Evolution and Gene Frequencies

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Evolution and Gene Frequencies

• Chapter 5

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Gene Frequencies

• Populations and gene pools
• Populations
• Groups of individuals of the same species that
  occupy a given area at the same time and share a
  unique set of genes
• Allele varying expressions of genes
• Ex. Hair or fur color

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Gene Frequencies

• Gene pool the sum of all alleles for all traits
  in a sexually reproducing population
• Reasons for variation
• Independent assortment
• Crossing-over
• Chance of a particular sperm fertilizing an egg
• Rearrangements of number and structure of
  chromosomes
• Mutations

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Gene Frequencies

• Hardy-Weinberg theorem
• The mixing of alleles at meiosis and their
  subsequent recombination do not alter the
  relative frequencies of the alleles in future
  generations
• Requires that certain assumptions be met

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Gene Frequencies

• Assumptions
• Population size must be large
• Sexual reproduction in the population must be
  random
• Migration can not occur
• Mutations must not occur

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Gene Frequencies

• Evolutionary mechanism
• Evolution is not bad, nor goodit is just a fact
• Evolution is a result of some individuals in a
  population surviving and being more effective at
  reproducing than others in the population
  (Fitness)
• This leads to changes in the allelic frequency,
  which is the definition of
• EVOLUTION!

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Gene Frequencies

• Population size
• The smaller the size the more significance chance
  may be
• Survival may not be because of being the best,
  but because of chance

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Gene Frequencies

• Genetic Drift
• Chance events influencing the frequencies of
  genes in populations Neutral Evolution
• Flipping a coin
• Founder Effect
• When individuals from a group colonize new
  habitats
• Seldom carry the same allelic frequencies
• New colony is likely to have distinctive genetic
  makeup

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Gene Frequencies

• Bottleneck effect
• Example of the Founder Effect
• Population becomes very small and reduces the
  amount of variation
• Traditional view bottlenecks decrease the
  genetic variation and makes the species less
  likely to withstand environmental stresses
• Populations with high diversity are likely to
  have individuals with genes that allow them to
  withstand environmental pressure

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Gene Frequencies

• Neutral evolution
• When gene frequencies change independently of
  natural selection
• These changes do not affect the fitness of the
  organisms

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Gene Frequencies

• Gene Flow
• Changes in relative allele frequency from the
  migration of individuals (Evolution)
• Immigrate
• Individuals enter a population from the outside
• Emigrate
• Individuals leave a population
• Can result in the separation of one species into
  two different species

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Gene Frequencies

• Mutation
• Changes in the structure of genes and chromosomes
• Mutations are a fact of life
• Source of variation
• Origin of all new alleles
• Make extinction less likely
• Most are harmful

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Gene Frequencies

• Modes of selection
• Directional selection
• When one extreme phenotypic group of individuals
  within a species are at a disadvantage to the
  other extreme
• Bad allele becomes extinct
• Peppered Moths

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Gene Frequencies

• Disruptive selection
• Selection in which individuals in the middle of
  the range of phenotypes is selected against or
  will become extinct

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Gene Frequencies

• Stabilizing selection
• When the two extreme phenotypes are not an
  advantage.

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Gene Frequencies

• Polymorphism
• Two or more distinct forms with no forms in
  between
• Balanced polymorphism
• Maintaining each distinct form at about the same
  amount

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Gene Frequencies

• Species
• A group of populations in which genes are
  actually, or potentially, exchanged through
  interbreeding

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Gene Frequencies

• Speciation
• The formation of new species
• Reproductive isolation
• Population must be isolated
• Premating isolation
• Prevents mating from taking place
• Temporal isolation
• Behavioral isolation
• Postmating isolation
• Prevents successful fertilization and development
  even though mating has occurred

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Gene Frequencies

• Allopatric speciation
• When subpopulations become geographically
  isolated
• Most common form of speciation
• Also results in adaptive radiation
• Ex. Gallapgos finches

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Gene Frequencies

• Parapatric speciation
• Speciation in a small, local population
• Demes

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Gene Frequencies

• Sympatric speciation
• Speciation that occurs in populations that have
  overlapping ranges