Header

UZH-Logo

Maintenance Infos

Adaptive Divergence across an Elevational Gradient in the Common Frog (Rana temporaria)


Bachmann, Judith. Adaptive Divergence across an Elevational Gradient in the Common Frog (Rana temporaria). 2017, University of Zurich, Faculty of Science.

Abstract

This thesis investigates adaptive divergence between populations of a widespread amphibian across its entire elevational distribution which is characterized by both strong divergent selection and high gene flow, two opposing evolutionary forces. It shows that gene flow opposes divergent selection, but is not strong enough to prevent the evolution of genetic divergence and adaptation to elevation. Four evolutionary forces influence genetic divergence between populations: Divergent selection, gene flow, random genetic drift and mutation. Whereas the two latter are random processes, divergent selection and gene flow have opposite impacts on genetic divergence. The differentiation of populations that encounter both strong divergent selection and exchange many alleles depends on the interplay between divergent selection and gene flow: if gene flow overrules divergent selection, phenotypic differences between populations are caused by phenotypic plasticity. Another important factor is the spatial scale across which the species is distributed in relation to the dispersal distance, this defines the grain of underlying environmental change and influences both divergent selection and gene flow between selective environments. In this thesis, I study divergence across the elevational distribution in the common frog (Rana temporaria). The grain of the environmental change between populations at different elevations is high and there is ample gene flow across the gradient, making this system a steep environmental gradient and ideal to study the evolution of genetic divergence in the face of gene flow. R. temporaria has been intensively studied across the latitudinal gradient, an environmental gradient which spans about the same temperature range on a larger spatial scale and has less gene flow between selective environments. This is a great contrast to the elevational gradient. First, I established how much genetic divergence exists across the elevational gradient, by raising tadpoles that originated from different elevations under common lab conditions. This revealed that genetic divergence evolved. The predominant pattern is countergradient variation, where growth and development rates are higher at the cold end of the environmental gradient. A comparison to the latitudinal gradient, showed that divergence is much weaker across elevation. These results suggest that gene flow is reducing the response to divergent selection by introducing alleles adaptive to other habitats. I then investigated if the divergence is adaptive by carrying out a reciprocal transplant experiment. Tadpoles originating from 500 m and above 2000 m of elevation were raised at four ponds at their home elevation and four ponds that differed in about 1800 m in elevation, which allowed to test adaptation to elevation and adaptation to the pond of origin. Tadpoles performed better at their home elevation, but not at their home pond suggesting that divergence across is indeed adaptive with respect to elevation. Third, I studied the physiological causes of countergradient variation. Tadpoles require more energy to achieve high growth and development rates. One hypothesis is that a higher resting metabolic rate provides higher rates of instantaneous energy releases provided for growth. This is indeed the case, there is divergence in resting metabolic rate and tadpoles from high elevations have higher metabolic needs, supporting the so-called metabolic cold adaptation hypothesis. This thesis established adaptive genetic divergence across elevation in a widespread amphibian and shows that adaptation is constrained by high gene flow despite strong divergent selection.

Abstract

This thesis investigates adaptive divergence between populations of a widespread amphibian across its entire elevational distribution which is characterized by both strong divergent selection and high gene flow, two opposing evolutionary forces. It shows that gene flow opposes divergent selection, but is not strong enough to prevent the evolution of genetic divergence and adaptation to elevation. Four evolutionary forces influence genetic divergence between populations: Divergent selection, gene flow, random genetic drift and mutation. Whereas the two latter are random processes, divergent selection and gene flow have opposite impacts on genetic divergence. The differentiation of populations that encounter both strong divergent selection and exchange many alleles depends on the interplay between divergent selection and gene flow: if gene flow overrules divergent selection, phenotypic differences between populations are caused by phenotypic plasticity. Another important factor is the spatial scale across which the species is distributed in relation to the dispersal distance, this defines the grain of underlying environmental change and influences both divergent selection and gene flow between selective environments. In this thesis, I study divergence across the elevational distribution in the common frog (Rana temporaria). The grain of the environmental change between populations at different elevations is high and there is ample gene flow across the gradient, making this system a steep environmental gradient and ideal to study the evolution of genetic divergence in the face of gene flow. R. temporaria has been intensively studied across the latitudinal gradient, an environmental gradient which spans about the same temperature range on a larger spatial scale and has less gene flow between selective environments. This is a great contrast to the elevational gradient. First, I established how much genetic divergence exists across the elevational gradient, by raising tadpoles that originated from different elevations under common lab conditions. This revealed that genetic divergence evolved. The predominant pattern is countergradient variation, where growth and development rates are higher at the cold end of the environmental gradient. A comparison to the latitudinal gradient, showed that divergence is much weaker across elevation. These results suggest that gene flow is reducing the response to divergent selection by introducing alleles adaptive to other habitats. I then investigated if the divergence is adaptive by carrying out a reciprocal transplant experiment. Tadpoles originating from 500 m and above 2000 m of elevation were raised at four ponds at their home elevation and four ponds that differed in about 1800 m in elevation, which allowed to test adaptation to elevation and adaptation to the pond of origin. Tadpoles performed better at their home elevation, but not at their home pond suggesting that divergence across is indeed adaptive with respect to elevation. Third, I studied the physiological causes of countergradient variation. Tadpoles require more energy to achieve high growth and development rates. One hypothesis is that a higher resting metabolic rate provides higher rates of instantaneous energy releases provided for growth. This is indeed the case, there is divergence in resting metabolic rate and tadpoles from high elevations have higher metabolic needs, supporting the so-called metabolic cold adaptation hypothesis. This thesis established adaptive genetic divergence across elevation in a widespread amphibian and shows that adaptation is constrained by high gene flow despite strong divergent selection.

Statistics

Additional indexing

Item Type:Dissertation
Referees:Van Buskirk Josh, Laurila Anssi, Vorburger Christoph, Blanckenhorn Wolf
Communities & Collections:07 Faculty of Science > Institute of Evolutionary Biology and Environmental Studies
Dewey Decimal Classification:570 Life sciences; biology
590 Animals (Zoology)
Language:English
Date:2017
Deposited On:06 Feb 2017 15:00
Last Modified:06 Feb 2017 15:18
Number of Pages:78

Download

Full text not available from this repository.