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Evolutionary Processes. Chapter 24. Populations. Group of individuals from the same species that live and breed together Smallest unit that can evolve four mechanisms can cause evolution in a population: natural selection genetic drift gene flow mutation.

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  • Group of individuals from the same species that live and breed together
  • Smallest unit that can evolve
  • four mechanisms can cause evolution in a population:
    • natural selection
    • genetic drift
    • gene flow
    • mutation
hardy weinberg principle
Hardy Weinberg Principle
  • Mathematical model to calculate what happens to allele frequencies when no evolution is taking place
  • For a gene with two alleles A1 and A2, three genotypes are possible:
    • A1A1, A1A2, and A2A2
  • If the frequency of A1=p and the frequency of A2=q, then:
    • the frequency of the A1A1 genotype in the new generation will be p2, the frequency of A2A2 will be q2, and the frequency of A1A2 will be 2pq
gene pools
Gene Pools
  • The total aggregate of genes in a population
    • All alleles and gene loci in all the individuals in a population
    • If there is only one allele in a population the gene is fixed for that population
      • All individuals are homozygous for that allele
    • If there is more than one allele for that gene than they can be heterozygous for either allele or can be heterozygous
hardy weinberg equation
Hardy Weinberg Equation
  • Sum of the three genotype frequencies must equal 1 (100% of the population):
    • p2 + 2pq + q2 = 1
    • Since all of the next generations progeny must be one of the three genotypes
  • States that unless acted on by a force
    • Allele frequency will not change, will always be p2, 2pq, and q2
    • Does not change under Mendelian inheritance
hardy weinberg equilibrium
Hardy-Weinberg Equilibrium
  • The Hardy-Weinberg theorem describes a hypothetical population
  • In real populations allele and genotype frequencies do change over time
  • In order for a population to be at Hardy-Weinberg equilibrium several conditions must be met
    • Among these are random mating and donating gametes at random
    • Extremely large population size
    • No gene flow
    • No mutations
hardy weinberg equilibrium1
Hardy Weinberg Equilibrium
  • Serves as a null hypothesis for determining whether evolution is acting on a particular gene in a population
  • We can use the Hardy-Weinberg equation
    • To estimate the percentage of the human population carrying the allele for an inherited disease
    • Use is to extimate the probabilities of births with certain diseases in a population
      • PKU (phenylketonurea)
hardy weinberg equilibrium2
Hardy Weinberg Equilibrium
  • When populations do not conform to Hardy-Weinberg proportions, evolution or nonrandom mating is occurring
  • Can figure this out by these steps:
  • Estimate genotype frequencies by observation
  • Calculate observed allele frequencies from the observed genotype frequencies
  • Use the observed allele frequencies to calculate the genotypes expected according to the Hardy-Weinberg principle
  • Compare the observed and expected values
patterns of natural selection1
Patterns of Natural Selection
  • Directional selection-natural selection increases the frequency of one allele
  • Reduces population genetic diversity over time
patterns of natural selection2
Patterns of Natural Selection
  • Stabilizing selection - individuals with intermediate traits reproduce more than others
  • Maintains intermediate phenotypes in a population
patterns of natural selection3
Patterns of Natural Selection
  • Disruptive selection is the opposite of stabilizing selection
  • Occurs when intermediate phenotypes are selected against and extreme phenotypes are favored.
  • Disruptive selection maintains genetic variation but does not change the mean value of a trait
patterns of natural selection4
Patterns of Natural Selection
  • Disruptive selection can cause speciation
  • If individuals with one extreme of a trait start mating preferentially with individual that have the same trait
sexual selection
Sexual Selection
  • Mate choice often plays an important role in speciation
  • Selection for enhanced ability to attract mates, and is a form of natural selection
  • Usually different in males and females
    • Females fitness is limited primarily by the ability to gain resources necessary to produce and rear young.
    • Male fitness is limited primarily by the ability to acquire mates
sexual selection1
Sexual Selection
  • Sexually selected traits that are useful in courtship are found primarily in males
  • Sexual selection stronger in males
  • Can cause sexual dimorphism
sexual selection via female choice
Sexual Selection via Female Choice
  • Females choose mates based on physical characteristics that signal male genetic quality, resources provided by the male, or both.
  • Girls preferred brighter beak
sexual selection via male male competition
Sexual Selection via Male-Male Competition
  • Males compete for breeding rights
  • Sexual selection is driven by male-male competition rather than by female choice
genetic drift1
Genetic Drift
  • Change in allele frequency due to chance
  • Causes allele frequencies to drift up and down randomly over time
  • Random with respect to fitness, not adaptive
  • Two main causes of genetic drift:
    • Founder effect
    • Bottleneck effect
founder effect
Founder Effect
  • Occurs when a group leaves a population, emigrates to a new area, and starts a new population
  • If it is a small populations, allele frequencies may differ from those of the source population
  • Common in the colonization of isolated habitats such as islands, mountains, caves, and ponds
founder effect1
Founder Effect
  • Can cause a recessive allele to be dominant in a population
  • Ex. Ellis-Van Crevald Syndrome
bottleneck effect
Bottleneck Effect
  • A sudden decrease in population size
  • Can lead to a geneticbottleneck—a sudden reduction in the number of alleles in a population
  • Commonly caused by disease outbreaks and natural catastrophes
  • Genetic drift often occurs in the resulting small population
bottleneck effect1

Elephant seals in California


Shaking just a few marbles through the narrow neck of a bottle is analogous to a drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new population and gold marbles are absent.







Bottleneck Effect
gene flow
Gene Flow
  • Movement of alleles from one population to another
  • Occurs whenever individuals leave one population, join another, and reproduce
  • Deduces genetic differences between the source and recipient populations
  • Lupines in Mt. St. Helens
  • Mutation restores genetic diversity and creates new alleles
  • Where most alleles come from
  • Adds new alleles into populations at all gene loci
  • Generally results in deleterious alleles butdoes occasionally produce advantageous alleles
  • Rates are too low to affect allele frequencies significantly
  • Must be acted on by another evolutionary mechanism before change can occur
  • Mutation provides the genetic variation upon which natural selection can act
  • Reduces the frequency of heterozygotes and increases the frequency of homozygotes in each generation
  • Affects genotype frequency, it does not change allele frequencies
  • Does not cause evolution
  • Reduces fitness in a population