Lecture 8 BIO 311D 1st Edition Outline of Last Lecture I Hardy Weinberg Equilibrium II Conditions for the Hardy Weinberg Equilibrium III Apply the Hardy Weinberg Principle IV Natural selection genetic drift and gene flow can alter allele frequencies in a population V Genetic Drift The founder effect and the bottleneck effect VI The Key Role of Natural Selection in Adaptive Evolution VII Sexual Selection Outline of Current Lecture I Frequency Dependent Selection II Why natural selection cannot fashion perfect organisms III Reproductive Isolation IV Speciation Current Lecture Frequency 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 fish Why Natural Selection Cannot Fashion Perfect Organisms 1 Selection can act only on existing variations 2 Evolution is limited by historical constraints 3 Adaptations are often compromises 4 Chance natural selection and the environment interact Two animals are considered members of different species if they A are members of different populations B Cannot mate and produce viable fertile offspring C Are geographically isolated D Live in different habitats E Look different Reproductive 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 or different 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 breakdown Speciation can take place with or without geographic separation Speciation can occur in two ways Allopatric speciation Populations separated geographically Sympatric speciation No physical barrier Which population characteristic would make the population more likely to undergo allopatric speciation A Having a large number of individuals B Having high mobile C Existing on the edge of the home range D Having a relatively homogeneous home range E Having a relatively homogeneous gene pool
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