The Evolution of Populations and Speciation

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Chapter 16

• The Evolution of Populations and Speciation

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Chapter 16

• Genetic Equilibrium
• Disruption of Genetic Equilibrium
• Formation of Species

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Genetic Equilibrium 16-1

• By the time of Darwins death in 1882, the idea
  of evolution by natural selection had gained wide
  acceptance among scientists. But with the birth
  of the field of genetics in the early 1900s,
  spurred by the rediscovery of Mendels work on
  the mechanics of inheritance, many questions
  about evolution and natural selection resurfaced

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Variation of Traits in a Population

• Population Genetics is the study of evolution
  from a genetic point of view.
• A population consists of a collection of
  individuals of the same species that routinely
  interbreed.

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Variation of Traits in a Population

• Populations are important to the study of
  evolution because a population is the smallest
  unit in which evolution occurs.
• Within a population, individuals may vary in
  observable traits.

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Variation of Traits in a Population
Average height of men
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Variation of Traits in a Population

• Because the shape of the curve looks like a bell,
  this is known as a bell curve.
• The bell curve shows that while a few men in a
  population are extremely short or extremely tall,
  most are average height.

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Variation of Traits in a Population

• Variation in traits
• 1. Environmental Factors
• Quality or amount of food available
• 2. Heredity

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Variation of Traits in a Population

• Variations in genotypes
• 1. Mutation
• This results from flawed copies of individual
  genes.

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Variation of Traits in a Population

• Variations in genotypes
• 2. Recombination
• The reassociation of genes in a diploid
  individual
• Occurs during meiosis by the independent
  assortment of genes on nonhomologous chromosomes
  and by crossing over between genes located on
  homologous chromosomes

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Variation of Traits in a Population
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Variation of Traits in a Population

• Variations in genotypes
• 3. Random fusion of gametes
• Hundreds of millions of sperm are involved in
  mating.
• The sperm that actually fertilizes the egg is all
  chance

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Variation of Traits in a Population
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Allele Frequencies and the Gene Pool

• Gene pool is the total genetic information
  available in a population

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Allele Frequencies and the Gene Pool

• Suppose there are 2 forms of an allele, A and a
  in a set of 10 gametes.
• If ½ the gametes in the set carry allele A, what
  would be the allele frequency?

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Allele Frequencies and the Gene Pool

• Allele frequency is determined by dividing the
  number of a certain allele by the total number of
  alleles of all types in the population.
• 5 instances of allele A / 10 gametes of a or A
• Remember that gametes are haploid and therefore
  carry only one form of the allele

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Allele Frequencies and the Gene Pool

• Phenotypes can change from generation to
  generation.
• Homozygous (dominant or recessive) contain two of
  the same allele (RR or rr).
• Heterozygous contain one of each Rr.

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Allele Frequencies and the Gene Pool

• Black hair is dominant in mice over brown hair.
• What are the 2 phenotypes?
• Black hair can be either BB or Bb
• Brown hair can only by bb
• Even though there are 3 genotypes, there are only
  2 phenotypes.

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Allele Frequencies and the Gene Pool

• In four oclock flowers that are homozygous RR
  are red.
• Homozygous rr flowers are white
• Heterozygous Rr flowers are pink.
• These flowers show incomplete dominance

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Allele Frequencies and the Gene Pool

• A phenotype frequency is equal to the number of
  individuals with a particular phenotype divided
  by the total number of individuals
• What is the allele frequency for the F2
  generation
• What is the phenotype frequency for the F2
  generation

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Hardy-Weinberg Genetic Equilibrium

• The Hardy-Weinberg equilibrium is based on a set
  of assumptions about an ideal hypothetical
  population that is not evolving

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Hardy-Weinberg Genetic Equilibrium

• 1. No net mutations occur that is, allele
  frequencies do not change overall because of
  mutation
• 2. Individuals neither enter nor leave the
  population.
• 3. Individuals mate randomly
• 4. Selection does not occur.

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Disruption of Genetic Equilibrium

• Evolution is the change in a populations genetic
  material over generations, that is a change of
  the populations allele frequencies or genotype
  frequencies. Any violation of the five
  conditions necessary for the Hardy-Weinberg
  equilibrium can result in evolution.

