Abstract
Local adaption through ecological niche specialization can lead to genetic structure between and within populations. In the Northeast Pacific, killer whales (Orcinus orca) of the same population have uniform specialized diets that are non‐overlapping with other sympatric, genetically divergent, and socially isolated killer whale ecotypes. However, killer whales in Iceland show intrapopulation variation of isotopic niches and observed movement patterns: some individuals appear to specialize on herring and follow it year‐round while others feed upon herring only seasonally or opportunistically. We investigated genetic differentiation among Icelandic killer whales with different isotopic signatures and observed movement patterns. This information is key for management and conservation purposes but also for better understanding how niche specialization drives genetic differentiation. Photo‐identified individuals (N = 61) were genotyped for 22 microsatellites and a 611 bp portion of the mitochondrial DNA (mtDNA) control region. Photo‐identification of individuals allowed linkage of genetic data to existing data on individual isotopic niche, observed movement patterns, and social associations. Population subdivision into three genetic units was supported by a discriminant analysis of principal components (DAPC). Genetic clustering corresponded to the distribution of isotopic signatures, mtDNA haplotypes, and observed movement patterns, but genetic units were not socially segregated. Genetic differentiation was weak (F ST < 0.1), suggesting ongoing gene flow or recent separation of the genetic units. Our results show that killer whales in Iceland are not as genetically differentiated, ecologically discrete, or socially isolated as the Northeast Pacific prey‐specialized killer whales. If any process of ecological divergence and niche specialization is taking place among killer whales in Iceland, it is likely at a very early stage and has not led to the patterns observed in the Northeast Pacific.
Highlights
Understanding patterns of population genetic structure and the processes driving those patterns is highly relevant for conservation purposes, since smaller population units are more vulnerable to ex‐ tinction (Stevick et al, 2006)
Much knowl‐ edge about killer whales comes from long‐term studies on two sympatric and socially segregated ecotypes in the Northeast Pacific (e.g., Ford et al, 1998; Ford, Ellis, & Balcomb, 2000): (a) the “resi‐ dent” fish‐eating killer whales, which feed primarily on salmon; and (b) the mammal‐eating, which feed on marine mammals
There is no social isolation between individuals with different observed movement patterns and isotopic signatures, that is, putative herring‐specialists remaining year‐round in Iceland have been photographed in close proximity with Icelandic‐Scottish killer whales (Tavares, Samarra, & Miller, 2017)
Summary
The socioecological characteristics of a species, such as fidelity to specific natal breeding or feeding grounds (e.g., Carroll et al, 2015; Kershaw et al, 2017; Valenzuela, Sironi, Rowntree, & Seger, 2009) and local adaptation to specific habitats, for example through niche specialization (e.g., Foote et al, 2016; Hoelzel, Dahlheim, & Stern, 1998; Smith & Skúlason, 1996), can determine genetic divergence of populations. Isotopic analyses of biopsy sampled individuals with different observed movement patterns show that these largely correspond to different isotopic niche widths (Samarra, Vighi, Aguilar, & Víkingsson, 2017) Individuals seen in both seasons had lower nitrogen stable isotope ratios (15N/14N, represented as δ15N), consistent with a diet predominantly composed of herring,. There is no social isolation between individuals with different observed movement patterns and isotopic signatures, that is, putative herring‐specialists remaining year‐round in Iceland have been photographed in close proximity with Icelandic‐Scottish killer whales (Tavares, Samarra, & Miller, 2017). It is unknown whether the apparent absence of social isolation in the Icelandic population corresponds to an absence of genetic divergence among individuals with different isotopic values and observed movement patterns. We interpret the identified genetic patterns among Icelandic killer whales, discuss potential ecological and behavioral processes driving them and how they correspond to the patterns of the best‐studied Northeast Pacific ecotypes
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