Lecture 8 BIO 311D 1st EditionOutline of Last Lecture I. Hardy Weinberg EquilibriumII. Conditions for the Hardy-Weinberg EquilibriumIII. Apply the Hardy-Weinberg PrincipleIV. Natural selection, genetic drift, and gene flow can alter allele frequencies in a populationV. Genetic Drift (The founder effect and the bottleneck effect)VI. The Key Role of Natural Selection in Adaptive EvolutionVII. Sexual SelectionOutline of Current Lecture I. Frequency-Dependent SelectionII. Why natural selection cannot fashion perfect organismsIII. Reproductive IsolationIV. SpeciationCurrent LectureFrequency-Dependent Selection:- The fitness of a phenotype declines if it becomes too common in the population- Selection can favor whichever phenotype is less common in a population- For example, frequency-dependent selection selects for approximately equal numbers of“right-mouthed” and “left-mouthed” scale-eating fishWhy Natural Selection Cannot Fashion Perfect Organisms:1. Selection can act only on existing variations2. Evolution is limited by historical constraints3. Adaptations are often compromises4. Chance, natural selection, and the environment interactTwo animals are considered members of different species if theyA. are members of different populationsB. Cannot mate and produce viable, fertile offspringC. Are geographically isolatedD. Live in different habitatsE. Look differentReproductive Isolation:• Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring• Hybrids are the offspring of crosses between different species• Reproductive isolation can be classified by whether factors act before or after fertilization• Prezygotic barriers block fertilization from occurring by:• Inhibiting the process of fertiliziation• Impeding different species from attempting to mate• Preventing the successful completion of mating• Hindering fertilization if mating is successful• Habitat isolation: Two species encounter each other rarely, or not at all, because they occupy different habitats, even though not isolated by physical barriers• Temporal isolation: Species that breed at different times of the day, different seasons, ordifferent years cannot mix their gametes• Behavioral isolation: Courtship rituals and other behaviors unique to a species are effective barriers• Mechanical isolation: Morphological differences can prevent successful mating• Gametic Isolation: Sperm of one species may not be able to fertilize eggs of another species• Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult:• Reduced hybrid viability• Reduced hybrid fertility• Hybrid breakdownSpeciation can take place with or without geographic separation• Speciation can occur in two ways:– Allopatric speciation• Populations separated geographically– Sympatric speciation• No physical barrierWhich population characteristic would make the population more likely to undergo allopatric speciation?A. Having a large number of individualsB. Having high mobileC. Existing on the edge of the home rangeD. Having a relatively homogeneous home rangeE. Having a relatively homogeneous gene
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