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Sources of Genetic Variation Factors increasing variation •Mutation •Migration Factors decreasing variation •Natural selection •Genetic drift in small populations (<1000) Evidence of Selection in Natural Plant Populations Selection Among Populations The Common Garden Experiments of Clauson, Keck and Heiesy (1948) Differences in phenotype across a gradient: Yarrow (Achiella spp) as an example What is the source of variation? • Different species – genetic variation? • Same species – phenotypic plasticity? Common Garden Experiment Step #1: Obtain Plants from Source Populations Stanford – 100’ Mather – 4600’ Timberline – 10,000’ Common Garden Experiment Step #2: Produce Clones Source Plant Clones (e.g., piece of root) Location #1 Location #2 Common Garden Experiment Step #3: Plant clones in common gardens Source Plant Clones (e.g., piece of root) Common Gardens Location #1 Location #1 Location #2 Location #2 Stanford Common Garden Mather Common Garden Timberline Common Garden Interpretation of Results: Pure Plastic Response Source Plant Clones Common Gardens ? Location #1 Location #1 ? Location #2 Location #2 Interpretation of Results: Pure Genetic Response Source Plant Clones Common Gardens ? Location #1 Location #1 ? Location #2 Location #2 Plastic response Genetic response Experimental Outcome: Growth of Mather Achiella Clones A Second Example Potentilla glandulosa Copyright © 1997-2001 by Jane Strong and Tom Chester http://tchester.org/sgm/conditions/blooms/idyellow.html Lowland Plant Lowland Ecotype ©Brother Alfred Brousseau, St. Mary's College Montane Plant ©Brother Alfred Brousseau, St. Mary's College Experimental Outcome: Growth of Potentilla Clones Interpretation Part I • Not a pure plastic response • Not a pure genetic response What is the relationship between these organisms? Separate experiments show that crosses between different source populations produce viable offspring Interpretation Part II • These are not different species What then are they? Ecotypes the middle ground • Genetically distinct organisms • Phenotypically distinct in terms of • Morphology • Physiology • Phenology • Occur in distinct habitats • Differences can be traced to ecological differences in home habitat • Plants are potentially interfertile (i.e., same biologicial species) An Interpretation Individuals or Ecotypes Selection Within a Population Purple loosestrife (Lythrum salicaria): an aggressive invasive species Purple Loosestrife and Tristyly Three flower types (morphs) • ♀ Pistal positions differ • ♂ Anther positions differ Pollination patterns • No self pollination • Each morph can pollinate the other two morphs Less frequent morphs have higher fitness Impact of Frequency-Dependent Selection on Invading Populations of Purple Loosestrife • Study system with 24 newly invaded sites censused over a 5 year period • Low evenness during year zero • Evenness predicted to increase due to frequency dependent selection among morphs No change line (y=x) Prediction is met, indicating a change in population due to natural selection Selection At a Global Scale Convergent Evolution Example #1: Desert plants Euphorbiaceae: Africa Cactaceae: N. America Example #2: Alpine plants Campanulaceae: Africa Asteraceae: S. America Life Histories and Tradeoffs Key Stages in the Life-History of a Plant Seed Maturation Flowering seed phase Growth Dispersal Dormancy Pollination Germination The Ideal Plant • Grow large rapidly • Live forever • Reproduce early and often Life Histories and Tradeoffs Impact of Limiting Resources General Scheme of Resource Allocation Reproduction Growth • Pollen • Nectar • Ovules • Seeds • Leaves • Stems • Roots • Rhizomes 3 2 Maintenance • Structural support • Storage • Defenses • Basal metabolism 1 General order in which resources are used Available resource General Scheme of Resource Allocation through time Time Resource Allocation for a Typical Annual Plant Time Resource Allocation over a typical year for a Stress Tolerating Plant Time Tradeoffs and Limiting Resources vs