Gene Flow Limits Adaptation along Steep Environmental Gradients.

Area of Science:

• Ecology
• Evolutionary Biology
• Population Genetics

Background:

• Dispersing individuals typically move to similar habitats, but steep environmental gradients can connect divergent habitats via gene flow.
• This gene flow is predicted to weaken adaptive clines, reducing population divergence.
• Countergradient variation, where populations adapt to local conditions, is a key evolutionary response.

Purpose of the Study:

• To compare quantitative genetic divergence in frog populations along elevational and latitudinal gradients.
• To investigate the impact of environmental gradient steepness on adaptive population divergence and countergradient variation.
• To test predictions about reduced adaptation on steep environmental gradients.

Main Methods:

• Experimental comparison of Rana temporaria populations along a 2,000-m elevational gradient and a 1,550-km latitudinal gradient.
• Genotyping of individuals at ~2,000 single-nucleotide polymorphism markers to assess dispersal and gene flow.
• Meta-analysis of 19 experimental studies on anuran populations across temperature gradients.

Main Results:

• Significant countergradient variation in larval development rate was found in both elevational and latitudinal studies.
• The adaptive cline was weaker along the elevational gradient compared to the latitudinal gradient.
• Dispersal distance was greater on the latitudinal gradient, but gene flow was more effective across environmental conditions on the elevational gradient.
• Meta-analysis confirmed that countergradient variation is weaker on steeper gradients.

Conclusions:

• Adaptive population divergence is less pronounced on steep environmental gradients.
• Steeper gradients facilitate greater maladaptation due to reduced effectiveness of natural selection.
• Environmental gradient steepness is a critical factor influencing evolutionary adaptation and population divergence.