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Mutation

• Spontaneous mutations occur constantly, at a very
  low rate and under normal conditions.
• Think of a worksheet that you photocopy a
  photocopythen photocopy that photocopy.
  Eventually the letters get a little smudged, but
  you still know whats going on. Mistaking a G
  for a C is not really life or death

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Mutations

• If an organism is exposed to mutagens mutation
  causing agents such as radiation and certain
  chemicals mutation rates can increase
  significantly.
• Now what if the word NO was smudgedthat would
  change everything

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Mutations

• Mutations can affect genetic equilibrium by
  producing totally new alleles for a trait
• Many, if not most, mutagens are harmful

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Migration

• Immigration is the movement of individuals into a
  population
• Emigration is the movement of individuals out of
  a population.
• Both either separate or together can change gene
  frequencies.

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Migration

• Common baboons live on the savannahs of Africa in
  social and breeding groups called troops. A
  troop is dominated by a few adult males, and it
  may have from 10-200 members. Females tend to
  remain with the troop they are born into,
  however, younger or less dominant males leave
  their birth troop, eventually joining another
  troop. This constant movement of male animals
  ensures gene flow.
• Gene Flow is the process of genes moving from one
  population to another

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Genetic Drift

• Genetic drift is the phenomenon by which allele
  frequencies in a population change as a result of
  random events, or chance.
• In small populations, the failure of even a
  single organism to reproduce can significantly
  disrupt the allele frequency of the population.

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Genetic Drift
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Nonrandom Mating

• Mate selection is often influenced by geographic
  proximity, and this can results in mates with
  some degree of kinship.
• Individuals often select a mate that has similar
  physical characteristics and therefore probably
  has some similar genes.
• Assortative mating is the selection of a mate
  that has similarity of characteristics

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Natural Selection

• Stabilizing Selection individuals with the
  average form of a trait have the highest fitness
• Directional Selection individuals that display
  a more extreme form of a trait have a greater
  fitness than individuals with an average form of
  the trait
• Disruptive Selection individuals with either
  extreme variation of a trait have greater fitness
  than individuals with the average form of the
  trait
• Sexual Selection females tend to choose the
  males they mate with based on certain traits

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Chapter 9

• Fundamentals of Genetics

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Chapter 9

• Mendels Legacy
• Genetic Crosses

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Mendels Legacy

• Genetics is the field of Biology devoted to
  understanding how characteristics are transmitted
  from parents to offspring. Genetics was founded
  with the work of Gregor Johann Mendel, an
  Austrian monk who experimented with garden peas.

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Gregor Mendel

• Heredity is the transmission of characteristics
  from parents to offspring.

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Gregor Mendel

• Mendel observed seven characteristics of pea
  plants. Each characteristic occurred in two
  contrasting traits

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Gregor Mendel

• Plant height
• Long or Short
• Flower position along stem
• Axial or Terminal
• Pod color
• Green or Yellow
• Pod appearance
• Inflated or constricted
• Seed texture
• Smooth or Wrinkled
• Seed color
• Yellow or Green
• Flower color
• Purple or White

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Gregor Mendel

• Mendel was able to document the traits of each
  generations parents by carefully controlling how
  the pea plants were pollinated.
• Pea plants tend to be self pollinated, but
  Mendels experiments involved cross pollination

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Mendels Experiments

• Mendel began growing plants that were pure for
  each trait (they always produce offspring with
  that trait)
• The term strain denotes plants that are pure for
  a specific trait
• Mendel eventually obtained 14 strains, one for
  each of the 14 traits he observed.

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Mendels Experiments

• He called each strain a parental generation , or
  P1 generation
• Mendel then cross pollinated these strains
• When the plants matured, he recorded the number
  of each type of offspring produced by each P1
  plant.

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Mendels Experiments

• Mendel called the offspring of the P1 generation
  the first filial generation, or F1 generation.
• He then allowed the flowers from the F1
  generation to self pollinate and collected the
  seeds.
• Mendel called the plants in this generation the
  F2 generation.

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Mendels Results and Conclusions

• Mendel crossed a pure plant for green pods with
  pure yellow pod plant.
• The F1 generation was only green-podded plants.
  No yellow pods developed even though one of the
  parents was pure for yellow pods.

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Mendels Results and Conclusions