Population Genetics and Speciation

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Population Genetics and Speciation

• Chapter 16

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Variation of Traits within a Population

• Microevolution is the evolution that occurs
  within a population or a change in the gene pool
  over a succession of generations
• Macroevolution is evolutionary change on a grand
  scale, encompassing the origin of novel designs,
  evolutionary trends and adaptive radiation and
  mass extinction

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Macroevolution

• Novel designs like feather wings
• Trends like increasing brain size in mammals
• Adaptive radiation is seen in flowering plants
• Mass extinctions like the dinosaurs

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Causes of Variation?

• Mutation
• Recombination
• Random pairing of gametes

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The Gene Pool

• Definition the total genetic information
  available in a population
• Allele frequency determined by dividing the
  number of a certain allele by the total number of
  alleles of all types in the population
• Ex. Two alleles A, a. If in a set of 100 gametes,
  half are carrying allele A, then the frequency of
  A is .5 or 50 . The total of all allele types
  must add up to 100.

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Hardy-Weinberg Theorem

• Before we can look at microevolution we must
  consider the H-W theorem. But first.. A few
  definitions
• Population-all the members of a single species
  occupying a particular area at the same time.
• Species-organisms that share a common gene pool,
  interbreed with one another
• Gene Pool- total of all the genes of all the
  individuals in a population.

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Hardy-Weinberg Genetic Equilibrium

• A set of assumptions about an ideal, hypothetical
  population that is NOT evolving.
• It states that the frequencies of alleles and
  genotypes in a populations gene pool remain
  constant over the generation unless acted upon by
  agents other than sexual recombination.
• Sexual shuffling of alleles due to meiosis and
  random fertilization have no effect on the
  overall genetic structure of a population

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5 conditions that must be met to maintain
equilibrium

• Population must be large
• Population must be isolated from others
• No mutations
• Random mating must occur
• No natural selection can occur

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Getting the Hardy-Weinberg Equilibrium Formula

• In a wildflower population there are two alleles
  for color. A-pink and a-white.
• 500 plants 1000 alleles
• 20 of those plants are white 40 a alleles
• 480 of those plants are pink
• 320 are AA 640 A alleles
• 160 are Aa 160 A alleles and 160 a alleles

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• So the frequency of allele A is 800/1000 .8
  80
• The frequency allele a is 200/1000 .2 20
• p the frequency of allele A
• q the frequency of allele a
• p q 1 (.8 .2 1)

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• If you consider genotypic frequencies
• AA 320/500 .64 64
• Aa 160/500 .32 32
• Aa 20/500 .04 4
• Hardy-Weinberg equation
• p2 2pq q2 1
• (frequency of (frequency of (frequency
  of
• AA genotype) Aa genotype) aa genotype)
• For our example .64 .32 .04 1

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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 exceptions to the five conditions necessary
  for H-W equilibrium can result in evolution.

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Causes of Microevolution

• Genetic drift
• Gene flow
• Mutations
• Nonrandom mating
• Natural selection
• If any of these occur then equilibrium is NOT
  present in the population!!

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Gene Flow

• Populations may gain or lose alleles by gene
  flow. This is genetic change due to the
  migration of fertile individuals or gametes
  between populations
• Ex. Human moving around the world.

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Mutations

• A change in an organisms DNA

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Genetic Drift-changes in a gene pool of a small
population due to chance

• Two situations that can lead to genetic drift
• Bottleneck effect disasters such as earthquakes
  or floods reduce the pop. drastically, killing
  victims unselectively- reduces genetic
  variability.
• Founder effect a small number of individuals
  colonize an isolated island, lake or other new
  habitat-reduced genetic variability.

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Nonrandom mating or assortive mating

• Individual select mates because of a particular
  phenotype.
• Ex. Cardinal with the brightest red feathers.
• Peacocks with the most eyes in tail.

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Natural Selection

• Differential success in reproduction because an
  organism is more fit for their environment.
• Which colored dot mouse became most common in
  your Adaptation Activity?

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Types of Selection

• Stabilizing individual with the average form of
  a trait have the highest fitness.

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• Disruptive Selection individual with either
  extreme variation of a trait have a greater
  fitness than individual with the average form of
  the trait.

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• Directional Selection individual that display a
  more extreme form of a trail have greater fitness
  than individuals with an average form of a trait.

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Formation of Species

• Biological concept of Species
• a population of organisms that can successfully
  interbreed but cannot breed with other groups.

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It all begins with being isolated.

• Geography barriers formed by canyons, mountains,
  water or deserts (cities and highways) can
  divide or fragment and isolate parts of
  populations from each other.
• Natural selection and genetic drift cause the two
  subpopulations to diverge, eventually making them
  incompatible for mating.

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Allopatric Speciation

• New species arise because of geographic
  isolation.
• More likely to happen in small populations where
  gene pool will change quickly.

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Reproductive Isolation

• May happen in the absence of geographic barriers.
• May be caused by disruptive selection
• Temporal different breeding times
• Behavioral different mating calls.

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Sympatric Speciation

• Two species develop reproductive isolation within
  the same geographic area by occupying different
  niches.

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Rates of Speciation

• Gradualism speciation occurs at a regular,
  gradual rate. Change happens slowly
• Punctuated equilibrium sudden, rapid change
  followed by long periods of equilibrium or little
  change.

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