Mechanisms for Evolution

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Mechanisms for Evolution
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Populations and Evolution

• Population
• a group of organisms of one species that
  interbreed and live in the same area.
• shares a gene pool
• Gene pool -the different alleles present in the
  population
• Each population has a number of times the allele
  occurs in the gene pool.
• The frequency of alleles in a population tend not
  to change unless there is an outside force
  causing it

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Populations and Evolution
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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
• Mutation
• Migration
• Genetic Drift
• Non-random Mating
• Natural Selection

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1. Mutations

• inheritable changes to the genotype of an
  organism
• occur randomly and spontaneously within a
  population
• Most are harmful, but some are useful

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1. 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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2. Migration

• Immigration can ADD individuals with variations
  to the population
• Emigration can REMOVE individuals with variations
  from a population

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2. Migration

• Many species encourage migration which can cause
  more gene flow which is the process of
  transferring genes among different populations

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3. Genetic Drift

• Genetic drift is the random changes in the number
  of alleles due to population size.

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3. Genetic Drift

• In smaller populations the number of alleles
  frequency can change more rapidly than in larger
  populations

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3. Genetic Drift contd

• The founder effect
• occurs when a few
• individuals from a larger population colonize a
  new area.
• Ex. Amish community

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3. Genetic Drift contd
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4. Non-Random Mating

• Having a limited number of individuals can also
  impact mating. (Inbreeding)
• Non random mating can influence the number of
  alleles because
• Mates can be limited by geography
• Mates can be chosen for their traits
• Mates can be more closely related to one another

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5. Natural Selection

• Explains how evolution can occur
• the mechanism in which favorable heritable traits
  are passed from one generation to another

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Key insights to Darwins Theory
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Artificial Selection

• AKA Selective breeding
• Breeding a species for desired traits

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Heritability

• The ability of a trait to be passed from one
  generation to another.

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Struggle for survival

• Resources and disease limit a population size.

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4 Main principles of natural selection
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Variation

• Differences that exist in a population
• Basis for natural selection
• Heritable

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Peppered Moths
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Overproduction

• Large number of offspring increases the chances
  of survival.
• Can result in competition for resources

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• http//www.youtube.com/watch?veJP3RxzuHCo
• http//www.youtube.com/watch?v0TbzP0AT1Bk

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Adaptation

• A variation that allows an individual to survive
  better than those it competes against

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Descent with modification

• Heritablity of adaptations

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5. Natural Selection

• There are 4 different types of natural selection
• a. Stabilizing Selection
• b. Directional Selection
• c. Disruptive Selection
• d. Sexual Selection

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5a. Stabilizing Selection

• occurs when individuals with the average form of
  the trait are most fit for the environment and
  extreme traits are eliminated
• most common form
• works in all populations at all times

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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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Example of stabilizing selection
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5b. Directional Selection

• occurs when individuals with one extreme of
  variations are the most fit in the environment.
• causes a gradual shift in allele frequency to
  that extreme.

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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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Example of Directional Selection
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5c. Disruptive Selection

• Disruptive selection occurs when both extremes of
  variations are the most fit
• There is selection against the middle variations

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5c. Disruptive Selection

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

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5c. Disruptive Selection
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5d. Sexual Selection

• the competition for mates within a population
  causing differences to occur in the allele
  frequencies of the 2 genders
• Mates tend to be chosen for their phenotypes and
  females tend to choose the males

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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 selected
  for
• http//www.pbs.org/wgbh/evolution/library/01/6/l_0
  16_09.html

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Questions Answer on separate paper

• What is the main similarity between the processes
  of artificial selection and natural selection?
• Could natural selection work on a trait that is
  not heritable? Explain.
• Could natural selection work on a population that
  has no variation? Explain

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What happens if a population does not evolve?
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Hardy Weinberg Theorem

• In the absence of other factors the segregation
  and recombination of alleles during meiosis and
  fertilization will not alter the overall genetic
  make-up of a population

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Hardy-Weinberg Theorem

• Calculate genotype frequencies with a binomial
  expansion
• (pq)2 p2 2pq q2
• pq 1

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What do the letters and numbers represent?

• Using the alleles A and a, list the different
  combinations.

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• Imagine an isolated wildflower population with
  the following characteristics
• Pink is dominant A and white is recessive a
• There are 480 pink flowers and 20 white
• 320 are AA
• 160 are Aa

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• Calculate q2 first
• There are 1000 alleles
• AA -- 320 x 2/plant 640
• Aa --160 x 1/plant 160
• 800
• aa -- 20 x 2/plant 40
• Aa 160 x 1/plant 160
• 200
• Frequency of A 80 and a 20

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Condition for Hardy-Weinburg

• Large population
• No net mutation
• Isolated population
• Random mating
• No natural selection