I am interested in the evolutionary ecology of vertebrates (especially fish). My main interest lies in how organisms adapt to different or changing environments and how this influences speciation and adaptive radiation. Currently, I am trying to resolve why some cichlid species have diversified in some lineages whereas other lineages occuring in the same or similar environment refuse to do so. Supervisors: Ole Seehausen
Freshwater species biodiversity is under threat. The average global decline for migratory fish sp... more Freshwater species biodiversity is under threat. The average global decline for migratory fish species is estimated to be more than 75% since 1970. Atlantic salmon is one of these species with a steep decline in northwestern Europe and it even went extinct in the river Rhine in the 1950s. The causes for this decline have been posted to habitat loss, pollution, climate change and overfishing. Annual stocking in the Rhine since the late 1980s resulted in an initial increase in the Atlantic salmon numbers after which numbers collapsed again. In this paper, we lay out the recent decline, estimate losses of smolts and adults at different sections in the freshwater habitat and elaborate on potential causes of the recent decline and these losses. We found
Freshwater species biodiversity is under threat. The average global decline for migratory fish sp... more Freshwater species biodiversity is under threat. The average global decline for migratory fish species is estimated to be more than 75% since 1970. Atlantic salmon is one of these species with a steep decline in northwestern Europe and it even went extinct in the river Rhine in the 1950s. The causes for this decline have been posted to habitat loss, pollution, climate change and overfishing. Annual stocking in the Rhine since the late 1980s resulted in an initial increase in the Atlantic salmon numbers after which numbers collapsed again. In this paper, we lay out the recent decline, estimate losses of smolts and adults at different sections in the freshwater habitat and elaborate on potential causes of the recent decline and these losses. We found
Studying phenotypic and genetic differentiation between very young species can be
very informativ... more Studying phenotypic and genetic differentiation between very young species can be very informative with regard to learning about processes of speciation. Identifying and characterizing genetic species structure and distinguishing it from spatial genetic structure within a species is a prerequisite for this and is often not given sufficient attention. Young radiations of cichlid fish are classical speciation study systems. However, it is only during the past decade that population genomics based on nextgeneration sequencing has begun to provide the power to resolve species and distinguish speciation from spatial population structure for the youngest of these radiations. The Lake Victoria haplochromine cichlids constitute the youngest large cichlid fish radiation, probably <20,000 years old. Earlier work showed that communities of rocky reef cichlids are composed of many reciprocally monophyletic species despite their very recent origins. Here, we build on this work by studying assemblages of offshore demersal cichlids, adding analyses of within-species spatial structure to the sympatric species structure. We sampled seven multispecies communities along a 6-km-long transect from one side of the Mwanza Gulf to the other side. We investigated whether phenotypically diagnosed putative species are reciprocally monophyletic and whether such monophyly is stable across species geographic ranges. We show that all species are genetically strongly differentiated in sympatry, that they are reciprocally monophyletic, and that monophyly is stable across distribution ranges. We found significant differentiation between geographically distinct populations in two species, but no or weak isolation by distance. We further found subtle but significant morphological differences between all species and a linear relationship between genomic and morphological distance which suggests that differences in morphology begin to accumulate after speciation has already affected genome-wide restrictions of gene flow.
Although fishing with electricity is illegal in the European Union, a number of temporary licence... more Although fishing with electricity is illegal in the European Union, a number of temporary licences allowed converting beam trawlers to pulse trawling. To analyse how the adaption of pulse trawling changed this fishery, we studied fishing speeds and landings per unit effort as proxies for catch efficiencies for the main target species. Compared to conventional tickler chain beam trawls, pulse trawls were towed at lower speeds (small vessels −10%, large vessels −23%). Large vessels that switched from conventional beam trawls to pulse trawls at the end of 2009 gradually increased catch efficiency for sole over the period of almost 1 year. While pulse trawling was found to have higher catch rates (kg/h) for sole (small vessels +74%, large vessels +17%), lower catch rates were observed for plaice (small vessels −31%, large vessels −32%). Vessels that switched later achieved immediate gains in catch efficiency for sole. The change in catch efficiency is likely due to the difference in cra...
