Skúli Skúlason

• PhD
• Professor (Full) at Hólar University

Climate change is causing rapid increases in temperature which drives genomic changes tied to adaptation. However, predicting the outcomes of climate change presents challenges as the anticipated conditions have yet to be experienced by natural populations. Modelling and lab experiments suggest that natural populations will experience shifts in lif...

In this paper, we argue that the concept of ‘species’ should not be the main focus of research and policies in biodiversity conservation. Diversity is important at all levels of life: within species as well as among them and within and among ecosystems. First, we give a brief overview of the debate about the necessity to find a unified concept of ‘...

Background A major goal in evolutionary biology is to understand the processes underlying phenotypic variation in nature. Commonly, studies have focused on large interconnected populations or populations found along strong environmental gradients. However, studies on small fragmented populations can give strong insight into evolutionary processes i...

Patterns in phenotypic and genotypic diversity within many species are becoming increasingly apparent, although there remain many species for which such patterns have yet to be described adequately. Fishes from recently glaciated ecosystems are likely to be particularly rich in intraspecific diversity, yet current conservation management strategies...

Given the threat of climate change to biodiversity, a growing number of studies are investigating the potential for organisms to adapt to rising temperatures. Earlier work has predicted that physiological adaptation to climate change will be accompanied by a shift in temperature preferences, but empirical evidence for this is lacking. Here, we test...

Within-clutch variation in the size or the amount of yolk in the egg has been widely studied because of its importance for both maternal and ofspring ftness. In particular, egg size has been used as a proxy for survival in first-feeding salmonids alevins. We tested the hypothesis that egg size has implications for the phenotypes (development, physi...

Gaining the ability to predict population responses to climate change is a pressing concern. Using a "natural experiment," we show that testing for divergent evolution in wild populations from contrasting thermal environments provides a powerful approach, and likely an enhanced predictive power for responses to climate change. Specifically, we used...

The costs and benefits of being social vary with environmental conditions, so individuals must weigh the balance between these trade-offs in response to changes in the environment. Temperature is a salient environmental factor that may play a key role in altering the costs and benefits of sociality through its effects on food availability , predato...

Maternal effects have the potential to alter early developmental processes of offspring and contribute to adaptive diversification. Egg size is a major contributor to offspring phenotype, which can influence developmental trajectories and potential resource use. However, to what extent intraspecific variation in egg size facilitates evolution of re...

David Lloyd George Noakes (1942–2020) is best known for his insatiable curiosity, his quick wit and dry sense of humor, his scientific contributions to the field of animal behaviour, and his ability to form and maintain long-lasting connections. His research interests were vast but remained grounded in early life history, behaviour, social behaviou...

Sympatric adaptive phenotypic divergence should be underlain by genomic differentiation between sub‐populations. When divergence drives similar patterns of phenotypic and ecological variation within species we expect evolution to draw on common allelic variation. We investigated divergence histories and genomic signatures of adaptive divergence bet...

Genetic variation in resistance against parasite infections is a predominant feature in host–parasite systems. However, mechanisms maintaining genetic polymorphism in resistance in natural host populations are generally poorly known. We explored whether differences in natural infection pressure between resource‐based morphs of Arctic charr (Salveli...

The topic investigated is the social-ecological system of Arctic charr (Salvelinus alpinus) fishing and aquaculture as a tourism product in an era of climate change. Arctic charr is a resilient salmonid species that was traditionally an important part of the sustenance economy in Arctic and Subarctic communities as a source of fresh food throughout...

Conceptual models of adaptive divergence and ecological speciation in sympatry predict differential resource use, phenotype–environment correlations, and reduced gene flow among diverging phenotypes. While these predictions have been assessed in past studies, connections among them have rarely been assessed collectively. We examined relationships a...

Parallelism, the evolution of similar traits in populations diversifying in similar conditions, provides strong evidence of adaptation by natural selection. Many studies of parallelism focus on comparisons of different ecotypes or contrasting environments, defined a priori, which could upwardly bias the apparent prevalence of parallelism. Here, we...

