Population Genetics

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• Population Genetics
• Explain the statement It is the population, not
  the individual, that evolves.
• Explain how Mendels particulate hypothesis of
  inheritance provided much-needed support for
  Darwins theory of evolution by natural
  selection.
• Distinguish between discrete and quantitative
  traits. Explain how Mendels laws of inheritance
  apply to quantitative traits.
• Define the terms population, species, and gene
  pool.
• Explain why meiosis and random fertilization
  alone will not alter the frequency of alleles or
  genotypes in a population.
• List the five conditions that must be met for a
  population to remain in Hardy-Weinberg
  equilibrium.
• Write the Hardy-Weinberg equation. Use the
  equation to calculate allele frequencies when the
  frequency of homozygous recessive individuals in
  a population is 25.   

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Mutation and Sexual Recombination

• Explain why the majority of point mutations are
  harmless
• Explain why mutation has little quantitative
  effect on allele frequencies in a large
  population
• Describe the significance of transposons in the
  generation of genetic variability
• Explain how sexual recombination generates
  genetic variability.   

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Natural Selection, Genetic Drift, and Gene Flow

• Explain the following statement Only natural
  selection leads to the adaptation of organisms to
  their environment.
• Explain the role of population size in genetic
  drift.
• Distinguish between the bottleneck effect and the
  founder effect.
• Describe how gene flow can act to reduce genetic
  differences between adjacent populations.  

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Genetic Variation, the Substrate for Natural
Selection

• Distinguish between average heterozygosity and
  nucleotide variability. Explain why average
  heterozygosity tends to be greater than
  nucleotide variability.
• Define a cline.
• Define relative fitness.a. Explain why relative
  fitness is zero for a healthy, long-lived,
  sterile organism.b. Explain why relative fitness
  could be high for a short-lived
  organism.21.Distinguish among directional,
  disruptive, and stabilizing selection. Give an
  example of each mode of selection.
• Explain how diploidy can protect a rare recessive
  allele from elimination by natural selection.
• Describe how heterozygote advantage and
  frequency-dependent selection promote balanced
  polymorphism.
• Define neutral variations. Explain why natural
  selection does not act on these alleles.
• Distinguish between intrasexual selection and
  intersexual selection.
• Explain how female preferences for showy male
  traits may benefit the female.
• Describe the disadvantages of sexual
  reproduction.
• Explain how the genetic variation promoted by sex
  may be advantageous to individuals on a
  generational time scale.
• List four reasons why natural selection cannot
  produce perfect organisms.

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CHAPTER 23THE EVOLUTION OF POPULATIONS
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The Modern Synthesis

• Integrates discoveries from different fields
  (paleontology, taxonomy, biogeography, and
  population genetics)

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• Population Genetics Emphasizes genetic
  variation with populations and recognizes
  importance of quantitative characters (polygenic
  inheritance)

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Emphasizes

• Importance of populations as units of evolution
• The central role of natural selection as the
  primary mechanism of evolutionary change
• Gradualism as the explanation of how large
  changes can result from an accumulation of small
  changes occurring over long periods of time

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A Populations Genetic Structure

• Population Localized group of organisms which
  belong to the same species

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Species Groups of interbreeding natural
populations, which are reproductively isolated
from such other groups

• Each population center is somewhat isolated from
  other population centers with only occasional
  gene flow among groups
• May be separated by boundaries
• Some populations not separated by boundaries
• 2 population centers may be connected by a
  sparsely populated range
• Gene flow between population centers reduced by
  intermediate range

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Gene Pool Total aggregate of genes in a
population at any one time

• All alleles at all gene loci in all individuals
  in a population. Alleles combine to produce next
  generation
• May be homozygous or heterozygous at each locus
• Fixed allele all individuals are homozygous
• Normally 2 or more alleles for a gene, each
  having a relative frequency

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Hardy-Weinberg Theorem Non-evolving Population

• In the absence of other factors, the segregation
  and recombination of alleles during meiosis and
  fertilization will not alter the overall genetic
  makeup of a population
• Allele frequencies in a gene pool will remain
  constant unless acted upon by other agents

NOTE Mendelian Genetics states that genetic
variation is from one generation to the next is
preserved
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Microevolution Generation-to-generation Change
in a Populations Allele or Genotype Frequency

• Hardy-Weinberg provides a base line from which
  evolutionary departure takes place
• At a locus with 2 alleles, the 3 possible
  genotypes will appear in the following
  proportions
• p2 2pq q2 1
• p2 homozygous dominant condition
• 2pq Heterozygous condition
• q2 homozygous recessive condition

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For Hardy-Weinberg equilibrium to be maintains,
5 conditions must be met

• Very large population size
• Isolation from other populations (no migration in
  or out of a population)
• No Mutations
• Random Mating
• No natural selection All genotypes are equal in
  survival and reproductive success

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Microevolution Small scale evolutionary change
represented by a generational shift in a
populations relative allelic frequencies