This chapter traces the history and trends in fisheries research for the Tanzanian part of Lake V... more This chapter traces the history and trends in fisheries research for the Tanzanian part of Lake Victoria and its basin back to the late nineteenth century when the haplochromine species were scientifically identified for the first time. Past studies in the early to mid-twentieth century were mainly designed to address issues related to the introduced species and the abundant haplochromines. Subsequent studies in the late twentieth to early twenty-first centuries integrated multiple disciplines such as fish biology, ecology, biodiversity, limnology, socio-economics and aquaculture. In the late 1990s and early 2000s, lake-wide research projects played a key role in further understanding of the lake-basin flora, fauna and related fisheries. Over the last half century, Lake Victoria has undergone dramatic ecosystem changes, partly driven by the introduction of the predatory Nile perch and the non-indigenous tilapiine species in the 1950s and early 1960s. The profound ecosystem transformation of the lake has also been attributed to cultural eutrophication, climatic variability and over-fishing. In the mid-1950s, the lake had a diverse fish fauna, which was thought to comprise about 29 genera and more than 650 species, with the haplochromines forming about 80% of the demersal fish stocks. Later discoveries in the early 1970s to the late 1990s confirmed that the haplochromine group alone consisted of over 500 endemic species. The introduction of the Nile perch was meant to utilize the abundant haplochromines that were considered commercially unimportant, and to extend fishing operations from the inshore to deeper offshore waters.
Adaptive radiation research typically relies on the study of evolution in retrospective, leaving ... more Adaptive radiation research typically relies on the study of evolution in retrospective, leaving the predictive value of the concept hard to evaluate. Several radiations, including the cichlid fishes in the East African Great Lakes, have been studied extensively, yet no study has investigated the onset of the intraspecific processes of niche expansion and differentiation shortly after colonization of an adaptive zone by cichlids. Haplochromine cichlids of one of the two lineages that seeded the Lake Victoria radiation recently arrived in Lake Chala, a lake perfectly suited for within-lake cichlid speciation. Here, we infer the colonization and demographic history, quantify phenotypic, ecological and genomic diversity and diversification, and investigate the selection regime to ask if the population shows signs of diversification resembling the onset of adaptive radiation. We find that since their arrival in the lake, haplochromines have colonized a wide range of depth habitats assoc...
Theory suggests that speciation with gene flow is most likely when both sexual and ecological sel... more Theory suggests that speciation with gene flow is most likely when both sexual and ecological selection are divergent or disruptive. Divergent sexual and natural selection on the visual system have been demonstrated before in sympatric, morphologically similar sister species of Lake Victoria cichlids, but this does not explain the subtle morphological differences between them. To investigate the significance of natural selection on morphology during speciation, we here ask whether the prevalence of disruptive ecological selection differs between sympatric sister species that are at different stages of speciation. Some of our species pairs do () and others do not () differ distinctively in sexually selected male nuptial coloration. We find that (i) evidence for disruptive selection, and for evolutionary response to it, is prevalent in traits that are differentiated between sister species; (ii) prevalence of both predicts the extent of genetic differentiation; and (iii) genetic differ...
Textbook examples of adaptive radiation often show rapid morphological changes in response to env... more Textbook examples of adaptive radiation often show rapid morphological changes in response to environmental perturbations. East Africa&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;s Lake Victoria, famous for its stunning adaptive radiation of cichlids, has suffered from human-induced eutrophication over the past decades. This cultural eutrophication is thought to be partly responsible for the dramatically reduced cichlid biodiversity, but climatic variability in itself might also have contributed to the eutrophication which resulted in low oxygen levels and decreased water transparency. To determine how recent environmental changes have influenced the lake and its cichlids over the past 50 years, we gathered environmental and meteorological variables and compared these with gill surface area of four cichlid species. We found that during the period of severe eutrophication and temperature increase (1980s), reduced wind speeds coincided with a reduction in oxygen levels and a decrease in both water temperature and transparency. The gill surface area in three out of the four cichlid species increased during this period which is consistent with adaptive change in response to increased hypoxia. During the 2000s, wind speeds, oxygen levels, water transparency and water temperature increased again, while cichlid gill surface area decreased. Our results imply that climatic changes and especially wind speed and direction might play a crucial role in tropical lake dynamics. The changes in Lake Victoria&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;s water quality coincide with fluctuations in cichlid gill surface area, suggesting that these fish can respond rapidly to environmental perturbations, but also that climatic variability, together with continued eutrophication, might be detrimental to the lake&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;s cichlid biodiversity.