Background Organismal fitness can be determined at early life-stages, but phenotypic variation at early life-stages is rarely considered in studies on evolutionary diversification. The trophic apparatus has been shown to contribute to sympatric resource-mediated divergence in several taxa. However, processes underlying diversification in trophic tr...

Background Organismal fitness can be determined at early life-stages, but phenotypic variation at these early life-stages has rarely been considered in studies on evolutionary diversification. The trophic apparatus has been shown to contribute to sympatric resource-mediated divergence in several taxa. However, processes underlying this diversificat...

Background Organismal fitness can be determined at early life-stages, but phenotypic variation at early life-stages is rarely considered in studies on evolutionary diversification. The trophic apparatus has been shown to contribute to sympatric resource-mediated divergence in several taxa. However, processes underlying diversification in trophic tr...

In light of global climate change, there is a pressing need to understand and predict the capacity of populations to respond to rising temperatures. Metabolic rate is a key trait that is likely to influence the ability to cope with climate change. Yet, empirical and theoretical work on metabolic rate responses to temperature changes has so far prod...

Eins og kunnugt er blasir við okkur alvarlegur umhverfisvandi sem ógnar lífi á jörðinni. Framferði mannsins á þar stærstan hlut að máli. Á líkingamáli má segja að orðið hafi misgengi milli viðhorfa okkar, siðferðis og lífshátta annars vegar og náttúrunnar hins vegar. Maðurinn hefur í krafti hraðvaxandi tæknilegrar getu sinnar og valds sagt sig úr l...

Parallelism, the evolution of similar traits in populations diversifying in similar conditions, provides good evidence of adaptation by natural selection. Many studies of parallelism have focused on comparisons of strongly different ecotypes or sharply contrasting environments, defined a priori, which could upwardly bias the apparent prevalence of...

The repeatability of adaptive radiation is expected to be scale-dependent, with determinism decreasing as greater spatial separation among "replicates" leads to their increased genetic and ecological independence. Threespine stickleback (Gasterosteus aculeatus) provide an opportunity to test whether this expectation holds for the early stages of ad...

This chapter covers Britain (England, Scotland, and Wales), Ireland (Northern Ireland and Republic of Ireland), Iceland, the Faroe Islands, and Greenland. Brown Trout Salmo trutta, which belongs to the Eurasian and North African species complex, is native to all regions in the North Atlantic Isles (NAI) with the exception of Greenland. Arctic Char...

In light of global climate change, there is a pressing need to understand and predict the capacity of populations to respond to rising temperatures. Metabolic rate is a key trait that is likely to influence the ability to cope with climate change. Yet, empirical and theoretical work on metabolic rate responses to temperature changes has so far prod...

A major goal of evolutionary science is to understand how biological diversity is generated and altered. Despite considerable advances, we still have limited insight into how phenotypic variation arises and is sorted by natural selection. Here we argue that an integrated view, which merges ecology, evolution and developmental biology (eco evo devo)...

Given the threat of climate change to biodiversity, a growing number of studies are investigating the potential for organisms to adapt to rising temperatures through changes in their physiology, morphology, and behaviour. Earlier work has predicted that evolutionary adaptation to climate change will be accompanied by a shift in temperature preferen...

In light of climate change, the ability to predict evolutionary responses to temperature changes is of central importance for conservation efforts. Studying parallel evolution in natural populations inhabiting contrasting thermal environments presents a powerful approach for understanding and predicting responses to increasing temperatures. In this...

Temperature and individual egg size have been long studied in the development of fishes because of their direct effects on individual fitness. Here we studied the combined effects of three important factors for fish development, i.e. egg size, social environment and water temperature. Arctic charr (Salvelinus alpinus), a coldwater fish known to be...

