Genes Within populations

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Chapter 20

• Genes Within populations

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Genetic Variation Evolution

• Darwin Descent With Modification
• On the Origin of Species
• Evolution- Change Over Time
• Genetic variation- 3 sources
• Mutations
• Gene Flow
• Genetic Shuffling-sex introduces new gene
  combinations

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Mechanisms of Evolution

• Natural Selection

• Inheritance of Acquired Characteristics

• Darwin
• Variation is genetic

• Jean-Baptiste Lamarck
• Variation by experience

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Terminology

• Population genetics- study of allele frequency
  distribution and change under the influence of
  the four evolutionary forces natural selection,
  genetic drift, mutation, and gene flow.
• Polymorphic (polymorphism)- more than one allele
  of a gene
• Enzyme Polymorphism
• Heterozygosity-different alleles (Tt)
• Homozygosity-same alleles (TT or tt)
• DNA Sequence Polymorphism

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Hardy-Weinberg Principle

• Hardy-English mathematician
• Weinberg-German physician
• 5 Assumptions
• No mutations
• No transfer of genes (No immigration or
  emigration)
• Random mating
• Population is large
• No selection
• If all are met, then the original proportions of
  the genotypes in a population will remain
  constant from generation to generation
• p2 2pq q2 1 and p q 1
• p frequency of the dominant allele in the
  populationq frequency of the recessive allele
  in the population
• p2 percentage of homozygous dominant
  individualsq2 percentage of homozygous
  recessive individuals2pq percentage of
  heterozygous individuals

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Five Agents of Evolutionary Change

• Any population NOT in Hardy-Weinberg equilibrium
  indicates one of the five agents of evolutionary
  change are taking place

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Mutation

• A change in a cells DNA sequence
• Changes frequencies of alleles
• Little effect on H-W Principle
• Gene mutates 1x 100,00 cell divisions
• Often does not change allele frequency
• However, mutations are the greatest source of
  genetic variation

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Gene flow

• Movement of alleles from 1 population to another
  population
• Occurs when individuals or gametes move to new
  populations
• Homogenize allele frequencies

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Nonrandom mating

• Occurs when individuals choose to mate with
  specific traits/characteristics of other
  individuals
• Causes a shift in genotype frequencies
• 2 types
• Assortative mating phenotypically similar
  individuals mate
• Causes an excess of homozygosity
• Disassortative mating phenotypically different
  individuals mate
• Causes an excess of heterozygosity

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Genetic Drift

• Random fluctuation in allele frequencies over
  time by chance
• important in small populations
• founder effect - few individuals found new
  population (small allelic pool)
• Island Biogeography
• Ex Amish community in Pennsylvania
• descendants of about 200 individuals who
  established the community after leaving Europe in
  the early 1700s.
• One of these founders carried an unusual allele
  that causes a rare kind of dwarfism.
• bottleneck effect - drastic reduction in
  population, and gene pool size
• Ex Florida Panther

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Selection

• Artificial Selection

• Natural Selection

• Breeder chooses desired characteristics
• Agriculture/Livestock- Early on

• Environment determines which individual produces
  the most offspring
• 3 conditions for NS to occur
• Variation must exist among individuals in a
  population
• Variation among individuals must result in
  differences in the number of offspring surviving
• Variation must be genetically inherited

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3 Examples of selection
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Fitness and Its Measurement

• Fitness A phenotype with greater fitness
  usually increases in frequency
• Most fit is given a value of 1
• Fitness is a combination of
• Survival how long does an organism live
• Mating success how often it mates
• Number of offspring per mating that survive

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Interactions Among Evolutionary Forces

• Mutation and genetic drift oppose selection
• How much drift occurs is inversely related to
  population size
• Gene flow can either promote or constrain
  evolutionary change

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Maintenance of Variation

• Frequency-dependent selection favors phenotypes
  on how often they occur
• Negative frequency- dependent selection favors
  rare phenotypes
• Predator searches for particular color of prey
• Positive frequency-dependent selection favors
  common phenotypes
• Predator searches for unique prey, stands out
  from other prey
• Oscillating selection favors one phenotype at one
  point in time and another phenotype at another
  point in time
• Example Bird beak size in drought vs. rainy
  season
• Heterozygote advantage favors individuals who are
  heterozygous
• Example Sickle cell allele and malaria in Africa

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Selection Acting on Traits Affected by Multiple
Genes

• 3 kinds of selection
• Disruptive selection- tries to eliminate
  intermediate forms
• Example Birds feeding on large/small seeds, need
  only large/small beaks
• Direction selection-tries to eliminate phenotypes
  at one end of the range
• Example Drosopholia moving away from light
• Stabilizing selection- favors individuals with
  intermediate forms
• Example Birth weight in humans

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Experimental Studies of Natural Selection

• Sometimes evolutionary change occurs rapidly
• Evolutionary studies
• High predation sites- males drab in color
• Low predation sties- males are brightly colored
• Both lab and field experiments were conducted
• Change occurring lt 12 yrs

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The Limits of Selection

• Pleitrophy- single gene influences several
  phenotypic traits
• Limits the variability in the phenotype
• Problem Evolution requires genetic variation
• Example
• Thoroughbred horses- race times have not improved
  since the 50s
• Epistasis- takes place when the action of one
  gene is modified by one or several other gene
• Affects fitness of gene