Evolution; international journal of organic evolution, 2015
Rapid morphological changes in response to fluctuating natural environments are a common phenomen... more Rapid morphological changes in response to fluctuating natural environments are a common phenomenon in species that undergo adaptive radiation. The dramatic ecological changes in Lake Victoria provide a unique opportunity to study environmental effects on cichlid morphology. This study shows how four haplochromine cichlids adapted their premaxilla to a changed diet over the past 30 years. Directly after the diet change toward larger and faster prey in the late 1980s, the premaxilla (upper jaw) changed in a way that is in agreement with a more food manipulating feeding style. During the 2000s, two zooplanktivorous species showed a reversal of morphological changes after returning to their original diet, whereas two other species showed no reversal of diet and morphology. These rapid changes indicate a potential for extremely fast adaptive responses to environmental fluctuations, which are likely inflicted by competition release and increase, and might have a bearing on the ability of...
Adaptive radiation research typically relies on the study of evolution in retrospective, leaving ... more Adaptive radiation research typically relies on the study of evolution in retrospective, leaving the predictive value of the concept hard to evaluate. Several radiations, including the cichlid fishes in the East African Great Lakes, have been studied extensively, yet no study has investigated the onset of the intraspecific processes of niche expansion and differentiation shortly after colonization of an adaptive zone by cichlids. Haplochromine cichlids of one of the two lineages that seeded the Lake Victoria radiation recently arrived in Lake Chala, a lake perfectly suited for within-lake cichlid speciation. Here, we infer the colonization and demographic history, quantify phenotypic, ecological and genomic diversity and diversification, and investigate the selection regime to ask if the population shows signs of diversification resembling the onset of adaptive radiation. We find that since their arrival in the lake, haplochromines have colonized a wide range of depth habitats assoc...
Demography and ecology were studied of a decreased lion population in Waza National Park, Cameroo... more Demography and ecology were studied of a decreased lion population in Waza National Park, Cameroon Home ranges of lions were large with an average of 1043 (MCP 100%) or 635 (HM 95%). Lions spent most of their time inside the park, especially in the hot dry season. Time spent outside the park increases in the wet season and peaks in the cold dry season. Lions were most active during the night with crepuscular peaks around sunrise and sunset and reduced activity during the hottest parts of the day. This nocturnal activity increases when lions were outside the park. Lions were least active in the cold dry season compared to the hot dry and the wet season. Transects showed an increasing number and congregation of wild ungulates on the floodplain in time with most of the observed lions in the vicinity of these congregations. Number of nights present at a GPS cluster presented the best model to predict whether or not a lion had killed or scavenged an animal successfully. Lions killed or s...
witte, f., kishe-machumu, m. a., mkumbo, o. c., wanink, J. h., goudswaard, P. c., van riJssel, J.... more witte, f., kishe-machumu, m. a., mkumbo, o. c., wanink, J. h., goudswaard, P. c., van riJssel, J.c. & van oiJen, m. J.P. The fish fauna of Lake Victoria during a century of human induced perturbations. In J. Snoeks & A. Getahun (eds), Proceedings of the Fourth International Conference on African Fish and Fisheries, Addis Ababa, Ethiopia, 22-26 September 2008. Tervuren: Royal Museum for Central Africa, ‘Zoological Documentation Online Series’, pp. 49-66. Lake Victoria, by area the largest tropical lake of the world, is well-known for its diverse native fish fauna, which comprised about 500 endemic haplochromine cichlid species, two tilapiine species and 46 other species belonging to 12 families. During the past decades, the fish species diversity in the lake has declined dramatically due to human induced perturbations in the ecosystem. Based on literature and our own research findings we provide an overview of these changes and their most likely causes. During the first half of the l...
