Climate-Induced Hybrid Zones and Species Boundaries

Abstract

Climate change is altering species distributions faster than reproductive isolation can maintain species boundaries, causing secondary contact between previously allopatric sister taxa whose ranges are shifting poleward or upslope in response to warming -- creating climate-induced hybrid zones that represent both evolutionary novelty and conservation complexity. As species track isotherms northward and upward (DBA papers #215, #231), they encounter congeners that have shifted from different directions, breaking down the geographic barriers that previously maintained reproductive isolation. This study characterised climate-induced hybrid zones in 14 European species pairs (7 plant; 4 invertebrate; 3 vertebrate pairs) using RADseq genomic data (2,840 genotyped individuals from 42 transect sites across Austria, Italy, and Sweden; 2020-2023), quantifying hybrid zone width, admixture proportions, and reproductive barrier strength (Isolation Index I; genomic cline analysis) and testing whether hybrid zone displacement rates correlate with climate velocity. Hybrid zones showed mean displacement rates of +18.4 +/- 4.4 km per decade -- significantly correlated with local climate velocity (r = +0.84; p < 0.001). Reproductive isolation was reduced in 84.4% of hybrid zones relative to historical reference data -- consistent with increased secondary contact reducing assortative mating and post-zygotic isolation in previously separated populations. A Hybrid Zone Climate Sensitivity Index (HZCSI) integrating hybrid zone displacement rate, genomic admixture rate, and reproductive isolation strength predicted species boundary stability under SSP2-4.5 with r = +0.84, identifying 5 of 14 species pairs as at risk of hybrid swarm formation and effective species boundary collapse by 2100.