Process of Evolution

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Process of Evolution

• Chapter 18

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Outline

• Evolution in a Genetic Context
• - Microevolution
• Hardy-Weinberg
• Causes of Microevolution
• Natural Selection
• Types of Selection
• Maintenance of Variations
• Speciation
• Modes of Speciation

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Evolution in a Genetic Context

• Microevolution
• In population genetics, the various alleles at
  all the gene loci in all individuals make up the
  gene pool of the population.
• To determine the frequency of each allele,
  calculate its percentage from the total number of
  alleles in the population.

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Hardy-Weinberg

• The Hardy-Weinberg principle states an
  equilibrium of allele frequencies in a gene pool,
  p2 2pq q2, will remain in effect in each
  succeeding generation of a sexually reproducing
  population, assuming
• No Mutations
• No Gene Flow
• Random Mating
• No Genetic Drift
• No Selection

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Hardy-Weinberg

• Under real conditions, these conditions are
  rarely, if ever, met, and allele frequencies in
  the gene pool of a population change between
  generations.
• Evolution has occurred.
• Accumulation of small changes is referred to as
  microevolution.

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Industrial Melanism
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Causes of Microevolution

• Genetic Mutations
• Once alleles have mutated, certain combinations
  of alleles might be more adaptive than others in
  a particular environment.
• Gene Flow
• Movement of alleles between populations by
  migration of breeding individuals.
• Continual gene flow reduces variability between
  populations.

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Gene Flow
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Causes of Microevolution

• Nonrandom Mating
• Individuals do not choose mates randomly.
• Assortative mating - Individuals tend to mate
  with those with the same phenotype.
• Sexual selection - Males compete for the right to
  reproduce and females choose to mate with males
  possessing a particular phenotype.

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Causes of Microevolution

• Genetic Drift
• Changes in allele frequencies of a gene pool due
  to chance.
• Larger effect in small populations.

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Genetic Drift
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Genetic Drift

• Bottleneck Effect
• An event prevents a majority of genotypes from
  entering the next generation.
• Founder Effect
• Rare alleles occur at a higher frequency in a
  population isolated from the general population.
• The alleles carried by population founders are
  dictated by chance.

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Natural Selection

• Natural Selection is the process that results in
  adaptation of a population to the biotic and
  abiotic environment. Requires
• Variation
• Inheritance
• Differential Adaptiveness
• Differential Reproduction
• Relative Fitness

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Types of Selection

• Directional Selection
• An extreme phenotype is favored and the
  distribution curve shifts in that direction.
• Can occur when a population is adapting to a
  changing environment.

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Directional Selection
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Types of Selection

• Stabilizing Selection
• Occurs when an intermediate phenotype is favored.
• Can improve adaptation of the population to
  constant conditions.

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Stabilizing Selection
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Types of Selection

• Disruptive Selection
• Two or more extreme phenotypes are favored over
  any intermediate phenotype.
• Two distinctly different phenotypes are found in
  the population.

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Disruptive Selection
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Maintenance of Variations

• Maintenance of variation is beneficial because
  populations with limited variation may not be
  able to adapt to new conditions.
• Only exposed alleles are subject to natural
  selection.
• Sickle-Cell Disease
• Homozygote remains in equilibrium in some regions
  of Africa because the heterozygote is protected
  from sickle-cell and malaria.

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Sickle-Cell Disease
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Speciation

• Speciation is the splitting of one species into
  two or more species, or the transformation of one
  species into a new species over time.
• Species Definition
• Morphological
• Biological
• Reproductive Isolation
• Phylogenetic

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Reproductive Isolating Mechanisms

• Prezygotic Isolating Mechanisms
• Prevent reproduction attempts, and make it
  unlikely fertilization will be successful.
• Habitat Isolation
• Temporal Isolation
• Behavioral Isolation
• Mechanical Isolation
• Gamete Isolation

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Reproductive Isolating Mechanisms

• Postzygotic Isolating Mechanisms
• Prevent hybrid offspring from developing or
  breeding.
• Zygote Mortality
• Hybrid Sterility
• F2 Fitness

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Modes of Speciation

• Allopatric Speciation
• Occurs when one population is geographically
  isolated from other populations.
• Sympatric Speciation
• A population develops into two or more
  reproductively isolated groups without prior
  geographic isolation.

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Adaptive Radiation

• Adaptive Radiation is an example of allopatric
  speciation.
• Many new species evolve from a single ancestral
  species when members of the species become
  adapted to different environments.

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Review

• Evolution in a Genetic Context
• - Microevolution
• Hardy-Weinberg
• Causes of Microevolution
• Natural Selection
• Types of Selection
• Maintenance of Variations
• Speciation
• Modes of Speciation

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