Freshwater species biodiversity is under threat. The average global decline for migratory fish sp... more Freshwater species biodiversity is under threat. The average global decline for migratory fish species is estimated to be more than 75% since 1970. Atlantic salmon is one of these species with a steep decline in northwestern Europe and it even went extinct in the river Rhine in the 1950s. The causes for this decline have been posted to habitat loss, pollution, climate change and overfishing. Annual stocking in the Rhine since the late 1980s resulted in an initial increase in the Atlantic salmon numbers after which numbers collapsed again. In this paper, we lay out the recent decline, estimate losses of smolts and adults at different sections in the freshwater habitat and elaborate on potential causes of the recent decline and these losses. We found
Freshwater species biodiversity is under threat. The average global decline for migratory fish sp... more Freshwater species biodiversity is under threat. The average global decline for migratory fish species is estimated to be more than 75% since 1970. Atlantic salmon is one of these species with a steep decline in northwestern Europe and it even went extinct in the river Rhine in the 1950s. The causes for this decline have been posted to habitat loss, pollution, climate change and overfishing. Annual stocking in the Rhine since the late 1980s resulted in an initial increase in the Atlantic salmon numbers after which numbers collapsed again. In this paper, we lay out the recent decline, estimate losses of smolts and adults at different sections in the freshwater habitat and elaborate on potential causes of the recent decline and these losses. We found
Studying phenotypic and genetic differentiation between very young species can be
very informativ... more Studying phenotypic and genetic differentiation between very young species can be very informative with regard to learning about processes of speciation. Identifying and characterizing genetic species structure and distinguishing it from spatial genetic structure within a species is a prerequisite for this and is often not given sufficient attention. Young radiations of cichlid fish are classical speciation study systems. However, it is only during the past decade that population genomics based on nextgeneration sequencing has begun to provide the power to resolve species and distinguish speciation from spatial population structure for the youngest of these radiations. The Lake Victoria haplochromine cichlids constitute the youngest large cichlid fish radiation, probably <20,000 years old. Earlier work showed that communities of rocky reef cichlids are composed of many reciprocally monophyletic species despite their very recent origins. Here, we build on this work by studying assemblages of offshore demersal cichlids, adding analyses of within-species spatial structure to the sympatric species structure. We sampled seven multispecies communities along a 6-km-long transect from one side of the Mwanza Gulf to the other side. We investigated whether phenotypically diagnosed putative species are reciprocally monophyletic and whether such monophyly is stable across species geographic ranges. We show that all species are genetically strongly differentiated in sympatry, that they are reciprocally monophyletic, and that monophyly is stable across distribution ranges. We found significant differentiation between geographically distinct populations in two species, but no or weak isolation by distance. We further found subtle but significant morphological differences between all species and a linear relationship between genomic and morphological distance which suggests that differences in morphology begin to accumulate after speciation has already affected genome-wide restrictions of gene flow.
Although fishing with electricity is illegal in the European Union, a number of temporary licence... more Although fishing with electricity is illegal in the European Union, a number of temporary licences allowed converting beam trawlers to pulse trawling. To analyse how the adaption of pulse trawling changed this fishery, we studied fishing speeds and landings per unit effort as proxies for catch efficiencies for the main target species. Compared to conventional tickler chain beam trawls, pulse trawls were towed at lower speeds (small vessels −10%, large vessels −23%). Large vessels that switched from conventional beam trawls to pulse trawls at the end of 2009 gradually increased catch efficiency for sole over the period of almost 1 year. While pulse trawling was found to have higher catch rates (kg/h) for sole (small vessels +74%, large vessels +17%), lower catch rates were observed for plaice (small vessels −31%, large vessels −32%). Vessels that switched later achieved immediate gains in catch efficiency for sole. The change in catch efficiency is likely due to the difference in cra...
