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Chapter 16

Chapter 16. Population Genetics and Speciation. Section 1 Genetic Equilibrium: Objectives. Identify traits that vary in populations and that may be studied Explain the importance of the bell curve to population genetics Compare three causes of genetic variation in a population

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Chapter 16

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  1. Chapter 16 Population Genetics and Speciation

  2. Section 1 Genetic Equilibrium: Objectives • Identify traits that vary in populations and that may be studied • Explain the importance of the bell curve to population genetics • Compare three causes of genetic variation in a population • Calculate allele frequency and phenotype frequency • Explain Hardy-Weinberg genetic equilibrium

  3. Variation of Traits within a Population • Population biologists study many different traits in populations, such as size and color • Population genetics is the study of evolution from a genetic point of view • EX: studying how dogwood trees in Middletown, Connecticut have changed over time would be a way to study evolution in a population

  4. Variation of Traits within a Population • Causes of Variation • Traits vary and can be mapped along a bell curve, which shows that most individuals have average traits whereas a few individuals have extreme traits • Variations in genotype arise by mutation, recombination, and the random pairing of gametes Short Average Tall

  5. The Gene Pool • The total genetic information available in a population is called the gene pool • Allele frequency is determined by dividing the total number of a certain allele by the total number of alleles of all types in the population • EX: AA, AA, Aa, aa, Aa (individuals in a population) Frequency of A allele = 6 ‘A’ / 10 ‘A or a’ = 6/10 = 60% Recall: an allele is one of the alternative forms of a gene

  6. The Gene Pool • Predicting Phenotype • Phenotypic frequency is equal to the number of individuals with a particular phenotype divided by the total number of individuals in the population • Allele frequencies in the gene pool do not change unless acted upon by certain forces…

  7. The Hardy-Weinberg Genetic Equilibrium • Hardy-Weinberg genetic equilibrium is a theoretical model of a population in which no evolution occurs and the gene pool of the population is stable • Occurs under certain conditions: • No mutations occur • The population is infinitely large • Individuals neither leave nor enter the population • Random mating

  8. Section 2 Disruption of Genetic Equilibrium: Objectives • Explain how migration can affect the genetics of equilibrium • Explain how genetic drift can affect populations of different sizes

  9. Gene Flow • Immigration – the movement of individuals into a population • Emigration – the movement of individuals out of a population • If individuals are moving in or out of a population, the genetics of that population will change… if individuals move, genes move with them Immigration Emigration

  10. Genetic Drift • Genetic drift is the phenomenon by which allele frequencies in a population change as a result of random events or chance • The larger the population, the more stable the frequencies will stay. The smaller, the more things can change.

  11. Section 3 Formation of Species: Objectives • Relate the biological species concept to the modern definitions of species • Explain how the isolation of populations can lead to speciation • Compare two kinds of isolation and the pattern of speciation associated with each • Contrast the model of punctuated equilibrium with the model of gradual change

  12. The Concept of Species • Speciation is the formation of new species as a result of evolution • Morphology is the study of the structure and form of an organism (a major way to classify organisms) • There are major limitations to the morphological concept: • A species may have a great deal of phenotypic variability • Organisms that can interbreed may have very different physical characteristics • It does not consider whether individuals of a species can mate and produce viable offspring

  13. The Concept of Species • According to the biological species concept, a species is a population of organisms that can successfully interbreed but cannot breed with other groups Flying Squirrel Fox Squirrel Photo Credit: Markus Krötzsch Californian Ground Squirrel Photo Credit: Howcheng

  14. Isolation and Speciation • Geographic isolation results from the separation of population subgroups by geographic barriers • EX: A canyon could form and divide a population • Speciation can occur as a result of geographic isolation because populations that live in different environment may be exposed to different selection pressures

  15. Isolation and Speciation • Geographic isolation may lead to allopatric speciation which is when new species arise as a result of geographic isolation

  16. Isolation and Speciation • Reproductive isolation results from the separation of population subgroups by barriers to successful breeding • Two types of reproductive isolation: • Prezygotic – before fertilization • Postzygotic – after fertilization • Reproductive isolation differs from geographic isolation because members of the same species are not physically separated in reproductive isolation like they are in geographic isolation Prezygotic Barrier: Behavior Blue-footed boobie courtship display Postzygotic Barrier: Reduced Hybrid Fertility Donkey + Horse = Mule (sterile)

  17. Isolation and Speciation • Sympatric speciation occurs when two subpopulations become reproductively isolated within the same geographic area • EX: A population of insects might live on an apple tree. If some of the individuals from this population begin to live on a hawthorne tree, they will no longer interbreed with the original population

  18. Rates of Speciation • Under punctuated equilibrium, new species arise abruptly, differ greatly from their ancestors, and then change little over long periods • In the gradual model of speciation (gradualism) species undergo small changes at a constant rate

  19. Homework Pp 320 #1-5 Pp 325 #2-4 Pp 330 #1-5

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