Data set for growth. (TXT)

Data set for behavioural variables. (TXT)

Random effects testing using Likelihood Ratio Test (LRT). The analysed variables are the foraging behaviours (bottom foraging, water column foraging, surface foraging and total foraging) of long-term isolated fish. (DOCX)

Random effects testing using Likelihood Ratio Test (LRT). The analysed variable is the immobility of long-term isolated fish. (DOCX)

The repeatability of adaptive radiation is expected to be scale dependent, with determinism decreasing as greater spatial separation among ″replicates″ leads to their increased genetic and ecological independence. Threespine stickleback ( Gasterosteus aculeatus ) provide an opportunity to test whether this expectation holds for the early stages of...

Gene expression during development shapes the phenotypes of individuals. Although embryonic gene expression can have lasting effects on developmental trajectories, few studies consider the role of maternal effects, such as egg size, on gene expression. Using qPCR, we characterize relative expression of 14 growth and/or skeletal promoting genes acro...

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 phenotyp...

Background: The Arctic charr, Salvelinus alpinus, is likely to be the first fish to colonize northernmost freshwater systems. It often exhibits sympatric morphotypes. Question: How do ecosystem characteristics and the life history of Arctic charr interact to generate food webs in subarctic lakes? Organisms: We focus on allometric dietary patterns o...

Intraspecific phenotypic diversity is the raw material for evolution, so understanding its origin and maintenance is critically important for conservation of biodiversity. Intraspecific diversity in a trait or a suite of traits can result from genetic diversity and/or phenotypic plasticity. The two are, however, not independent as plasticity has be...

Cases of evolutionary diversification can be characterized along a continuum from weak to strong genetic and phenotypic differentiation. Several factors may facilitate or constrain the differentiation process. Comparative analyses of replicates of the same taxon at different stages of differentiation can be useful to identify these factors. We esti...

A key question in evolutionary biology is to understand which factors shape biological diversity. This is especially true at the intra-specific level, where evolutionary and ecologically processes interact to shape the phenotypic and genetic structure of natural populations. Long-term monitoring studies on highly replicated wild populations are par...

The primary target of selection in most breeding programmes in aquaculture is to promote rapid growth while delaying maturation. In most cases, little is known about how such selection programmes may affect maternal provisioning of offspring and early development. Under favourable and predictable condition, which is typically the case in fish farmi...

When genetic constraints restrict phenotypic evolution, diversification can be predicted to evolve along so-called lines of least resistance. To address the importance of such constraints and their resolution, studies of parallel phenotypic divergence that differ in their age are valuable. Here, we investigate the parapatric evolution of six lake a...

The genetic variance that determines phenotypic variation can change across environments through developmental plasticity and in turn play a strong role in evolution. Induced changes in genotype-phenotype relationships should strongly influence adaptation by exposing different sets of heritable variation to selection under some conditions, while al...

Hér er gerð grein fyrir rannsóknum okkar á íslenskri dvergbleikju. Dvergbleikja hefur fundist víða um land og virðist hafa þróast endurtekið og aðlagast að hraun- og lindarbúsvæðum. Dvergbleikjur eru í öllum tilfellum smávaxnar, kubblsegar, undirmyntar og dökkar á lit. Þær bea fjölmörg einkenni ungviðis bleikju, sem bendir til sértækra þróunarleiða...

This study investigated how dietary habits vary with lake characteristics in a fish species that exhibits extensive morphological and ecological variability, the Arctic charr Salvelinus alpinus. Iceland is a hotspot of geological activity, so its freshwater ecosystems vary greatly in physical and chemical attributes. Associations of dietary items w...

Parasite communities of fishes are known to respond directly to the abiotic environment of the host, for example, to water quality and water temperature. Biotic factors are also important as they affect the exposure profile through heterogeneities in parasite distribution in the environment. Parasites in a particular environment may pose a strong s...

We evaluated hypotheses of intralacustrine diversification and plastic responses to two diet environments in Icelandic Arctic charr (Salvelinus alpinus). Full-sib families of progeny of wild polymorphic charr from two lakes where morphs vary in their degree of phenotypic and ecological divergence were split, with half of the offspring reared on a b...