This chapter traces the history and trends in fisheries research for the Tanzanian part of Lake V... more This chapter traces the history and trends in fisheries research for the Tanzanian part of Lake Victoria and its basin back to the late nineteenth century when the haplochromine species were scientifically identified for the first time. Past studies in the early to mid-twentieth century were mainly designed to address issues related to the introduced species and the abundant haplochromines. Subsequent studies in the late twentieth to early twenty-first centuries integrated multiple disciplines such as fish biology, ecology, biodiversity, limnology, socio-economics and aquaculture. In the late 1990s and early 2000s, lake-wide research projects played a key role in further understanding of the lake-basin flora, fauna and related fisheries. Over the last half century, Lake Victoria has undergone dramatic ecosystem changes, partly driven by the introduction of the predatory Nile perch and the non-indigenous tilapiine species in the 1950s and early 1960s. The profound ecosystem transformation of the lake has also been attributed to cultural eutrophication, climatic variability and over-fishing. In the mid-1950s, the lake had a diverse fish fauna, which was thought to comprise about 29 genera and more than 650 species, with the haplochromines forming about 80% of the demersal fish stocks. Later discoveries in the early 1970s to the late 1990s confirmed that the haplochromine group alone consisted of over 500 endemic species. The introduction of the Nile perch was meant to utilize the abundant haplochromines that were considered commercially unimportant, and to extend fishing operations from the inshore to deeper offshore waters.
Adaptive radiation research typically relies on the study of evolution in retrospective, leaving ... more Adaptive radiation research typically relies on the study of evolution in retrospective, leaving the predictive value of the concept hard to evaluate. Several radiations, including the cichlid fishes in the East African Great Lakes, have been studied extensively, yet no study has investigated the onset of the intraspecific processes of niche expansion and differentiation shortly after colonization of an adaptive zone by cichlids. Haplochromine cichlids of one of the two lineages that seeded the Lake Victoria radiation recently arrived in Lake Chala, a lake perfectly suited for within-lake cichlid speciation. Here, we infer the colonization and demographic history, quantify phenotypic, ecological and genomic diversity and diversification, and investigate the selection regime to ask if the population shows signs of diversification resembling the onset of adaptive radiation. We find that since their arrival in the lake, haplochromines have colonized a wide range of depth habitats assoc...
Theory suggests that speciation with gene flow is most likely when both sexual and ecological sel... more Theory suggests that speciation with gene flow is most likely when both sexual and ecological selection are divergent or disruptive. Divergent sexual and natural selection on the visual system have been demonstrated before in sympatric, morphologically similar sister species of Lake Victoria cichlids, but this does not explain the subtle morphological differences between them. To investigate the significance of natural selection on morphology during speciation, we here ask whether the prevalence of disruptive ecological selection differs between sympatric sister species that are at different stages of speciation. Some of our species pairs do () and others do not () differ distinctively in sexually selected male nuptial coloration. We find that (i) evidence for disruptive selection, and for evolutionary response to it, is prevalent in traits that are differentiated between sister species; (ii) prevalence of both predicts the extent of genetic differentiation; and (iii) genetic differ...
Textbook examples of adaptive radiation often show rapid morphological changes in response to env... more Textbook examples of adaptive radiation often show rapid morphological changes in response to environmental perturbations. East Africa&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;s Lake Victoria, famous for its stunning adaptive radiation of cichlids, has suffered from human-induced eutrophication over the past decades. This cultural eutrophication is thought to be partly responsible for the dramatically reduced cichlid biodiversity, but climatic variability in itself might also have contributed to the eutrophication which resulted in low oxygen levels and decreased water transparency. To determine how recent environmental changes have influenced the lake and its cichlids over the past 50 years, we gathered environmental and meteorological variables and compared these with gill surface area of four cichlid species. We found that during the period of severe eutrophication and temperature increase (1980s), reduced wind speeds coincided with a reduction in oxygen levels and a decrease in both water temperature and transparency. The gill surface area in three out of the four cichlid species increased during this period which is consistent with adaptive change in response to increased hypoxia. During the 2000s, wind speeds, oxygen levels, water transparency and water temperature increased again, while cichlid gill surface area decreased. Our results imply that climatic changes and especially wind speed and direction might play a crucial role in tropical lake dynamics. The changes in Lake Victoria&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;s water quality coincide with fluctuations in cichlid gill surface area, suggesting that these fish can respond rapidly to environmental perturbations, but also that climatic variability, together with continued eutrophication, might be detrimental to the lake&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;s cichlid biodiversity.
