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Biological Science Chapter 5


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Evolution and Gene Frequencies

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Biological Science Chapter 5

  1. 1. Chapter 5 Evolution and Gene Frequencies
  2. 2. Populations and Gene Pools Population - are groups of individuals of the same species that occupy a given area. - share unique set of genes  Alleles – varying expressions of genes at each locus
  3. 3. Gene pool - the sum of all the alleles for all traits in a sexually reproducing population is pool of hereditary resources for the entire population.
  4. 4. • These sources of variation include: 1. The independent assortment of chromosomes
  5. 5. 2. The crossing over
  6. 6. 3. the chance fertilization of an egg by a sperm cell
  7. 7. 4.Rearrangements in the number and structure of chromosomes.
  8. 8. 5. Mutations of existing alleles
  9. 9. MUST EVOLUTION HAPPEN? The rate of evolution is slow, and sometimes it is rapid. Theories of population genetics - the study of the genetic events in gene pools.
  10. 10. The Hardy- Weinberg Theorem Godfrey H. Hardy Wilhelm Weinberg - it states that the mixing of alleles at meiosis and their subsequent recombination do not alter the relative frequencies of the alleles in future generations, if ccertain assumption are met.
  11. 11. The asssumption of the Hardy- Weinberg theorem are as follows: 1.The population size must be large. 2.Sexual reproduction within the population must be random. 3.Individuals cannot migrate into, or out of, the population. 4.Mutations must not occur.
  12. 12. EVOLUTIONNARY MECHANISM Population size, Genetic drift, and Neutral evolution Population size - the smaller population, the more significant may be. Genetic drift - chance events influencing the frequencies of genes in populations Neutral evolution - gene frequencies are changing independently of natural selection
  13. 13. Two special cases of genetic drift have influenced the genetic makeup of some populations. Founder effect - the new colony that emerges from the founding individuals is likely to have distinctive genetic makeup with far less variation than the larger population.
  14. 14. Bottleneck effect - if population sizes are restored, they will have only the remnant of the original gene pool.
  15. 15. Gene flow -changes in relative allelic frequency from the migration of individuals.
  16. 16. Mutation -changes in the structure of genes and the chromosomes. -origin of all new alleles and a source of variation that may prove adaptive for an animal. MUTATION PRESSURE -measure of the tendency for gene frequencies to change through mutation.
  17. 17. mutation
  18. 18. Natural selection reexamined: MODES OF SELECTION Natural selection may affect a range of phenotypes in tree ways: 1. Directional selection- occurs when individuals at one phenotypic extreme are at a disadvantage compared to all other individuals in the population.
  19. 19. 2.Disruptive selection- occurs when natural selection favors both extremes of continuous variation. -an intermediate phenotype is selected against.
  20. 20. 3. Stabilizing selection- occurs when individuals at both phenotypic extremes are selected against. -narrows the phenotypic range.
  21. 21. Neutralist/ Selectionist Controversy  Both natural selection and neutral evolution occur, but they may not be equally important in all circumstances.
  22. 22. Balanced Polymorphism and Heterozygote Superiority Balanced polymorphism- occurs when different phenotypes are maintained at relatively stable frequencies in the population. Heterozygote superiority- when the heterozygote is more fit than either homozygote.
  23. 23. Species and Speciation  Species – a group of populations in which genes are actually, or potentially, exchanged through interbreeding.
  24. 24.  Speciation – the formation of new species
  25. 25.  Reproductive isolation- gene flow among populations or subpopulations does not occur.  Occurs in different ways:  Premating isolation – prevents mating from taking place.
  26. 26.  Postmating isolation – prevents successful fertilization and development, even though mating may have occured.
  27. 27. Allopatric speciation -occurs when subpopulations become geographically isolated from one another.
  28. 28. Parapatric Speciation - occurs in small population called demes Sympatric Speciation - occurs within a single population
  29. 29. Rates of Evolution - it is a measurement of the change in an evolutionary lineage over time Phyletic Gradualism - evolutionary change as occuring over millions of years - changes are gradual over long periods Punctuated Equilibrium - rapid periods of change interrupt long periods of stasis
  30. 30. Molecular Biology All evolutionary change results from changes in the base sequences in DNA and amino acids in proteins Gene Duplication - the accidental duplication of a gene on a chromosome is one way that extra genetic material can arise Mosaic Evolution - the occurence within a given population of organisms of different rates of evolutionary change various body structures aand functions