Natural selection for body shape in resource polymorphic Icelandic Arctic charr.

Area of Science:

• Ecology
• Evolutionary Biology
• Fisheries Science

Background:

• Resource polymorphisms drive intraspecific diversity, often maintained by diversifying selection.
• Phenotypic trade-offs can create ecological barriers to gene flow between morphs.
• Linking morphology, performance, and fitness in natural populations is challenging due to confounding factors.

Purpose of the Study:

• Investigate how morphology affects organismal performance and generates diversifying selection in polymorphic Arctic charr (Salvelinus alpinus).
• Examine direct and indirect pathways (trophic resource use, parasitism) linking morphology to growth rate.
• Determine if phenotypic trade-offs influence fitness variation in wild populations.

Main Methods:

• Employed the extended selection gradient path analytical approach over three years in Icelandic lakes.
• Estimated trophic resource use via stable isotopic signatures.
• Assessed growth rate and size-dependent mortality to infer fitness consequences.

Main Results:

• Diversifying selection was detected in Thingvallavatn, favoring streamlined planktivores and smaller benthic charr.
• Selection was mediated by parasitism and non-trophic pathways, contrary to expectations of trophic performance.
• Size-dependent mortality differed between morphs and lakes, influenced by non-trophic effects of shape.

Conclusions:

• Morphological trophic performance alone is insufficient to explain Arctic charr polymorphism.
• A complex interplay of local factors, including non-trophic effects and parasitism, shapes fitness.
• Understanding early diversification requires considering multiple interacting pathways beyond intuitive trade-offs.