Adaptation to elevation but limited local adaptation in an amphibian.

We performed a reciprocal transplant experiment to estimate "parallel" adaptation to elevation and "unique" adaptation to local sites at the same elevation, using the frog Rana temporaria in the Swiss Alps. It is important to distinguish these two processes because they have different implications for population structure and ecological specialization. Larvae were reared from hatching to metamorphosis within enclosures installed in their pond of origin, in three foreign ponds at the same elevation, and in four ponds at different elevation (1500-2000 m higher or lower). There were two source populations from each elevation, and adults were held in a common environment for 1 year before they were crossed to produce offspring for the experiment. Fitness was a measure that integrated larval survival, development rate, and body size. Parallel adaptation to elevation was indicated by an advantage at the home elevation (11.5% fitness difference at low elevation and 47% at high elevation). This effect was stronger than that observed in most other studies, according to a survey of previous transplant experiments across elevation (N = 8 animal species and 71 plants). Unique local adaptation within elevational zones was only 0.3-0.7 times as strong as parallel adaptation, probably because gene flow is comparatively high among nearby wetlands at the same elevation. The home-elevation advantage may reduce gene flow across the elevational gradient and enable the evolution of habitat races specialized on elevation.

Gene Flow Limits Adaptation along Steep Environmental Gradients.

When environmental variation is spatially continuous, dispersing individuals move among nearby sites with similar habitat conditions. But as an environmental gradient becomes steeper, gene flow may connect more divergent habitats, and this is predicted to reduce the slope of the adaptive cline that evolves. We compared quantitative genetic divergence of Rana temporaria frog populations along a 2,000-m elevational gradient in eastern Switzerland (new experimental results) with divergence along a 1,550-km latitudinal gradient in Fennoscandia (previously published results). Both studies found significant countergradient variation in larval development rate (i.e., animals from cold climates developed more rapidly). The cline was weaker with elevation than with latitude. Animals collected on both gradients were genotyped at ∼2,000 single-nucleotide polymorphism markers, revealing that dispersal distance was 30% farther on the latitudinal gradient but 3.9 times greater with respect to environmental conditions on the elevational gradient. A meta-analysis of 19 experimental studies of anuran populations spanning temperature gradients revealed that countergradient variation in larval development, while significant overall, was weaker when measured on steeper gradients. These findings support the prediction that adaptive population divergence is less pronounced, and maladaptation more pervasive, on steep environmental gradients.

Real-time social selection maintains honesty of a dynamic visual signal in cooperative fish.

Our understanding of animal communication has been largely driven by advances in theory since empirical evidence has been difficult to obtain. Costly signaling theory became the dominant paradigm explaining the evolution of honest signals, according to which communication reliability relies on differential costs imposed on signalers to distinguish animals of different quality. On the other hand, mathematical models disagree on the source of costs at the communication equilibrium. Here, we present an empirical framework to study the evolution of honest signals that generates predictions on the form, function, and sources of reliability of visual signals. We test these predictions on the facial color patterns of the cooperatively breeding Princess of Burundi cichlid, Neolamprologus brichardi. Using theoretical visual models and behavioral experiments we show that these patterns possess stable chromatic properties for efficient transmission in the aquatic environment, while dynamic changes in signal luminance are used by the fish to communicate switches in aggressive intent. By manipulating signal into out-of-equilibrium expression and simulating a cheater invasion, we demonstrate that social costs (receiver retaliation) promote the honesty of this dynamic conventional signal. By directly probing the sender of a signal in real time, social selection is likely to be the mechanism of choice shaping the evolution of inexpensive, yet reliable context-dependent social signals in general.