Evolution; international journal of organic evolution, 2015
Rapid morphological changes in response to fluctuating natural environments are a common phenomen... more Rapid morphological changes in response to fluctuating natural environments are a common phenomenon in species that undergo adaptive radiation. The dramatic ecological changes in Lake Victoria provide a unique opportunity to study environmental effects on cichlid morphology. This study shows how four haplochromine cichlids adapted their premaxilla to a changed diet over the past 30 years. Directly after the diet change toward larger and faster prey in the late 1980s, the premaxilla (upper jaw) changed in a way that is in agreement with a more food manipulating feeding style. During the 2000s, two zooplanktivorous species showed a reversal of morphological changes after returning to their original diet, whereas two other species showed no reversal of diet and morphology. These rapid changes indicate a potential for extremely fast adaptive responses to environmental fluctuations, which are likely inflicted by competition release and increase, and might have a bearing on the ability of...
Adaptive radiation research typically relies on the study of evolution in retrospective, leaving ... more Adaptive radiation research typically relies on the study of evolution in retrospective, leaving the predictive value of the concept hard to evaluate. Several radiations, including the cichlid fishes in the East African Great Lakes, have been studied extensively, yet no study has investigated the onset of the intraspecific processes of niche expansion and differentiation shortly after colonization of an adaptive zone by cichlids. Haplochromine cichlids of one of the two lineages that seeded the Lake Victoria radiation recently arrived in Lake Chala, a lake perfectly suited for within-lake cichlid speciation. Here, we infer the colonization and demographic history, quantify phenotypic, ecological and genomic diversity and diversification, and investigate the selection regime to ask if the population shows signs of diversification resembling the onset of adaptive radiation. We find that since their arrival in the lake, haplochromines have colonized a wide range of depth habitats assoc...
Demography and ecology were studied of a decreased lion population in Waza National Park, Cameroo... more Demography and ecology were studied of a decreased lion population in Waza National Park, Cameroon Home ranges of lions were large with an average of 1043 (MCP 100%) or 635 (HM 95%). Lions spent most of their time inside the park, especially in the hot dry season. Time spent outside the park increases in the wet season and peaks in the cold dry season. Lions were most active during the night with crepuscular peaks around sunrise and sunset and reduced activity during the hottest parts of the day. This nocturnal activity increases when lions were outside the park. Lions were least active in the cold dry season compared to the hot dry and the wet season. Transects showed an increasing number and congregation of wild ungulates on the floodplain in time with most of the observed lions in the vicinity of these congregations. Number of nights present at a GPS cluster presented the best model to predict whether or not a lion had killed or scavenged an animal successfully. Lions killed or s...
witte, f., kishe-machumu, m. a., mkumbo, o. c., wanink, J. h., goudswaard, P. c., van riJssel, J.... more witte, f., kishe-machumu, m. a., mkumbo, o. c., wanink, J. h., goudswaard, P. c., van riJssel, J.c. & van oiJen, m. J.P. The fish fauna of Lake Victoria during a century of human induced perturbations. In J. Snoeks & A. Getahun (eds), Proceedings of the Fourth International Conference on African Fish and Fisheries, Addis Ababa, Ethiopia, 22-26 September 2008. Tervuren: Royal Museum for Central Africa, ‘Zoological Documentation Online Series’, pp. 49-66. Lake Victoria, by area the largest tropical lake of the world, is well-known for its diverse native fish fauna, which comprised about 500 endemic haplochromine cichlid species, two tilapiine species and 46 other species belonging to 12 families. During the past decades, the fish species diversity in the lake has declined dramatically due to human induced perturbations in the ecosystem. Based on literature and our own research findings we provide an overview of these changes and their most likely causes. During the first half of the l...
