Abstract
Divergence in phenotypic traits may arise from the interaction of different evolutionary forces, including different kinds of selection (e.g., ecological), genetic drift, and phenotypic plasticity. Sensory systems play an important role in survival and reproduction, and divergent selection on such systems may result in lineage diversification. Such diversification could be largely influenced by selection in different environments as a result of isolation by environment (IbE). We investigated this process using geographic variation in the resting echolocation frequency of the horseshoe bat species, Rhinolophus damarensis, as a test case. Bats were sampled along a latitudinal gradient ranging from 16°S to 32°S in the arid western half of southern Africa. We measured body size and peak resting frequencies (RF) from handheld individual bats. Three hypotheses for the divergence in RF were tested: (1) James’ Rule, (2) IbE, and (3) genetic drift through isolation by distance (IbD) to isolate the effects of body size, local climatic conditions, and geographic distance, respectively, on the resting frequency of R. damarensis. Our results did not support genetic drift because there was no correlation between RF variation and geographic distance. Our results also did not support James' Rule because there was no significant relationship between (1) geographic distances and RF, (2) body size and RF, or (3) body size and climatic variables. Instead, we found support for IbE in the form of a correlation between RF and both region and annual mean temperature, suggesting that RF variation may be the result of environmental discontinuities. The environmental discontinuities coincided with previously reported genetic divergence. Climatic gradients in conjunction with environmental discontinuities could lead to local adaptation in sensory signals and directed dispersal such that gene flow is restricted, allowing lineages to diverge. However, our study cannot exclude the role of processes like phenotypic plasticity in phenotypic variation.
Highlights
Every region has its own geological past, with unique flora and fauna shaped by environmental variation and barriers to gene flow that exist between geographic areas (Neuweiler, 2000)
The calculations required the following information: (1) climatic conditions (e.g., annual mean temperature (AMT), relative humidity (RH), and atmospheric pressure) at each sampled site, where atmospheric pressure was kept at that for normal atmospheric conditions, taken as 101.325 pascal; (2) resting frequency (Hz) of each individual bat; (3) the dynamic range, which is the difference between peak intensity measured at 1 m and the auditory threshold of the bat (assumed to be 0 dB SPL for horseshoe bats (Holderied & von Helversen, 2003; Long & Schnitzler, 1975); (4) reflection loss, C1, which accounts for the fraction of the energy reflected, and (5) geometric spreading, C2, which quantifies the loss of energy due to spreading multiplied by 2 for both outgoing emitted call and the returning echo
We further explored whether selection or stochastic processes were responsible for resting frequency (RF) variation by running separate linear mixed-effects models (LMEs) on just the southern populations
Summary
Every region has its own geological past, with unique flora and fauna shaped by environmental variation and barriers to gene flow that exist between geographic areas (Neuweiler, 2000). Atmospheric conditions (i.e., climate) can exert a strong influence on geographic variation of complex signals, such as bird song (Lengagne & Slater, 2002), frog mating calls (Lingnau & Bastos, 2007), and the echolocation calls of bats (Lawrence & Simmons, 1982; Luo, Kosel, Zsebok, Siemers, & Goerlitz, 2014) These acoustic signals may diverge along climatic gradients as a result of variation in atmospheric attenuation of sound. Other studies have suggested that bat echolocation call frequency diverges along climatic gradients (Snell- Rood, 2012) Such environmentally mediated differences in sensory systems leading to lineage diversification can be facilitated by geographic or environmental isolation of populations (Schluter, 2001). We currently lack data on gene flow and dispersal to adequately test the role of phenotypic plasticity and other evolutionary processes, but we discuss their potential influence in promoting phenotypic variation
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