The mechanisms behind speciation have been subject of debate for centuries. The presence of resource polymorphism has been discovered to play a significant part in this process, and has been proven to induce phenotypic and genetic divergence. Although resource polymorphism has been intensely studied during the last few decades, there is a gap of information as to why this can be observed in some systems but not in others. Recent studies of European whitefish (Coregonus lavaretus) in Scandinavian lakes have shown that predation, in this case by Northern pike (Esox lucius), could be the factor that induces resource polymorphism. European whitefish is known to diverge into several ecomorphs in Scandinavian lakes, but only in the presence of pike. Divergence is assumed to be caused by the size selectivity of pike, and the following niche separation and eventually reproductive isolation. In this study, pike prey selectivity was studied in the field through sample fishing using hooks baited with whitefish of different sizes. The hypothesis was that pike prefers smaller prey over larger and mainly hunts in the littoral zone. This should causes smaller whitefish ecomorphs to be prone to predation in the littoral and thus utilize refuge spawning grounds with low predation pressure. However, no pike were caught on whitefish spawning grounds, and fishing at two additional pike rich sites displayed a preference towards medium-sized whitefish (p < 0.05). This indicates a size selectivity, although further and improved studies would be required to answer the question of the pike’s role in resource polymorphism.

Phenotypic plasticity in Eurasian perch (Perca fluviatilis) can be driven by a trade-off for ecological specialisation to littoral and pelagic resources. Previous studies on perch have found that this specialisation can have different effects on linkage between the littoral and pelagic food web depending on water transparency. In this study I aimed to answer how foraging efficiency and prey preference of phenotypic divergent perch are affected by high and low water transparency, and the presence of a predator in a series of aquarium experiments. Two different phenotypes of perch were kept in littoral and pelagic environments in the lab. By presenting perch with Daphnia sp. and Ephemeroptera, either separately or combined. I found that in clear water the littoral and pelagic phenotypes were comparatively more efficient on resources that were representative of their habitats (Ephemeroptera and Daphnia, respectively) and that both phenotypes prefer Ephemeroptera over Daphnia. In low visibility the differences in foraging efficiency between phenotypes when feeding on Daphnia disappeared but remained similar to clear water when feeding on Ephemeroptera. When vision was constrained littoral and pelagic perch showed no sign of prey preferences. In the presence of a predator the difference in foraging efficiency between the phenotypes, and also prey preference disappeared. I found that littoral phenotypes interacted more with other group members than did pelagic phenotypes, when foraging on littoral prey. And for perch in general, when foraging for Daphnia the interaction among group members was markedly reduced compared to when foraging for Ephemeroptera. In this study I show that morphological adaptation and prey choice is affected by visibility and predation. I also give suggestions how and argue why this can affect linkage of food webs and the community composition in littoral and pelagic habitats.

<p>This thesis deals with the evolution of individuals within a species adapted to utilize specific resources, i.e. resource polymorphism. Although a well-known phenomenon, the understanding of the mechanisms behind is not complete. Considering the ruling theories, resource polymorphism is suggested to depend on severe competition for resources, the presence of open niches to be occupied leading to a reduction in competition, and disruptive selection where generalist are out-competed due trade-offs in foraging efficiency for different prey. In order to study resource polymorphism, I have used fish as the animal group in focus and the methods I have used range over laboratory experiments, field experiments, literature surveys and theoretical modelling.</p><p>In my work, I have showed that different resource use induces different body shapes and that the rate of change is dependent of the encounter rate of different resources. The induced body changes partly led to increased foraging efficiency but surprisingly I did not find any trade-offs due to specialization. However, when studying predation risk in relation to resource polymorphism, my studies point towards that resource use and predation risk may act as balancing factors in such a way that disruptive selection can take place.</p><p>My work also shows that population feedbacks have to be explored when considering the evolution of resource polymorphism. In pond and field experiments, I found that changes in resource densities affected the actual resource use despite previous adaptations to certain resources. By performing a literature survey, I found that cannibalism indirectly by its effect on population dynamics seems to facilitate the evolution of resource polymorphism. Modelling a size-structured population, I found that resource dynamics were stabilized, and the relative availability of different resources was levelled out due to cannibalism.</p><p>Taken together, my studies strongly suggest that to understand the development of resource polymorphism in consumer populations, future studies have to include the effect of a dynamic environment both with respect to resources and predators.</p>

