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Evolution of Populations. 23.1 – Mutation & sexual reproduction produce genetic variation that makes evolution possible. 1) Microevolution Change in the allele frequencies of a population over generations Evolution on the smallest scale. 2) Mutations

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23 1 mutation sexual reproduction produce genetic variation that makes evolution possible
23.1 – Mutation & sexual reproduction produce genetic variation that makes evolution possible
  • 1) Microevolution
    • Change in the allele frequencies of a population over generations
    • Evolution on the smallest scale
slide3

2) Mutations

    • The only source of NEW genes & NEW alleles
    • Only mutations in cell lines that produce gametes can be passed on to offspring
types of mutations
Types of Mutations
  • A) Point Mutation
    • Change in one base in a gene
    • Can impact phenotype
      • Sickle cell anemia
  • B) Chromosomal Mutation
    • Delete, disrupt, duplicate, or rearrange many loci at once
      • Most are harmful, but not always
slide5

3) Variations due to sexual reproduction

    • Rearranges alleles into new combinations in every generation
    • 3 mechanisms for this shuffling:
        • Next slide
slide6

1) Crossing over

    • During Prophase I of meiosis
  • 2) Independent assortment
    • During meiosis (223 different combinations possible)
  • 3) Fertilization
    • 223 x 223 for sperm and egg
23 2 the hardy weinberg equation can be used to test whether a population is evolving
23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving
  • Population genetics
    • Study of how populations change genetically over time
  • Population
    • Group of individuals of the same species that live in the same area
    • Interbreed & produce fertile offspring
slide9

Gene pool

    • All of the alleles at all loci in all the members of a population
    • In diploids, each individual has 2 alleles for a gene & the individual can be heterozygous or homozygous
    • If all are homozygous for an allele, the allele is FIXED – only one allele exists at the locus in the population
    • The greater the # of FIXED alleles, the lower the species’ diversity
slide10

Hardy-Weinberg

    • Used to describe a population that is NOT evolving
    • Frequencies of alleles & genes in a gene pool will remain constant over generations
5 conditions for hardy weinberg
5 Conditions for Hardy-Weinberg
  • 1) No mutations
  • 2) Random mating
  • 3) No natural selection
  • 4) The population size must be large (no genetic drift)
  • 5) No gene flow (Emigration, immigration, transfer of pollen, etc.)
slide13

If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then

    • p2 + 2pq + q2 = 1
    • where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype
practice
Practice
  • Suppose in a plant population that red flowers (R) are dominant to white flowers (r). In a population of 500 individuals, 25% show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait?
23 3 natural selection genetic drift gene flow can alter allele frequencies in a population
23.3 – Natural Selection, genetic drift, & gene flow can alter allele frequencies in a population
  • Mutations can alter gene frequency, but are rare
  • 3 major factors alter allelic frequencies
    • 1) Natural selection
      • Alleles are passed to the next generation in proportions different from their frequencies to the present generation
      • Those that are better suited produce more offspring than those that are not
slide16

2) Genetic Drift

    • Unpredictable fluctuation in frequencies from one generation to the next
    • The smaller the population, the greater chance
    • Random & nonadaptive
      • A) Founder effect = individuals are isolated and establish a new population – gene pool is not reflective of the source population
      • B) Bottleneck effect = a sudden change in the environment reduces population size – survivors have a gene pool that no longer reflects original
slide18

Genetic drift is significant in small populations

  • Genetic drift causes allele frequencies to change at random
  • Genetic drift can lead to a loss of genetic variation within populations
  • Genetic drift can cause harmful alleles to become fixed
slide19

3) Gene Flow

    • Populations loses or gains alleles by genetic additions or subtractions
    • Results from movement of fertile individuals or gametes
    • Reduces the genetic differences between populations, makes populations more similar
23 4 natural selection is the only mechanism that consistently causes adaptive evolution
23.4 Natural Selection is the only mechanism that consistently causes adaptive evolution
  • Relative fitness
    • The contribution an organism makes to the gene pool of the next generation relative to the contributions of the other members
    • Does NOT indicate strength or size
    • Measured by reproductive success
slide22

Natural selection acts more directly on the phenotype and indirectly on the genotype

  • Can alter the frequency distribution of heritable traits in 3 ways:
    • 1) Directional selection
    • 2) Disruptive selection
    • 3) Stabilizing selection
slide23

1) Directional selection

    • Individuals with one extreme of a phenotypic range are favored, shifting the curve toward this extreme
      • Example: Large black bears survived periods of extreme cold better than small ones, so they became more common during glacial periods
slide24

2) Disruptive Selection

    • Occurs when conditions favor individuals on both extremes of a phenotypic range rather than individuals with intermediate phenotypes
      • Example: A population has individuals with either large beaks or small beaks, but few with intermediate – apparently the intermediate beak size is not efficient in cracking either the large or small seeds that are available
slide25

3) Stabilizing Selection

    • Acts against both extreme phenotypes and favors intermediate variations
      • Example: Birth weights of most humans lie in a narrow range, as those babies who are very large or very small have higher mortality rates
slide27

How is genetic variation preserved in a population?

    • Diploidy
      • Capable of hiding genetic variation (recessive alleles) from selection
    • Heterozygote advantage
      • Individuals that are heterozygous at a certain locus have an advantage for survival
        • Sickle cell anemia – homozygous for normal hemoglobin are more susceptible to malaria, homozygous recessive have sickle-cell, but those that are heterozygotes are protected from malaria and sickle-cell
slide29

Why Natural Selection cannot produce perfect organisms:

    • 1) Selection can only edit existing variations
    • 2) Evolution is limited by historical constraints
    • 3) Adaptations are often compromises
    • 4) Chance, natural selection, & the environment interact