Mechanisms for Evolution

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Mechanisms for Evolution

• Chapter 15

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Populations and Evolution

• Population a group of organisms that interbreed
• Each population shares a gene pool (the different
  alleles present in the population)
• Each population has a relative frequency of each
  allele, or the number of times the allele occurs
  in the gene pool.
• The frequency of alleles in a population tends
  not to change unless there is an outside force
  causing the change

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Evolution occurs because of changes to the
equilibrium

• There are 5 mechanisms that can change the allele
  frequencies in a population
• Natural Selection
• Genetic Drift
• Mutation
• Migration
• Non-random Mating

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1. Natural Selection

• Natural selection is a process by which
  individuals who are more fit for their
  environment survive and reproduce more often
• Allele frequencies change because of interactions
  between the population and the environment
• There are 4 different types of natural selection
• a. Stabilizing Selection
• b. Directional Selection
• c. Disruptive Selection
• d. Sexual Selection

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1a. Stabilizing Selection

• Stabilizing selection occurs when individuals
  with the average form of a trait are most fit for
  the environment and extreme traits are eliminated
• This is the most common form of natural selection

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Example of stabilizing selection

• Lizard body size
• Large lizards are easily seen by predators, but
  smaller lizards cannot run as fast to escape the
  predators
• Mid sized lizards are most fit in the
  environment, so they survive and reproduce more
  often, changing the allele frequencies in the
  population

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1b. Directional Selection

• Directional selection occurs when individuals
  with one extreme variation of a trait are the
  most fit in the environment.
• This causes a gradual shift in allele frequency
  to that extreme (shift in one direction).

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Example of Directional Selection

• Anteater tongue length
• Anteaters with long tongues are most fit because
  of the depth of the nests of the termites they
  eat.

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1c. Disruptive Selection

• Disruptive selection occurs when both extreme
  variations of a trait are the most fit.
• There is selection against the middle variations.
• This type of selection often results in new
  species being created.

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Example of Disruptive Selection

• Ex Limpet shells
• Dark limpets blend with bare rocks
• Light limpets blend with barnacle covered rocks
• Tan limpets are visible in both situations and
  get preyed upon more often by birds

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1d. Sexual Selection

• Sexual selection is the competition for mates
  within a population causing differences to occur
  in the allele frequencies of the two genders
• Often results in extreme differences in the
  physical appearance of males and females.
• Mates tend to be chosen for their phenotypes and
  females tend to choose the males.

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1d. Sexual Selection contd

• Ex Peacocks
• Male peacocks have large tail feathers that make
  it difficult to fly and escape from predators.
• Female peacocks choose males based on their tail
  feather length and fullness.
• Over time males with larger tail feathers
  reproduce more causing large tails to be more
  common.
• Sexual Selection in Peacocks

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2. Genetic Drift

• Genetic drift describes changes in allele
  frequency due to population size.
• Allele frequencies can change more rapidly in
  smaller populations

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Genetic Drift Demo

• 1. What do the marbles in the bottle represent?
• 2. Describe the allele frequencies of the
  original population in the bottle.
• 3. How are the allele frequencies in the final
  population different from the original
  population?
• 4. What sorts of events might cause genetic drift
  to occur?

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2. Genetic Drift contd

• The founder effect
• occurs when a few
• individuals from a larger
• population colonize a
• new area.
• Ex. Amish community
• The allele frequency
• of this population may
• differ from the larger
• population because of the
• limited number of individuals

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3. Mutations

• Mutations are inheritable changes to the
• genotype of an organism
• Mutations occur randomly and spontaneously within
  a population
• Many mutations are harmful, but some are useful
• Mutations can affect allele frequency in a
  population by
• 1. Adding new alleles for a trait
• 2. Changing the amount of each allele
  present
• It can take a long time to eliminate a mutation
  and a long time for a new mutation to become
  prevalent

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4. Migration

• Movement into and out of a population can change
  the allele frequency in a gene pool
• Immigration can ADD individuals with variations
  to the population
• Emigration can REMOVE individuals with variations
  from a population
• Gene flow which is the process of transferring
  genes among different populations

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5. Non-Random Mating

• Having a limited number of individuals can also
  impact mating.
• Non-random mating can influence allele
  frequencies because
• Mates can be limited by geography
• Mates can be chosen for their traits
• Mates can be more closely related to one another