15.3 Shaping Evolutionary Theory

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15.3 Shaping Evolutionary Theory

• Main idea The theory of evolution continues to
  be refined as scientists learn new information.
• Objectives
• Discuss patterns observed in evolution.
• Describe factors that influence speciation.
• Compare gradualism with punctuated equilibrium.
• Review Vocabulary
• Allele alternative forms of a character trait
  that can be inherited.

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15.3 Shaping Evolutionary Theory

• New Vocabulary

Hardy-Weinberg Principle Prezygotic isolating mechanism
Genetic drift Allopatric speciation
Founder effect Postzygotic isolating mechanism
Bottleneck Sympatric speciation
Stabilizing selection Adaptive radiation
Directional selection Gradualism
Disruptive selection Punctuated Equilibrium
Sexual selection
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Mechanisms of Evolution

• Evolution occurs at the population level, with
  genes as the raw material.
• Hardy-Weinberg Principle states that when allelic
  frequencies remain constant, a population is in
  genetic equilibrium. (Even though the number of
  owls doubled, the ratio of gray to red owls
  remained the same).

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Hardy-Weinberg Principle

• p2 2pq q2 1
• This equation allows us to determine the
  equilibrium frequency of each genotype in the
  population homozygous dominant (p2),
  heterozygous (2pq), and homozygous recessive
  (q2). Note the sum of these frequencies equals
  one.

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Hardy-Weinberg Principle

• A population in genetic equilibrium must meet
  five conditions there must be no genetic drift,
  no gene flow, mating must be random, mutation,
  and there must be no natural selection.

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Genetic Drift

• A change in the allelic frequencies in a
  population that is due to chance and random
  mating.
• In smaller populations, the effects of genetic
  drift become more pronounced, and the chance of
  losing an allele becomes greater.
• Examples Founder Effect Bottleneck

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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory
Genetic Drift

• A change in the allelic frequencies in a
  population that is due to chance and random mating

• In smaller populations, the effects of genetic
  drift become more pronounced, and the chance of
  losing an allele becomes greater.
• Marble Example

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Founder Effect

• Occurs when a small sample of a population
  settles in a location separated from the rest of
  the population
• Alleles that were uncommon in the original
  population might be common in the new population.
• Examples Amish French Canadians

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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory
Founder Effect

• The loss of genetic variation that occurs when a
  new population is established by a very small
  number of individuals from a larger population

Island 1
Island 2
Mainland
Island 3
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Bottleneck

• Occurs when a population declines to a very low
  number and then rebounds

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Evolution
15.3 Shaping Evolutionary Theory
Bottleneck

• a significant percentage of a population or
  species is killed or otherwise prevented from
  reproducing and can rebound later

• Often caused by a natural disaster

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Gene Flow

• A population in genetic equilibrium experiences
  no gene flow, however, few populations are
  isolated
• Random movement of individuals between
  populations, or migration increases genetic
  variation within a population and reduces
  differences between populations

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Gene Flow

• Genes entering or leaving a population through
  migration
• Emigration? Genes LEAVING a population
• Immigration? INCOMING genes in a population

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Nonrandom mating

• Rarely is mating completely random in a
  population.
• Usually individuals mate with individuals in
  close proximity.
• This promotes inbreeding and could lead to a
  change in allelic proportions favoring
  individuals that are homozygous for particular
  traits

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Mutations

• A random change in genetic material.
• Collective mutations violate genetic equilibrium.
• Occasionally mutations provide an advantage to
  organisms and become more common in subsequent
  generations.
• Basis upon which natural selection works.

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What is a gene pool?

• Gene pool combined genetic info of all members
  of a population
• Contains two of more alleles (genes) for the same
  trait
• Allele frequency number of times an allele
  occurs in a gene pool compared to the number of
  times another allele occurs (expressed in
  percents)

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Relative Frequencies of Alleles
Section 16-1
allele for brown fur
allele for black fur
Sample Population
48 heterozygous black
Frequency of Alleles
16 homozygous black
36 homozygous brown
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Natural Selection

• Acts to select the individuals that are best
  adapted for survival and reproduction
• Stabilizing selection operates to eliminate
  extreme expressions of a trait when the average
  expression leads to higher fitness. (Birth
  Weights)
• Directional selection makes an organism more fit.
  (Peppered Moths)
• Disruptive selection is a process that splits a
  population into two groups. (African Finches)

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Chapter 15
15.3 Shaping Evolutionary Theory

• Stabilizing selection operates to eliminate
  extreme expressions of a trait when the average
  expression leads to higher fitness.

• Ex. Siberian Husky

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Evolution
Chapter 15
15.3 Shaping Evolutionary Theory

• Directional selection makes an organism more fit.

• Favors the extremes

• Ex. Greyhound Dog

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Sexual Selection

• Sexual selection operates in populations where
  males and females differ significantly in
  appearance. (Males are largest and most colorful
  in the group).
• Qualities of sexual attractiveness appear to be
  the opposite of qualities that might enhance
  survival. (More likely target for predators yet
  bodies enhance reproductive success).
• Examples Peacocks

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Reproductive Isolation

• Two types of reproductive isolating mechanism
  prevent gene flow among populations
• Prezygotic isolating mechanisms operate before
  fertilization occurs.
• Postzygotic isolating mechanisms operate after
  fertilization has occurred to ensure that the
  resulting hybrid remains infertile.

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Prezygotic Isolation

• prevents reproduction by making fertilization
  unlikely.
• prevents genotypes from entering a populations
  gene pool through geographic, ecological,
  behavioral, or other differences
• Eastern and Western meadowlarks similar in
  appearance but their songs separate them
  behaviorally.

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Postzygotic Isolation

• Occurs when fertilization has occurred but a
  hybrid offspring cannot develop or reproduce.
• Prevents offspring survival or reproduction.
• A Liger is the offspring of a male lion and a
  female tiger. Ligers are sterile.

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Speciation

• A population must diverge and then be
  reproductively isolated for speciation to occur.
• Two types of speciation
• Allopatric speciation - A physical barrier
  divides one population into two or more
  populations. (The Grand Canyon separates the
  Abert and Kaibab squirrels).
• Sympatric speciation - A species evolves into a
  new species without a physical barrier. The
  ancestor species and the new species live side by
  side during the speciation process. (Occurs
  frequently in plants due to polyploidy).

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Patterns of Evolution

• Adaptive Radiation - Can occur in a relatively
  short time when one species gives rise to many
  different species in response to the creation of
  new habitat or some other ecological opportunity.
• Follows large-scale extinction events

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Coevolution

• The relationship between two species might be so
  close that the evolution of one species affects
  the evolution of the other species.
• Mutualism occurs when two species benefit each
  other. (Moth and the comet orchid it pollinates
  exist in a mutualistic relationship).
• Coevolutionary arms race one species can evolve
  a parasitic dependency on another species.
• Example A plant and an insect pathogen that is
  dependent on the plant for food. The plant
  evolves a chemical defense the insect evolves a
  biochemistry to resist the defense response
  continues to escalate.

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Convergent Evolution

• Unrelated species evolve similar traits even
  though they live in different parts of the world.
• Occurs in environments that are geographically
  far apart but have similar ecology and climate.

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Rate of Speciation

• Evolution proceeds in small, gradual steps
  according to a theory called gradualism.
• Punctuated equilibrium explains rapid spurts of
  genetic change causing species to diverge quickly.