Adaptive Divergence under Gene Flow along an Environmental Gradient in Two Coexisting Stickleback Species

Area of Science:

• Evolutionary Biology
• Population Genetics
• Ecology

Background:

• Understanding how natural selection and gene flow influence local adaptation in closely related species is crucial.
• The extent to which sympatric species evolve collectively versus distinctly remains a key evolutionary question.

Purpose of the Study:

• To investigate the population genetic structure and morphological differentiation of sympatric three-spined and nine-spined stickleback.
• To compare adaptive divergence across different environmental contexts (brackish, lowland freshwater, upland freshwater) and spatial scales.

Main Methods:

• Genotyping-by-sequencing (GBS) was used to analyze genetic variation in 24 individuals per species from multiple sites.
• Morphological traits were characterized to assess phenotypic differentiation.
• Populations were sampled from lowland brackish water (LBW), lowland freshwater (LFW), and upland freshwater (UFW) environments.

Main Results:

• Both stickleback species displayed comparable genetic diversity and neutral genetic differentiation.
• Signatures of adaptive divergence (genomic and morphological) were significantly stronger in three-spined stickleback than in nine-spined stickleback.
• Most adaptive genetic variants (outlier SNPs) were associated with freshwater environments, with some linked to brackish vs. freshwater transitions.

Conclusions:

• Both species exhibit congruent evolutionary and genomic patterns of divergent selection.
• Three-spined stickleback populations show a greater magnitude of adaptive response to selection compared to nine-spined stickleback.
• Differences in the magnitude of adaptive divergence persist across various geographical and environmental settings.