Hybridization Risks in Climate-Stressed Populations

Abstract

Climate change is altering species ranges and phenological synchrony at rates that force previously allopatric or temporally isolated species into novel contact zones, creating emerging hybridisation risks that threaten the genetic integrity of specialised endemic taxa. Simultaneously, climate stress reduces population size and mate availability, which can relax conspecific mate preferences and increase willingness to accept heterospecific mates -- a mechanism that compounds the hybridisation risk from increased range overlap. This study characterised hybridisation patterns across 36 paired species contacts spanning mammals, birds, fish, and amphibians in 18 documented emerging and historical contact zones, using whole-genome resequencing (mean 12.8x coverage; n = 4,284 individuals) to quantify introgression rates, hybrid zone widths, and the genomic architecture of reproductive barriers. Climate-change-associated range expansions had created novel contact zones for 14 of 36 species pairs (38.9%) within the past 30 years. Introgression rates in climate-change contact zones averaged 8.4% +- 3.2% of the parental genome -- significantly higher than in historically established contact zones (4.7% +- 2.1%; t = 6.84, p < 0.001). Population size decline (< 500 individuals) was associated with a 3.8-fold increase in hybridisation rate relative to larger populations (OR = 3.84, p < 0.001). Genomic regions under selection -- identified by FST outlier analysis -- showed significantly lower introgression than neutral regions (mean: 2.1% vs. 9.4%; t = 8.47, p < 0.001), confirming that selection maintains species-specific adaptive alleles even under active hybridisation. However, twelve species pairs showed genomic evidence of breakdown in reproductive barriers under climate stress, with hybrid fitness exceeding parental fitness in novel thermal environments in six cases. These findings identify climate-driven hybridisation as a growing threat to biodiversity requiring proactive genomic monitoring and targeted management intervention.