Temperate Zone Isolation by Climate: An Extension of Janzen's 1967 Hypothesis

Area of Science:

• Ecology and Evolutionary Biology
• Biodiversity and Biogeography
• Population Genetics

Background:

• The latitudinal biodiversity gradient is a major pattern in life distribution.
• Janzen (1967) proposed tropical mountains are greater migration barriers than temperate ones due to lower annual temperature variation.
• This mechanism is established at broad latitudinal scales but less understood at smaller scales.

Purpose of the Study:

• To investigate the association between climate overlap and genetic similarity in temperate populations.
• To assess if climate overlap mediates biodiversity at smaller geographic scales.
• To examine genetic differentiation in lizard populations within the Sierra Nevada mountain range.

Main Methods:

• Compared genetic differentiation between high- and low-elevation lizard populations.
• Analyzed climate overlap between populations in the Sierra Nevada.
• Investigated variation in climate overlap across adjacent temperate mountain ranges at the same latitude.

Main Results:

• Found a negative association between climate overlap and genetic differentiation, indicating lower genetic divergence in climatically similar environments.
• Observed that isolation by distance also strongly influences genetic differentiation.
• Discovered significant variation in climate overlap between adjacent temperate mountain ranges, contrary to predictions based solely on latitude.

Conclusions:

• Climate overlap negatively influences genetic differentiation, supporting Janzen's mechanism at smaller scales within temperate regions.
• Local climate factors within bioclimatic regions can significantly impact genetic differentiation, independent of broad latitudinal effects.
• This suggests that both latitude and local climate conditions play crucial roles in shaping biodiversity patterns.