Natural selection for body shape in resource polymorphic Icelandic Arctic charr

Abstract

Resource polymorphisms exhibit remarkable intraspecific diversity and in many cases are expected to be maintained by diversifying selection. Phenotypic trade-offs can constrain morphologically intermediate individuals from effectively exploiting both alternate resources, resulting in ecological barriers to gene flow. Determining if and how phenotypic trade-offs cause fitness variation in the wild is challenging because of phenotypic and environmental correlations associated with alternative resource strategies. We investigated multiple pathways through which morphology could affect organismal performance, as measured by growth rate, and whether these effects generate diversifying selection in polymorphic Icelandic Arctic charr (Salvelinus alpinus) populations. We considered direct effects of morphology on growth and indirect effects via trophic resource use, estimated by stable isotopic signatures, and via parasitism associated with trophic resources. We sampled over 3 years in (lakes) Thingvallavatn and Vatnshlíðarvatn using the extended selection gradient path analytical approach and estimating size-dependent mortality. We found evidence for diversifying selection only in Thingvallavatn: more streamlined and terminally mouthed planktivore charr experienced greater growth, with the opposite pattern in small benthic charr. However, this effect was mediated by parasitism and nontrophic pathways, rather than trophic performance as often expected. Detection of between-morph differences in the presence (Vatnshlíðarvatn) and direction (Thingvallavatn) of size-dependent mortality, together with nontrophic effects of shape, suggests that a morphological trophic performance explanation for polymorphism is insufficient. This rare insight into selection during early diversification suggests that a complex of interacting local factors must be considered to understand how phenotype influences fitness, despite morphological variation reflecting intuitive trade-off explanations.

A way forward with eco evo devo: an extended theory of resource polymorphism with postglacial fishes as model systems

“…specific ecological factors) that structure local selection regimes (e.g. MacColl, ), they show that a range of biotic and abiotic agents of selection (and their interactions) are associated with phenotypic divergence (Robinson & Wilson, ; Smith & Skúlason, ; Robinson & Schluter, ; Knudsen, Amundsen, & Klemetsen, ; Siwertsson et al ., ; Bartels et al ., ; Keller & Seehausen, ; Woods et al ., ; Franklin et al ., ). Biotic factors include low levels of interspecific but high levels of intraspecific competition, as well as interactions with prey, predators and parasites.…”

Multivariate distributions of behavioural, morphological, and ontogenetic traits in hybrids bring new insights into the divergence of sympatric Arctic charr morphs

“…However, numerous differentiated regions scattered across the genome of the PL and the SB charr have been identified recently [47], suggesting that a large number of traits may have diverged in response to divergent selection. More precisely, the differences in head and body shape between SB and PL charr were shown to result from divergent selection related to trophic and non-trophic environmental variables [89]. Thus, it may be reasonable to assume that the phenotypic values we observe in these two morphs approximated their respective fitness optima, and that deviation from these values as observed in the hybrids may be detrimental.…”

Temporal instability of lake charr phenotypes: synchronicity of growth rates and morphology linked to environmental variables?

“…Growth rate in fishes has been shown to drive intraspecific morphological differentiation (Tonn et al 1994;Olsson et al 2007;Chivers et al 2008). Growth rate seems to play a key role in regulating morphological expression, but underlying mechanisms remain uncertain (Olsson et al 2006;Svanbäck et al 2017;Franklin et al 2018). A possible explanation is that at higher growth rates, energy is allocated to somatic growth and morphology modulation in addition to metabolic maintenance, but that at lower growth rates, energy is used almost exclusively for metabolic maintenance and/or reproduction (Olsson et al 2006;Svanbäck et al 2017).…”