Morphological, dietary and life-history variation in Arctic charr Salvelinus alpinus were characterized from three geographically proximate, but isolated lakes and one large lake into which they drain in south-western Alaska. Polymorphism was predicted to occur in the first three lakes because S. alpinus tend to become polymorphic in deep, isolated...

Background: The mechanisms causing resource polymorphism are not well understood, but likely include frequency-dependent selection. However, other selection mechanisms could also explain the development of resource polymorphism. Comparative analyses of polymorphic and monomorphic systems are uncommon, making it difficult to distinguish the effects...

Background: Finite-mixture models allow sub-populations to be described by separate parameter sets, but do not require the members of sub-populations to be identified. Although the accessibility of these methods has greatly increased in recent years, they are rarely used in studies of ecology and evolution, despite their ability to address basic an...

It is critical to study factors that are important for origin and maintenance of biological diversity. A comparative approach involving a large number of populations is particularly useful. We use this approach to study the relationship between ecological factors and phenotypic diversity in Icelandic Arctic charr (Salvelinus alpinus). Numerous popu...

Fish constitute the most species-rich group of the vertebrates. They have adapted to highly diverse habitats in the sea and fresh waters of the world and display great phenotypic variability. Some fish groups have gone through extremely rapid local radiations with clear connections to ecological factors. Such extensive radiations are, for example,...

Current data on the Y-specific sex-determining region of salmonid fishes from genera Salvelinus, Salmo, and Oncorhynchus indicate variable polymorphisms in the homologous chromosomal locations of the sex-specific determining region. In the majority of the Atlantic lineage Arctic charr, including populations from the Fraser River, in Labrador Canada...

Salmonid exhibit complex life histories ranging from populations displaying multiple saltwater migrations to populations that are freshwater resident or landlocked. Such diversity has been of interest for evolutionary biologists in understanding the maintenance of diversity but in turn it has been a constant challenge for management of populations....

The common occurrence of parallel phenotypic patterns suggests that a strong relationship exists between ecological dynamics and micro-evolution. Comparative studies from a large number of populations under varying sets of ecological drivers could contribute to a better understanding of this relationship. We used data on morphology of arctic charr...

The high commercial value from the aquaculture of salmonid fishes has prompted many studies into the genetic architecture of complex traits and the need to identify genomic regions that have repeatable associations with trait variation both within and among species. We searched for quantitative trait loci (QTL) for body weight (BW), condition facto...

Early behaviour can determine food intake and growth rate with important consequences for life history and survival in fishes. Egg size is known to affect growth rate of young Arctic charr but its influence on the development of behaviour is poorly documented. It is believed that egg size influence on growth and potentially on the behaviour of youn...

Phenotypic plasticity is a developmental process that plays a role as a source of variation for evolution. Models of adaptive divergence make the prediction that increasing ecological specialization should be associated with lower levels of plasticity. We tested for differences in the magnitude, rate and trajectory of morphological plasticity in tw...

Resource polymorphism has been proposed as an important phase of diversification and speciation in vertebrates. Studies of fish in young lakes of the Northern Hemisphere indicate variably advanced cases of adaptive trophic diversification. We have previously proposed a scheme describing this variation in terms of a gradient of resource-based polymo...

Most salmonids, as well as many other freshwater fish species in the Northern Hemisphere, have been reported to show some form of daytime sheltering behaviour over the winter. Previous work has shown that temperatures around 6-8°C trigger the onset of this sheltering behaviour. However, fish from colder environments would be expected to respond dif...

Four morphs of arctic char, Salvelinus alpinus, which differ substantially in morphology, behaviour, and life-history characteristics, occupy different habitats in the landlocked lake Thingvallavatn, Iceland. Progeny of wild morphs reared in a common environment showed early size differences among morphs that were maintained throughout the 3-year e...

The common occurrence of parallel phenotypic patterns suggests that a strong relationship exists between ecological dynamics and micro-evolution. Comparative studies from a large number of populations under varying sets of ecological drivers could contribute to a better understanding of this relationship. We used data on morphology of arctic charr...