This thesis deals with the evolution of individuals within a species adapted to utilize specific resources, i.e. resource polymorphism. Although a well-known phenomenon, the understanding of the mechanisms behind is not complete. Considering the ruling theories, resource polymorphism is suggested to depend on severe competition for resources, the presence of open niches to be occupied leading to a reduction in competition, and disruptive selection where generalist are out-competed due trade-offs in foraging efficiency for different prey. In order to study resource polymorphism, I have used fish as the animal group in focus and the methods I have used range over laboratory experiments, field experiments, literature surveys and theoretical modelling. In my work, I have showed that different resource use induces different body shapes and that the rate of change is dependent of the encounter rate of different resources. The induced body changes partly led to increased foraging efficiency but surprisingly I did not find any trade-offs due to specialization. However, when studying predation risk in relation to resource polymorphism, my studies point towards that resource use and predation risk may act as balancing factors in such a way that disruptive selection can take place. My work also shows that population feedbacks have to be explored when considering the evolution of resource polymorphism. In pond and field experiments, I found that changes in resource densities affected the actual resource use despite previous adaptations to certain resources. By performing a literature survey, I found that cannibalism indirectly by its effect on population dynamics seems to facilitate the evolution of resource polymorphism. Modelling a size-structured population, I found that resource dynamics were stabilized, and the relative availability of different resources was levelled out due to cannibalism. Taken together, my studies strongly suggest that to understand the development of resource polymorphism in consumer populations, future studies have to include the effect of a dynamic environment both with respect to resources and predators.

Individuals within a single population can vary widely in their phenotype e.g. in their body shape. These differences are an important source of biodiversity and they can precede evolutionary divergence within a population. In this thesis we use the biological invasion of the zebra mussels into Swedish lakes to investigate which processes create or maintain phenotypic diversity within populations of the two native fish species perch and roach and the mussel itself. Both fishes have specially adapted body shapes that depend on whether they feed in the near-shore or open-water habitat of lakes. This habitat-specific divergence was more pronounced in lakes with zebra mussels, probably because resources in both habitats were in higher supply due to the mussels’ effects on the lakes. Divergence in perch body shapes between habitats was also higher in lakes with a higher water clarity, suggesting that visual conditions can affect the resource use and thus also the expression of a habitat-specific body shape. When investigating the diversity of body shapes in the mussel itself we found that mussels from one lake changed their shell shape when exposed to different predators: fish predators induced a more elongated shell shape while crayfish predators induced a rounder shell. These specific shell shapes probably serve as two alternative predator defenses protecting the mussel from predation. We conclude that the availability and use of distinct resources is an important source of diversity within populations. Abiotic conditions can play a previously underappreciated role by promoting or impairing the use of the distinct resources thus affecting the divergence. The diversity of shell shapes we found in the zebra mussels complements our study by demonstrating that not only consumer responses to resources but also resources’ responses to predators can generate phenotypic diversity.

Predation by northern pike is believed to have initiated a divergence in whitefish into several different morphs, differing in size, habitat use and growth rate.  The development of a small pelagic zooplankton feeding morph is expected to have large impacts on the zooplankton community. In this study the effect of a changing predation regime on Bosmina, before and after introduction of pike in Valsjön, was investigated.  By looking at the change in carapace length (and indication of the level of predation pressure from fish) and mucro index (an indication of the level of invertebrate predation) of Bosmina remains in lake sediment the changing predation pressure from invertebrates and fish could be investigated. These features proved to be good proxys for the level of defense against fish and invertebrate predation.  However, other species than whitefish, and unknown interactions seems to have affected the zooplankton community. This makes it hard to tell which effect is due to diversification in whitefish and which is not. Also it is not clear that it is pike that has induced the divergence in the whitefish population. Other species like brown trout might also have been involved.

