Diversity in the internal functional feeding elements of sympatric morphs of Arctic charr (Salvelinus alpinus).

The diversity of functional feeding anatomy is particularly impressive in fishes and correlates with various interspecific ecological specializations. Intraspecific polymorphism can manifest in divergent feeding morphology and ecology, often along a benthic-pelagic axis. Arctic charr (Salvelinus alpinus) is a freshwater salmonid known for morphological variation and sympatric polymorphism and in Lake Þingvallavatn, Iceland, four morphs of charr coexist that differ in preferred prey, behaviour, habitat use, and external feeding morphology. We studied variation in six upper and lower jaw bones in adults of these four morphs using geometric morphometrics and univariate statistics. We tested for allometric differences in bone size and shape among morphs, morph effects on bone size and shape, and divergence along the benthic-pelagic axis. We also examined the degree of integration between bone pairs. We found differences in bone size between pelagic and benthic morphs for two bones (dentary and premaxilla). There was clear bone shape divergence along a benthic-pelagic axis in four bones (dentary, articular-angular, premaxilla and maxilla), as well as allometric shape differences between morphs in the dentary. Notably for the dentary, morph explained more shape variation than bone size. Comparatively, benthic morphs possess a compact and taller dentary, with shorter dentary palate, consistent with visible (but less prominent) differences in external morphology. As these morphs emerged in the last 10,000 years, these results indicate rapid functional evolution of specific feeding structures in arctic charr. This sets the stage for studies of the genetics and development of rapid and parallel craniofacial evolution.

Adaptive cellular evolution or cellular system drift in hares.

Adaptations occur at many levels, for example, from DNA sequence of regulatory elements and cellular homeostatic systems to organismal physiology and behaviour (Mayr, 1997). Established adaptations are maintained by purifying and stabilizing selection. Students of animal diversity tend to focus on higher order traits, anatomy, physiology, organismal function and interactions. The core cellular and metabolic systems of metazoans evolved early in their history and are assumed to be rather similar between groups. The housekeeping functions and core metabolic functions of cells are generally considered relatively static, especially among closely related species. The extent to which evolution shapes core cellular metabolism and physiology in animals is largely unexplored. Ecological opportunities or strong positive selection can alter basal metabolic rate, activity levels and life-history traits (e.g., life span, age of maturity, offspring number) and potentially lead to divergence in core cellular and metabolic trait systems (Norin & Metcalfe, 2019; Speakman, 2005). Furthermore, systems under stabilizing selection can also change. Developmental systems of related species may produce the same phenotype or structure, but experience drift that can alter connections and even lead to turnover of cogs in the system (True & Haag, 2001). Are the cellular functions of animals highly constrained, subject to cellular system drift or affected by positive selection? This was tackled by a new study by Kateryna Gaertner and colleagues in a From the Cover manuscript in this issue of Molecular Ecology (Gaertner et al., 2022), using fibroblasts from the closely related but ecologically distinct brown and mountain hares.