Sexual Selection

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Sexual Selection

• Mate choice often plays important role in
  speciation
• Sexual selection selection for enhanced ability
  to attract mates (nonrandom mating)
• form of NS for ability to attract mates
• directed by ? choice or ?-? competition
• violate assumptions of Hardy-Weinberg principle
  (b/c not random)
• affects both allele and genotype frequencies
• usually acts on ? more strongly than ?
• thus traits that respond to sexual selection more
  elaborated in ?

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Sexual Selection

• Fundamental asymmetry of sex ? usually invest
  more in their offspring than ?
• ? typically produce relatively few offspring
• fitness limited by ability to gain resources
  necessary to produce and rear young
• produce relatively few young over course of
  lifetime
• eggs are expensive, but sperm are cheap
• typically invest more in offspring than ?
• ? can father many offspring
• fitness limited by ability to acquire ?
• sexual selection more intense ?
• thus exaggerated sex. dimorphic traits

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Sexual Selection

• Sexual selection results in sexual dimorphism
• ? and ? look different
• exaggeration of sexually dimorphic traits
• ?
• ?

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Inbreeding

• In nature, mating btwn individuals are seldom
  random
• mating/courtship behavior influences mating
• In small populations, mating btwn relatives
  (inbreeding) common
• violates Hardy-Weinberg (b/c not random)
• ? proportion of heterozygotes and ? proportion of
  homozygotes
• affects genotype frequency, not allele
  frequencies
• thus, does not cause evolution

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Inbreeding

• How inbreeding affects populations
• homozygous parents produce homozygous offspring
• heterozygous parents produce homozygous and
  heterozygous offspring in genotypic 121 ratio
• heterozygotes rare after 4 generations
• no evolution occurred
  b/c allele
  frequencies
  unchanged

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Inbreeding

• Inbreeding depression ? fitness due to ?
  homozygosity
• b/c ? frequency of deleterious homozygous
  recessive alleles
• homozygotes have ? fitness
• can ? rate at which NS removes deleterious
  recessive alleles
• thus indirect cause of evolution
• Mechanisms to prevent inbreeding
• self-incompatibility loci in plants
• cultural taboos/laws

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Genetic Drift

• Genetic drift change in allele frequencies in
  population due to chance
• causes allele frequencies to randomly drift up
  and down over time
• random w/ respect to fitness
• allele frequency changes are not adaptive
• more pronounced in small populations
• can lead to random loss or fixation of alleles (?
  genetic variation)
• populations of endangered species
• countered by manipulating migration
• wildlife reserves designed so natural populations
  linked through habitat corridors
• captive breeding programs exchange semen

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Genetic Drift

• Founder event occurs when group emigrates to new
  area and starts new population
• if founding group is small, allele frequencies
  likely differ from source population (founder
  effect)
• common in colonization of isolated habitats
• islands, mountains, caves, ponds
• Population bottleneck sudden constriction in
  population size that ? allelic diversity due to
  drift
• genetic bottleneck sudden ? in of alleles in
  population
• commonly caused by disease outbreaks and natural
  catastrophes

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Migration

• Migration (gene flow) movement of alleles btwn
  populations
• occurs when indiv. leave one population and breed
  in another
• ? genetic differences btwn source and recipient
  populations
• tends to eliminate genetic differences among
  populations
• thus conservation biologists cautious about
  reintroduction programs
• if wild population has alleles that do well in
  wild, gene flow from captive stock might
  introduce alleles less adapted to wild habitat
• can be used to ? genetic diversity/fitness in
  small, isolated populations that tend to lose
  alleles due to genetic drift

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Mutation

• Mutation (change in genome) is evolutionary
  mechanism that ? genetic diversity in populations
  by introducing new alleles
• violates HW assumptions
• ultimate source of genetic variability for NS to
  act on thus evolution can occur
• 1 mutation/2000 gametes
• 1/1000 individuals carries mutation at particular
  locus
• generally results in deleterious alleles but
  occasionally produce advantageous alleles
• not important cause of evolutionary change b/c
  infrequent
• does little to change allele frequencies but when
  combined w/ NS can be evolutionary force