Evolution, Ch 23 U203PP

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Evolution, Ch 23 U203PP
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• Overview The Smallest Unit of Evolution
• One common misconception about evolution is that
  individual organisms evolve, in the Darwinian
  sense, during their lifetimes
• Natural selection acts on individuals, but
  populations evolve

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We know there is considerable genetic variation
in populations

• Electrophoresis
• DNA sequencing
• Chemical analysis of proteins

so how does this variation translate into allele
frequency changes in a population, i.e., how does
microevolution occur?
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• Concept 23.1 Population genetics provides a
  foundation for studying evolution
• Microevolution
• Is change in the genetic makeup of a population
  from generation to generation

Darwin would suggest that small changes
(microevolution) within a population would
accumulate and lead to bigger changes that would
result in the development of new species
(macroevolution)
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An organism exposes its phenotype to the
environment

• Acting on phenotypes, though, natural selection
  adapts a population by increasing or maintaining
  favored genotypes in the gene pool

Uh. Phenotype? Genotype? I should know these
from freshman bio, right?
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• Phenotype behavior, physical traits, appearance,
  physiology, metabolism
• Genotype the combination of alleles that make up
  the trait

Alleles? Isnt that another word I should know?
Alleles are VERSIONS of genes you may have for a
particular trait for which there is one gene (two
alleles)
If the alleles are the same homozygous If the
allele are different heterozygous
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Gene Pools and Allele Frequencies

• A population
• Is a localized group of individuals that are
  capable of interbreeding and producing fertile
  offspring

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• The gene pool
• Is the total aggregate of genes in a population
  at any one time
• Consists of all gene loci in all individuals of
  the population

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The Modern Synthesis

• Population genetics
• Is the study of how populations change
  genetically over time
• Reconciled Darwins and Mendels ideas
• Integrates Mendelian genetics with the Darwinian
  theory of evolution by natural selection
• Focuses on populations as units of evolution

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The Hardy-Weinberg Theorem

• The Hardy-Weinberg theorem
• Describes a population that is not evolving
• States that the frequencies of alleles and
  genotypes in a populations gene pool remain
  constant from generation to generation provided
  that only Mendelian segregation and recombination
  of alleles are at work

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• Mendelian inheritance
• Preserves genetic variation in a population

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Preservation of Allele Frequencies

• In a given population where gametes contribute to
  the next generation randomly, allele frequencies
  will not change

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Hardy-Weinberg Equilibrium

• Hardy-Weinberg equilibrium
• Describes a population in which random mating
  occurs
• Describes a population where allele frequencies
  do not change

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• A population in Hardy-Weinberg equilibrium

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• If p and q represent the relative frequencies of
  the only two possible alleles in a population at
  a particular locus, then
• p2 2pq q2 1
• And p2 and q2 represent the frequencies of the
  homozygous genotypes and 2pq represents the
  frequency of the heterozygous genotype

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Conditions for Hardy-Weinberg Equilibrium

• The Hardy-Weinberg theorem
• Describes a hypothetical population
• In real populations
• Allele and genotype frequencies do change over
  time

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• The five conditions for non-evolving populations
  are rarely met in nature
• Extremely large population size
• No gene flow
• No mutations
• Random mating
• No natural selection

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Population Genetics and Human Health

• We can use the Hardy-Weinberg equation
• To estimate the percentage of the human
  population carrying the allele for an inherited
  disease