Elevational speciation in action? Restricted gene flow associated with adaptive divergence across an altitudinal...

Area of Science:

• Evolutionary Biology
• Speciation Research
• Amphibian Ecology

Background:

• Ecological speciation theory posits that divergent selection pressures across environmental gradients can lead to reproductive isolation.
• While adaptive divergence along elevational gradients is documented, evidence for restricted gene flow and subsequent speciation is less common.
• Previous research on the boreal chorus frog (Pseudacris maculata) identified adaptive divergence in traits across a significant elevational gradient in the Colorado Front Range.

Purpose of the Study:

• To investigate if adaptive divergence in Pseudacris maculata across an elevational gradient is associated with restricted gene flow.
• To determine if behavioral isolation, specifically through acoustic signaling, contributes to reproductive isolation in this species.
• To assess the potential for incipient ecological speciation in Pseudacris maculata.

Main Methods:

• Analysis of 12 microsatellite loci from 797 Pseudacris maculata individuals across 53 populations.
• Statistical controls for geographic distance and topography to assess gene flow.
• Examination of male advertisement call characteristics (dominant frequency, pulse number, pulse duration) across elevations.
• Phonotaxis experiments to test female mating call preference using playback experiments.

Main Results:

• Significant restriction of gene flow was detected across the elevational gradient, independent of geographic distance and topography.
• Male advertisement calls exhibited significant variation in dominant frequency, pulse number, and pulse duration across elevations, partly explained by body size and temperature.
• Female phonotaxis experiments showed a non-significant trend towards preferring calls from their own elevation, indicating weak behavioral isolation.

Conclusions:

• Adaptive divergence across elevational gradients demonstrably restricts gene flow in Pseudacris maculata.
• While genetic divergence suggests progress towards speciation, the current level of behavioral isolation via acoustic signals is insufficient to fully explain reproductive isolation.
• The mechanisms driving incipient ecological speciation in Pseudacris maculata remain an open area for further investigation.