Resource variation and species interactions require organisms to respond behaviorally, physiologically, and morphologically within and among generations to compensate for spatial and temporal environmental variation. One successful evolutionary strategy to mitigate environmental variation is phenotypic plasticity: the production of alternative phenotypes in response to environmental variation. Phenotypic plasticity yields multiple characters that may enable organisms to better optimize phenotypic responses across environmental gradients. In this thesis, I trace the development of thought on phenotypic plasticity and present two empirical studies that implicate phenotypic plasticity in producing morphological variation in response to resource variation. The first empirical study addresses trophic plasticity, population divergence, and the effect of fine-scale environmental variation in western mosquitofish (Gambusia affinis). Offspring from two populations were fed either attached or unattached food items offered in three orientations: (1) water surface, (2) mid-water, (3) benthic, and (4) a daily rotation of the former three (fine-grained variation). Attached food induced wide heads, blunt snouts and rounded pectoral fins relative to morphology in the unattached treatment. Mid-water feeding induced elongated heads and deeper mid-bodies relative to benthic and surface feeding induced morphologies. The rotating treatment produced intermediate morphologies. Population divergence seemed related to both trophic and predation ecology. Ecomorphological consequences of induced morphologies and the need for inclusion of greater ecological complexity in studies of plasticity are discussed. The second study examines induced morphological plasticity and performance in red drum (Sciaenops ocellatus). I fed hatchery fish either hard or soft food for two months. Performance trials were designed to measure their ability to manipulate and consume hard food items. External morphology and the mass of pharyngeal crushing muscles were assessed for variation among treatments. A hard food diet induced deeper bodies and larger heads, more massive pharyngeal muscles, and initially more efficient consumption of hard food than fish receiving soft food. The observed morphological variation is in accordance with variation among species. Determining evolutionary mechanisms operating within red drum populations should eventually aid in developing and optimizing conservation efforts and ease the transition from hatchery facilities to estuaries.

Resource variation and species interactions require organisms to respond behaviorally, physiologically, and morphologically within and among generations to compensate for spatial and temporal environmental variation. One successful evolutionary strategy to mitigate environmental variation is phenotypic plasticity: the production of alternative phenotypes in response to environmental variation. Phenotypic plasticity yields multiple characters that may enable organisms to better optimize phenotypic responses across environmental gradients. In this thesis, I trace the development of thought on phenotypic plasticity and present two empirical studies that implicate phenotypic plasticity in producing morphological variation in response to resource variation. The first empirical study addresses trophic plasticity, population divergence, and the effect of fine-scale environmental variation in western mosquitofish (Gambusia affinis). Offspring from two populations were fed either attached or unattached food items offered in three orientations: (1) water surface, (2) mid-water, (3) benthic, and (4) a daily rotation of the former three (fine-grained variation). Attached food induced wide heads, blunt snouts and rounded pectoral fins relative to morphology in the unattached treatment. Mid-water feeding induced elongated heads and deeper mid-bodies relative to benthic and surface feeding induced morphologies. The rotating treatment produced intermediate morphologies. Population divergence seemed related to both trophic and predation ecology. Ecomorphological consequences of induced morphologies and the need for inclusion of greater ecological complexity in studies of plasticity are discussed. The second study examines induced morphological plasticity and performance in red drum (Sciaenops ocellatus). I fed hatchery fish either hard or soft food for two months. Performance trials were designed to measure their ability to manipulate and consume hard food items. External morphology and the mass of pharyngeal crushing muscles were assessed for variation among treatments. A hard food diet induced deeper bodies and larger heads, more massive pharyngeal muscles, and initially more efficient consumption of hard food than fish receiving soft food. The observed morphological variation is in accordance with variation among species. Determining evolutionary mechanisms operating within red drum populations should eventually aid in developing and optimizing conservation efforts and ease the transition from hatchery facilities to estuaries.

Recent research has suggested that individual specialization within populations could be substantial and more common than previously acknowledged. Eurasian perch is one of many species of fish in lakes of postglacial origin that displays a morphological and dietary variation tightly coupled to the littoral and pelagic habitats of the lake. The occurrence of such resource polymorphisms might have important consequences for local adaptation and might also be an important initial step in speciation. I have investigated the importance of a number of factors for the development of resource polymorphisms using perch as a study organism. I found a weak genetic basis for morphological differences, and the environmental influence on morphology was of such a magnitude that an induced morphology could be reversed. The results nevertheless suggested that genetic differentiation could be substantial at small spatial and temporal scales, even within habitats. Several environmental factors were shown to influence the morphological development, and the results also suggest that behavioral differences could mediate a morphological response. I also found evidence for that competition-driven divergence might only occur when divergence in resource use is favoured at the same time as growth rates are kept sufficiently high for character divergence to be effective. The results finally indicate that divergence in the gut length of individuals might co-vary with habitat and diet use in resource polymorphic populations. This might enhance habitat fidelity and possibly also facilitate the persistence of resource polymorphisms since individuals should experience a cost of switching diets due to a too specific digestive system. Based on these findings I conclude that small scaled genetic differentiation might be more common than currently acknowledged, that more multi-factorial studies are needed if we are to fully understand the mechanisms behind trait diversity, and that competition not always favors divergence.