This chapter traces the history and trends in fisheries research for the Tanzanian part of Lake V... more This chapter traces the history and trends in fisheries research for the Tanzanian part of Lake Victoria and its basin back to the late nineteenth century when the haplochromine species were scientifically identified for the first time. Past studies in the early to mid-twentieth century were mainly designed to address issues related to the introduced species and the abundant haplochromines. Subsequent studies in the late twentieth to early twenty-first centuries integrated multiple disciplines such as fish biology, ecology, biodiversity, limnology, socio-economics and aquaculture. In the late 1990s and early 2000s, lake-wide research projects played a key role in further understanding of the lake-basin flora, fauna and related fisheries. Over the last half century, Lake Victoria has undergone dramatic ecosystem changes, partly driven by the introduction of the predatory Nile perch and the non-indigenous tilapiine species in the 1950s and early 1960s. The profound ecosystem transformation of the lake has also been attributed to cultural eutrophication, climatic variability and over-fishing. In the mid-1950s, the lake had a diverse fish fauna, which was thought to comprise about 29 genera and more than 650 species, with the haplochro-mines forming about 80% of the demersal fish stocks. Later discoveries in the early 1970s to the late 1990s confirmed that the haplochromine group alone consisted of over 500 endemic species. The introduction of the Nile perch was meant to utilize the abundant haplochromines that were considered commercially unimportant, and to extend fishing operations from the inshore to deeper offshore waters.
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Papers by Jacco van Rijssel
very informative with regard to learning about processes of speciation. Identifying
and characterizing genetic species structure and distinguishing it from spatial genetic
structure within a species is a prerequisite for this and is often not given sufficient
attention. Young radiations of cichlid fish are classical speciation study systems.
However, it is only during the past decade that population genomics based on nextgeneration sequencing has begun to provide the power to resolve species and distinguish speciation from spatial population structure for the youngest of these radiations.
The Lake Victoria haplochromine cichlids constitute the youngest large cichlid fish
radiation, probably <20,000 years old. Earlier work showed that communities of rocky
reef cichlids are composed of many reciprocally monophyletic species despite their
very recent origins. Here, we build on this work by studying assemblages of offshore
demersal cichlids, adding analyses of within-species
spatial structure to the sympatric species structure. We sampled seven multispecies communities along a 6-km-long
transect from one side of the Mwanza Gulf to the other side. We investigated whether
phenotypically diagnosed putative species are reciprocally monophyletic and whether
such monophyly is stable across species geographic ranges. We show that all species
are genetically strongly differentiated in sympatry, that they are reciprocally monophyletic, and that monophyly is stable across distribution ranges. We found significant
differentiation between geographically distinct populations in two species, but no or
weak isolation by distance. We further found subtle but significant morphological differences between all species and a linear relationship between genomic and morphological distance which suggests that differences in morphology begin to accumulate
after speciation has already affected genome-wide
restrictions of gene flow.
very informative with regard to learning about processes of speciation. Identifying
and characterizing genetic species structure and distinguishing it from spatial genetic
structure within a species is a prerequisite for this and is often not given sufficient
attention. Young radiations of cichlid fish are classical speciation study systems.
However, it is only during the past decade that population genomics based on nextgeneration sequencing has begun to provide the power to resolve species and distinguish speciation from spatial population structure for the youngest of these radiations.
The Lake Victoria haplochromine cichlids constitute the youngest large cichlid fish
radiation, probably <20,000 years old. Earlier work showed that communities of rocky
reef cichlids are composed of many reciprocally monophyletic species despite their
very recent origins. Here, we build on this work by studying assemblages of offshore
demersal cichlids, adding analyses of within-species
spatial structure to the sympatric species structure. We sampled seven multispecies communities along a 6-km-long
transect from one side of the Mwanza Gulf to the other side. We investigated whether
phenotypically diagnosed putative species are reciprocally monophyletic and whether
such monophyly is stable across species geographic ranges. We show that all species
are genetically strongly differentiated in sympatry, that they are reciprocally monophyletic, and that monophyly is stable across distribution ranges. We found significant
differentiation between geographically distinct populations in two species, but no or
weak isolation by distance. We further found subtle but significant morphological differences between all species and a linear relationship between genomic and morphological distance which suggests that differences in morphology begin to accumulate
after speciation has already affected genome-wide
restrictions of gene flow.