Ecomorphological convergence following niche shifts in montane ground beetles (Carabidae: Nebria)

Aim: A critical step towards uncovering generalizable patterns of phenotype-niche relationships is understanding how functional traits have evolved as species occupy new habitats. Ecomorphological traits impact how organisms function in their environment and are predictive of habitat use and niche. Studying ecomorphological variation in the context of strong environmental filtering can provide opportunities to understand the role of convergent evolution in forming trait-habitat use patterns. By integrating a molecular phylogeny, habitat use, and morphometrics, this study aimed to understand the role of ancestry and convergent evolution in ecomorphological trait evolution.

Location: Montane and alpine streambeds, talus, and snowfields in Holarctic mountain ranges (0-5000 m. above sea level).

Time period: 1970-2021

Major Taxa Studied: Seventy-nine species comprising three species complexes in the ground beetle genus Nebria (Carabidae: Nebriini).

Methods: Morphological measurements including pronotal ratio (widest point divided by the base), elytral length, elytral ratio (length divided by width), antennal scape length, and pronotal and elytral shape (Fourier decomposition) were used in this study, in conjunction with measurements of habitat use habitat where specimens were collected. Morphological variation was examined in relation to habitat use and phylogenetic relatedness, and morphological trait evolution was tested for convergence.

Results: Ecomorphological traits are evolving slower than expected under a null model of Brownian motion evolution. Nebria species cluster in multivariate morphospace according to relatedness, but habitat use and relatedness together are the best predictors of morphological variation. There is evidence for convergence in riparian species based on morphologicy alone, and additional evidence for morphological convergence in riparian and alpine species when phylogenetic distance is considered. 

Main conclusions: In species assemblages of Nebria, we found evidence of rapid diversification followed by a slow rate of ecomorphological evolution, with convergent evolution playing a significant role in shaping trait-habitat use patterns and niche acquisition.
Methods
Specimens from the California Academy of Sciences were used. Morphometrics were collected by photographing specimens on a Leica camera (Leica AG, 2020) and measured in ImageJ. Silhouettes of morphological features were created in Inkscape (Inkscape, 2020) and analyzed using Fourier decomposition analysis using the R package momocs (Bonhomme et al., 2014). Methods from Sansalone et al., 2020 and Stayton, 2015 were used to test for convergence. The R package RRphylo (Castiglione et al., 2019) was used to assess the rate of trait evolution. The online database Worldclim (Fick & Hijmans, 2017) was used to extract bioclimatic data for species. Google earth (Google Maps 2021) was used to estimate lat/long and elevation of species based on museum locality data. All analyses were performed using R statistical software v4.2.0.1 (R Core Team, 2021).