Natural selection requires genetically based phenotypic variation to facilitate its action and cause adaptive evolution. It has become increasingly recognized that morphological development can become canalized likely as a result of selection. However, it is largely unknown how selection may influence canalization over ontogeny and differing enviro...

Abstract Five groups of Arctic charr, Salvelinus alpinus (L.), were studied, each group consisting of 13 females. The fish in four of the groups received a single intramuscular (IM) injection: three groups were injected with LHRHa in various forms and dosages, and one was injected with saline (control). The females in the fifth group were implanted...

We studied the salmonid fish Arctic charr, Salvelinus alpinus, in a small and shallow landlocked lake in NW Iceland. The lake is productive but die only fish present is Arctic charr. Despite the apparent absence of discrete benthic and limnetic habitats for fish, two forms of Arctic charr are found in the lake. They show subde differences in morpho...

Abstract – This study examined the degree and pattern of variability in trophic morphology in Arctic charr (Salvelinus alpinus L.) at three spatial scales: across 22 populations from Scotland and between and within two adjacent catchments (Laxford and Shin) in northern Scotland. In addition, the variability at six microsatellite loci between and wi...

Arctic charr (Salvelinus alpinus L.) from Lake Thingvallavatn, Iceland occur as four distinct morphs: large benthivorous (LB), dwarf benthivorous (DB), piscivorous (PI) and planktonivorous (PL). The morphs differ with respect to body size, head morphology, growth rate, and life history. The aim of this study was to investigate the paired box protei...

There is now increasing acceptance that divergence of phenotypic traits, and the genetic structuring that underlie such divergence, can occur in sympatry. Here we report the serendipitous discovery of a sympatric polymorphism in the upper Forth catchment, Scotland, in a species for which high levels of phenotypic variation have been reported previo...

An experiment was conducted to test whether handling, anaesthesia or repeated formalin bathing had any effect on the growth rate in first feeding Arctic charr held in separate containers. The results show that these treatments had no effect on the growth rate.

Restriction fragment length polymorphisms in mitochondrial DNA (mtDNA) were examined in progeny of four sympatric morphs of Arctic charr (Salvelinus alpinus L.) from Thingvallavatn, Iceland. The mtDNA analysis with 46 hexanucleotide restriction enzymes indicates that the Thingvallavatn morphs are very closely related. Sequence divergence was less t...

The relationships among time of spawning, incubation temperature, timing of first feeding and early growth were examined in four sympatric morphs of Arctic charr in Thingvallavatn, Iceland. Large benthivorous charr spawn in July-August at sites with cold ground-water flow. Planktivorous and piscivorous charr spawn in September-November and are not...

Threespine stickleback most often have 10 pectoral fin rays, and it seems to be a highly canalized trait. We observed an unusually high frequency of stickleback with 11 pectoral fin rays in a population recently isolated from the marine environment in a freshwater lagoon in Iceland. These new morphologies may be beneficial for the fish in the new h...

Planktivorus Arctic charr had larger eggs than small benthivorous charr and the progeny of the former were longer (total length) at days 125, 145 and 159 after fertilization. Size differences remained significant after the removal of egg size effect on embryo size. Size of hybrid progeny tended to be similar to their maternal pure progeny group, su...

Thingvallavatn, the largest and one of the oldest lakes in Iceland,contains four morphs of Arctic charr Salvelinus alpinus. Dwarf benthic (DB), large benthic (LB), planktivorous (PL) and piscivorous (PI)morphs can be distinguished and differ markedly in head morphology,colouration and maximum fork length (FLmax), reflecting their different resource...

Icelandic freshwater systems are geologically young and contain only six species of freshwater fish. As these species colonized Icelandic fresh waters they were presented with a diversity of unique, uncontested habitats and food resources, promoting the evolution of new behaviour strategies crucial to the formation of new morphs and specia-tion. To...

