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Natural Selection and Speciation. Another way to look at it…. Directional Selection. Fitness. Trait. Trait. Negative Directional Selection. Positive Directional Selection. Stabilizing selection. Fitness. average trait value. Egg laying wasp that eventually kills the cactus.
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Another way to look at it… • Directional Selection Fitness Trait Trait Negative Directional Selection Positive Directional Selection
Stabilizing selection Fitness average trait value
Disruptive Selection Fitness average trait value
Smith, T. B. 1990. Evolution. 44(4):832 • Black-bellied seed crackers (Pyrenestes) in Cameroon
Same birds different studyOpen portion of bar – Number hatchedBlack portion of bar – Number survived
Two more types of selection • Correlational – working on combinations of traits at once. (body size) (Wing length)
Frequency Dependent Selection – Level of selection dependent upon most common phenotype (at the time) Genyochromis Scale eater
Scalebiters – frequency dependent selection Right “mouthedness” Left “mouthedness”
Frequency of left “mouthedness” Frequency of right “mouthedness” Time
Selection can lead to speciation • Speciation: • Splitting of one species into two or more species • Transformation of one species into a new species over time
B B A TIME TIME A A PHENOTYPIC VARIATION PHENOTYPIC VARIATION Anagenetic (gradual) versus Cladistic (punctuated) Speciation
Species • Many, many definitions • Biological species definition: A group of actually or potentially breeding individuals which are reproductively isolated from all other groups. • Doesn’t work in all situations • Fossils • Asexual organisms
Isolating mechanisms • Prezygotic – • Postzygotic -
Pre-zygotic barriers include • Temporal isolation • Behavioral • Mechanical • Gametic
Prezygotic I.M.s • Habitat isolation • Dendroica warblers in Eastern US
Modes of speciation • Allopatric – allo = different, patri = fatherland • Sympatric – sym = together • Has to do with location or habitat
Sympatric speciation • Polyploidy in plants • Hybrids
Adaptive Radiation • Evolution of ecological (& morphological) diversity within a rapidly speciating lineage.