's lemurs, the clades with high diversification rates also have high rates of genomic introgression (Everson et al. 2025).

Theoretical opinion remains divided: some models conclude that hybridization may rarely promote speciation (Servedio, Hermisson and van Doorn 2013), others that its importance for speciation is limited but real (Barton 2013), and the major synthesis of Abbott et al. (2013) treats the net macroevolutionary consequence as unsettled. What no study has delivered is a comparative, quantitative synthesis across animal groups that jointly estimates (a) how often and in which direction distinct taxa exchange genes in nature, (b) the demographic and phylogenetic limits of that exchange, and (c) the net outcome for species formation versus boundary erosion. Every existing net-outcome test is confined to a single clade, and the debate is explicitly restated as open in work from 2019 through 2025 (Patton et al. 2020; Ottenburghs 2023; Everson et al. 2025).

Problem Statement

The problem, as framed by the source literature (Mallet et al. 2007; Everson et al. 2025), is a single composite question: does natural interspecific hybridization in animals, on balance, promote the formation of new species or erode existing species boundaries? A complete answer requires jointly delivering, across at least three phylogenetically independent animal groups (no two of the groups drawn from the same radiation or taxonomic class): (1) a quantitative estimate of the incidence of natural interspecific gene exchange - the fraction of species or clades involved, per-individual hybrid frequencies where sympatric, and the genomic extent and direction of introgression; (2) the demographic and phylogenetic limits of that exchange - how long after divergence, at what genetic distance, and under what demographic conditions gene flow between diverging animal lineages persists and where it stops; and (3) the net macroevolutionary outcome - whether hybridizing lineages, compared with non-hybridizing ones across animal groups, show higher or lower net diversification (speciation minus extinction), or whether the outcome is conditional and on what factors (for example ecological opportunity, habitat disturbance, or divergence time). These three components serve one overarching objective - settling the promote-versus-erode balance for animal diversity - and an answer is complete when all three components are jointly delivered at a comparative multi-clade scale with quantitative support, establishing the balance or its explicit conditionality, not when any single component is delivered for a single clade.

References

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