To examine the population genetic structure of lake-resident Arctic charr, Salvelinus alpinus from northwest Europe on multiple spatial scales, 2367 individuals from 43 lakes located in three geographical regions (Iceland, the British Isles and Scandinavia) were genotyped at six microsatellite loci. On a large scale, data provided little evidence t...

Young Arctic charr Salvelinus alpinus, derived from one male and one female only from Lake Ölvesvatn in northern Iceland (a stock that has been popular in Icelandic aquaculture), showed large variance in body size, primarily as a consequence of variable egg size. Shortly after the onset of exogenous feeding, large fish (0·11–0·14 g) were more activ...

Previous studies have found no consistent relationship between egg size and the rate of development in fish. This is surprising since there are several hypothesis as to why such a relationship should exist. By using eggs from Arctic charr, Salvelinus alpinus, we examined several developmental features repeatedly on live individuals. In all measured...

In 1987, a fjord in the Snæfellsnes peninsula, north-west Iceland, was dammed and a fresh- water lagoon formed. There is a large population of threespine stickleback in this lagoon. We compared morphological features of stickleback in the lagoon population to those of their marine ancestor, and morphological polymorphism within the lagoon in relati...

Icelandic threespine sticklebacks show parallel sympatric morphological differences related to different substrate habitats in four Icelandic lakes. The level of morphological diversification varies among the lakes, ranging from a population with a wide morphological distribution to a population with clear resource morphs, where morphological diver...

Resource polymorphism has been proposed as an important phase of diversification and speciation in vertebrates. Studies of fish in young lakes of the Northern Hemisphere indicate variably advanced cases of adaptive trophic diversification. We have previously proposed a scheme describing this variation in terms of a gradient of resource-based polymo...

The astounding breadth of diversity of life on earth intrigues and amazes many people, while the future of world biodiversity is a cause for widespread concern. Within the current context of global interest in biological diversity, this is a timely review of the most recent research into the evolutionary origins of biological diversity and the proc...

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Resource polymorphism in vertebrates is generally under-appreciated as a diversifying force and is probably more common than is currently recognised. Research across diverse taxa suggest that they may play a more important roles in population divergence and speciation. They may involve various kinds of traits, including morphological and behavioura...

Discrete resource polymorphisms occur in various vertebrate species and probably occur more frequently than is generally appreciated. They are manifested in a number of ways, including morphological, behavioral and life history characters. Research on a number of unrelated taxa suggests that resource polymorphisms may be underestimated as a diversi...

Lake Thingvallavatn supports four trophic morphs of Arctic charr, Salvelinus alpinus (L.); two of the morphs are benthic (small and large benthivorous charr) one exploits pelagic waters (planktivorous charr) and the fourth is found in both habitats (piscivorous charr). The morphological variation among these morphs was analysed by use of principal...

Lake Thingvallavatn supports four trophic morphs of Arctic charr, Salvelinus alpinus (L.); two of the morphs are benthic (small and large benthivorous charr) one exploits pelagic waters (planktivorous charr) and the fourth is found in both habitats (piscivorous charr). The morphological variation among these morphs was analysed by use of principal...

Four morphs of arctic charr, Salvelinus alpinus, coexist in the landlocked lake Thingvallavatn, Iceland. The morphs occupy different habitats and are segregated in diet year round. Feeding trials, involving laboratory-reared progeny of three morphs, indicated that several behavioural differences are genetically based, supporting the claim that the...

The coexistence of four morphs of arctic charr in Thingvallavatn, Iceland. was demonstrated by investigations of morphology, habitat use, diet, endoparasitic fauna, life history, time and place of spawning, early ontogeny, and population genetics. Head morphology in embryos, juveniles and adults revealed the presence of two charr morphotypes, each...

The annual yield of the commercial fishery for planktivorous charr ("murta") in Thingvallavatn has varied between zero and 73 tonnes since 1958, with two periods of high catches: 1963-1965 and 1976-1984. The commercial fishery is performed with 22 mm bar mesh gill nets on the spawning grounds. The nets selectively catch mature fish over 17.5 cm in...