L6 Speciation Mechanisms 2 2024 PDF

Introduction to Origins and Distribution of Biodiversity L6: Mechanisms of speciation 2 benthic...

Introduction to Origins and Distribution of Biodiversity L6: Mechanisms of speciation 2 benthic insect & snail eater Jakob Brodersen & Institute of Ecology & Evolution, EAWAG Centre of Ecology, Aquatic Ecology & Macroevolution, Ole Seehausen Evolution and Biogeochemistry, University of Bern www.fishecology.ch Kastanienbaum Mechanisms of speciation: 1. evolution of genic or behavioural incompatibilities as a by-product of neutral divergence (genetic drift) 2. evolution of genic or behavioural incompatibilities as a by-product of adaptation (selection) 3. origin (or reinforcement) of ecological and behavioural incompatibilities under direct selection 4. divergence of mating preferences as a by-product of divergent or disruptive sexual selection  Adaptive Speciation Origin of ecological incompatibility under direct selection in geographical contact Requires disruptive selection (b) Disruptive selection Forms of selection: Unimodal fitness landscape → directional or..................... Fitness landscape resulting from distribution of ecological agent, e.g prey size selection Phenotype, e.g. size Distribution of predator phenotypes: position of mode matters Forms of selection: Unimodal fitness landscape →.................... stabilizing selection Fitness landscape resulting from distribution of ecological agent, e.g prey size Phenotype, e.g. size Distribution of predator phenotypes: position of mode matters Forms of selection: Multimodal fitness landscape → directional,.................................... selection Fitness landscape resulting from distribution of ecological agent, e.g prey size Phenotype, e.g. size mode and variance matter Forms of selection: Multimodal fitness landscape →..................... stabilizing or................ selection Fitness landscape resulting from distribution of ecological agent, e.g prey size Phenotype, e.g. size mode and variance matter Forms of selection: Multimodal fitness landscape →.......................................... disruptive selection Fitness landscape resulting from distribution of ecological agent, e.g prey size Phenotype, e.g. size mode and variance matter Forms of selection: Multimodal fitness landscape →.......................................... disruptive selection Fitness landscape resulting from distribution of ecological agent, e.g prey size Phenotype, e.g. size mode and variance matter  Disruptive selection on multimodal fitness landscapes can generate polymorphisms as a solution to low average fitness Trophic polymorphisms are common in vertebrates. For example, 4 sympatric morphs of Arctic char, and 2 beak types in black-bellied seedcrackers (larger- beaked birds are more efficient when feeding on hard seeds and smaller- beaked birds are more efficient when feeding on soft seeds). Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Johnston et al. 2004). J. Exp. Biol. 207: 4343-4360.  Polymorphisms can be a step towards adaptive speciation under disruptive selection 1) Resource distribution favours individuals at the extremes of the phenotype distribution (disruptive selection) 2) a polymorphism emerges 3) The ecological advantage of extreme over modal phenotypes selects for mating preferences for own “ecotype” (e.g. small x small; large x large) 4) Assortative mating reduces the variance in phenotype distributions and causes reproductive isolation between the incipient species  Mathematical and computer simulation models of sympatric speciation as a consequence of adaptation to alternative ecological resources John Maynard-Smith (1965) developed one of the first mathematical models for sympatric speciation by assortative mating among ecological morphs. This was a one-gene per trait model involving 3 genes and 3 traits: (1) ecological performance, (2) resource choice, (3) choice of mating site. For speciation the allelic status at the 3 genes must become associated. The model demonstrated that it is difficult to create and maintain associations between matching alleles at these 3 gene loci because of re-mixing during sexual reproduction. Hence, sympatric speciation was thought to be unlikely. Several recent models simulated the 3 traits as quantitative traits (each determined by many genes) and concluded that under these assumptions sympatric speciation is not unlikely. However, the biological realism of other assumptions in these models is contested. Good empirical examples are found among phytophagous insects, lacustrine fish, several flowering plants and perhaps some birds  Sympatric speciation through adaptation to alternative resources in the apple maggot fly (Rhagoletis pulmonella) cherry apple hawthorne Hawthorne apple cherry original host new host (1864) new host (1960) Chemical cues to select host plant/fruit, host choice altered by single mutation, second mutation for larval survival in new host, hosts differ in phenology spatial and temporal reproductive isolation Feder et al., Bush et al. Environment creating underlying fitness landscape for fish in lake ecosystems Warm Cold Environment creating underlying fitness landscape for fish in lake ecosystems Light Dark Environment creating underlying fitness landscape for fish in lake ecosystems Structured Unstructured Environment creating underlying fitness landscape for fish in lake ecosystems Benthic Pelagic Environment creating underlying fitness landscape for fish in lake ecosystems Size, behaviour and habitat of food items  Sympatric speciation through adaptation to alternative dietary resources in fish populations of postglacial lakes: e.g. Arctic Char trophic morphs pelagic plankton eater benthic small insect eater benthic insect & snail eater fish eater Skulasson & Snorrason Sympatric speciation through adaptation to alternative dietary resources in fish populations of postglacial lakes: e.g. Arctic Char trophic morphs Sympatric speciation through adaptation to alternative dietary resources in fish populations of postglacial lakes: e.g. Arctic Char trophic morphs the red crossbill Loxia curvirostra is being devided into up to 20 phylogenetic species on the basis of unique song characteristics. Of nine different call types, seven are specialized on different species of conifers. Hence, they are not just phylogenetic but also ecological and cohesion species http://research.amnh.org/ornithology/crossbills/nathist.html Classical cases of speciation through divergent selection on resource use without geographical isolation  Disruptive selection in migratory direction of blackcaps (Sylvia atricapilla) SE vs SW migratory parents: Migratory direction is a heritable trait Cross mating experiments show that crosses migrate in intermediate directions. Intermediate phenotypes potentially unfavourable. Helbig 1991 Beh. Ecol. Sociobiol. Random mating Random mating Offspring Non-random mating Non-random mating Offspring Mechanisms of speciation II: Disruptive selection Mechanisms of speciation II: Disruptive selection Mechanisms of speciation: 1. evolution of genic or behavioural incompatibilities as a by- product of neutral divergence (genetic drift) 2. evolution of genic or behavioural incompatibilities as a by- product of adaptation (selection) 3. origin (or reinforcement) of ecological and behavioural incompatibilities under direct selection 4. divergence of mating preferences as a by-product of divergent or disruptive sexual selection