• Microevolution can be caused by genetic drift,
  gene flow, mutation, nonrandom mating, and
  natural selection deviation from Hardy-Weinberg
  
• Only natural selection leads to an accumulation
  of favorable adaptations in a population
• Other four Non-adaptive and non-Darwinian
  changes

A Short Overview of Microevolution Click to watch
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Genetic Drift

• Definition Changes in the gene pool of a small
  population due to chance
• Chance events have greatest effects on small
  population (natural disasters)
• Chance events may cause the frequencies of
  alleles to drift randomly from generation to
  generation

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Bottleneck Effect The size of a population may
be reduced drastically by natural disasters that
kill organisms nonselectively

• Small surviving population is unlikely to
  represent genetic makeup of original population
• Reduces overall genetic variability in a
  population since some alleles may be entirely
  absent
• Examples Northern elephant seals and south
  African cheetahs

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Founder Effect Genetic Drift occurring when a
few individuals colonize a new habitat

• The smaller the founding population, the less
  likely its gene pool will represent the original
  populations genetic makeup

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Gene Flow

• Definition Migration of fertile individuals, or
  the transfer of gametes, between populations
• Natural populations may gain or lose alleles
• Reduces between-population differences and may
  eventually group neighboring populations into a
  single population

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Mutations

• A new mutation transmitted in gametes immediately
  changes the gene pool of a population by
  substituting one allele for another
• Mutation has little quantitative effect on large
  populations in a single generation, since
  mutation at any given locus is very rare
• It is important to evolution since it is the
  original source of genetic variation, which is
  the raw material for natural selection

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• Non-Random Mating Increases the number of
  homozygous loci in a population, but does not
  alter the frequencies of alleles in a
  populations gene pool

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Inbreeding individuals usually mate with close
relatives

• Self fertilization most extreme form and causes
  build up of homozygous alleles
• Inbreeding results in relative genotypic
  frequencies that deviate from predicted
  Hardy-Weinberg, but does not alter allele
  frequencies more homozygous individuals, less
  heterozygotes

Inbreeding of Shetland ponies
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• Assortive Mating Individuals mate with partners
  that are like themselves in certain phenotypic
  characters

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Natural Selection

• Variations among individuals exist and some
  variants leave more offspring than others
• Natural selection is differential success in
  reproduction
• Alleles are passed on to next generation in
  disproportionate numbers relative to frequencies
  in present generation
• Only adaptive agent of microevolution

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Genetic Variation

• Polygenic Ranges with a trait due to influences
  from several genes
• Discrete characteristics determined by only 1
  locus
• Morphs contrasting Mendelian characteristics
• Polymorphic two or more morphs present in
  noticeable frequencies

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Geographical Variation

• Cline graded change in some trait along a
  geographic transect

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Mutations

• Mutations that alter protein function usually are
  harmful (point)
• Occasionally, a mutant allele is beneficial,
  which is more probable when environmental
  conditions are changing (DDT and mosquitoes)
• Translocation (rarely) may produce a cluster of
  genes with cooperative function

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• Sexual recombination recombines old alleles into
  fresh assortments

A short clip describing the causes of genetic
variation through sexual reproduction
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Preservation of genetic variation

• Diploidy hides much genetic variation from
  selection by presence of recessive alleles
  hidden recessive alleles may be beneficial if
  environmental conditions change

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Balanced polymorphism Ability of natural
selection to maintain diversity in a population

• Heterozygote advantage natural selection will
  maintain 2 or more alleles at a locus if
  heterozygotes have greater reproductive success
• Sickle cell anemia
• Hybrid vigor segregation of deleterious
  recessives

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Normal RBC
Sickle RBC
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• Patchy Environments different phenotypes
  favored in different sub-regions
• Frequency dependent selection reproductive
  success of a morph declines if that form becomes
  too common Papilio dardanus

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• Neutral Variation genetic variations that
  confer no selective advantage

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Natural selection Mechanism for Adaptive
Evolution

• Adaptive Evolution results from
• Chance events that produce genetic variation
• Natural selection favoring a variation
• Fitness measured by the relative contribution
  an individual makes to the gene pool of the next
  generation reproductive success

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Peppered moths of the industrial revolution
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Modes of Natural Selection

• Stabilizing Selection favors intermediate
  variants by selecting against extreme phenotypes
• Directional Selection favors variants of one
  extreme
• Diversifying (Disruptive) Selection Opposite
  phenotypic extremes favored over intermediate

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Sexual Selection

• Sexual dimorphism distinction between the
  secondary sexual characteristics of males and
  females

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• Adornments have no adaptive advantage but
  increase reproductive success by attracting
  females

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Does Evolution Fashion the Perfect Organisms?

• NO!!!
• Organisms are locked into historical constraints
• Adaptations are often compromises
• Not all evolution is adaptive
• Selection can only edit variations that exist