Modes of Natural Selection

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Modes of Natural Selection

• Directional one end of range selected for gt
  mean shifts range may i or stay the same

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• Stabilizing extremes selected against gt mean
  stays same range ?

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• Disruptive extremes selected for gt mean stays
  the same range ?

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Experimental evidence for the effects of selection

• Yoo et al. (1980s) series of selection expts on
  abdominal bristle in fruit flies
• Directional selection selected 6 lines (pops)
  for largest bristle - only the 50 flies with
  the highest s were allowed to mate
• Mean should shift to larger and range should
  decline strong selection can gt monomorphy (loss
  of low-bristle genes)

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• Baseline bristle
• Males 6.95 2.99
• Females 9.35 2.07
• After 86 generations, bristle reached 25-35
  among the 6 populations
• none became monomorphic all retained some
  variability in bristle
• one line plateaued at lower s
• Had they selected out alleles for low bristle ?
  
• How could you tell?
• Relaxed selection random mating gt all reversed
  to lower bristle s

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Conclusions for Directional selection experiments

• Strong directional selection produces rapid
  responses even in small pops. Responses are too
  rapid to result from new mutations gt pre-existing
  variation
• Mean can shift far beyond original range of
  variation gt norm of reaction (all genotypes can
  produce a range of phenotypes depending on
  response to environment potential vs. realized
  expression)
• Genetic variation retained in spite of strong
  selection
• (didnt deplete variation/low bristle genes)

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• 4) New mutations can ? variation to sustain
  response to selection indefinitely
• 5) As intense selection shifts the mean, other
  factors that are associated with the trait may ?
  fitness gt plateaued lineages
• correlated response
• Why?
• Pleiotropy one gene affects many phenotypes
• gene phenotypes
• 1 (bristle )
• A product 2 (leg length)
• 3 (fecundity)

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Stabilizing selection on bristle

• Problem most characters are polygenic (multiple
  loci produce the phenotype epistasis)
• - - - - - - -
• - - - - - - - -
• If mean character derives from heterozygosity at
  multiple loci, variation cannot be depleted
• You can get monomorphic phenotype
• Relaxing selection will reform full range of
  variation

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Disruptive selection on bristle

• Again, polygenic characters will confound our
  ability to change frequency distribution
• - - - - - - -
• - - - - - - - -
• If you select for low and high bristle , you can
  get divergent phenotypes
• But random mating reconstitutes heterozygotes

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Adaptation (Ch. 10)

• Produced by Natural Selection
• Change that correlates the genetic make-up of a
  population with its functions in the environment
• An adaptation is any character that increases
  fitness

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How do we recognize adaptation?

• High correlation between structure and function
• Complexity implies an organizing force like
  natural selection
• Cautions!
• Not all aspects of character may be adaptive,
  e.g., the eye is complex and most of its parts
  are adaptive, but color may not be
• some aspects of structure may be historic rather
  than adaptive

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Cautions (cont.)

• does the character really have a function or is
  it the product of some other process, e.g.,
  autumn leaves, gray hair
• careful of pleiotropy gt correlated response!
• e.g., vestigial mutant in fruit flies also gt ?
  halteres and ? egg production
• e.g., hairlessness in humans may be correlated
  response to developmental pattern of neoteny

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Giraffe Revisited

• Careful not to choose the most obvious selection
  scenario!
• Giraffes seldom forage at maximum height
• When competition is highest (dry season) they
  actually forage lower in the brush
• Real story may be more complex
• In competition for mates, male giraffes joust
  with their necks gt longer, stronger necks
• How do we test this (or any) adaptive hypothesis?
• determine functions of adaptive character
• show increased fitness according to functions

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Testing Adaptive Hypotheses

• Does adaptive feature function as proposed and
  does it increase fitness?
• Experimental method direct manipulation of
  feature, e.g., tephritid flies (read) mustache
  mark on flickers modeling - Nicklas model of
  paleozoic cones
• Observational method if you cant manipulate,
  observe in nature, e.g., giraffe
  hypothesis that male-male competitiongtlong,
  strong necks

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How do we observe giraffes and test the male
competition hypothesis?

• males are taller than females of same age, but
  could gt foraging competition
• tallest males with strong necks are dominant to
  shorter males gt beat them in jousting bouts
  (getting warmer)
• females mate more frequently with taller males
  (bingo!)
• Our observations support the idea that male neck
  size gt dominance gt higher fitness
• Why do females have long necks?

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• In observational studies, it is critical to show
  that feature you think is adaptive is not used at
  random that its function(s) is consistent with
  your hypothesis and that using the feature in
  this manner increases fitness

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Testing adaptive hypotheses (cont.)

• Comparative method oldest relies on
  convergent evolution other species living in
  similar environments using similar niches will
  develop similar adaptive characters, e.g.,
• all warm bloods living in cold climates should
  limit surface area gt large body size Bergmans
  rule
• fast fliers have long, narrow wings (swallows,
  swifts, hummers)
• sperm competition among social species gt ? size
  of testes tested among solitary vs. social
  fruit bats

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Limitation on adaptation

• Variation is limited necessary mutations have
  never occurred at right time place
• Phylogenetic (historical) constraints as we
  noted earlier in constraints on perfection you
  are limited by what you inherited from your
  ancestors e.g., llama toes pandas thumb
  tracheal system in insects
• Genetic correlations co-adapted gene complexes
  (pleiotropy epistasis)

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3) Genetic correlations (cont.)

• Interactions among genes to produce adaptive
  phenotypes gt constraint
• Sometimes due to epistasis stick insect must
  look like a stick act like one
• Pleiotropy Irish elk Tyrannosaurus examples
  of allometric growth

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Allometry
isometry
isometry
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• 4) Developmental constraints
• canalization (may be due to epistasis) groups
  of develop genes regulate final phenotypic
  expression so even different alleles gt same
  phenotype
• developmental pattern may be influenced by both
  co-adapted genes and phylogenetic history
• loss of digits in frogs vs. salamanders
• Frogs preaxial toe lost
• Salys postaxial toe lost
• Strongly correlated features will resist change
  embryonic induction

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• 5) Trade-offs a character or group of
  characters cannot adapt optimally because they
  are under conflicting selection, e.g.,
• Human brain vs. human pelvis
• frog feet must work for both hopping swimming
• 6) Architectural/mechanical/physiological
  constraints materials and/or processes limit
  ways in which character can adapt
• strength of bone, tendon, ligament
• growth of pollen tubes in fuscia (read in text)
• Read Spandrels of San Marco on web

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Sexual selection

• Why do we see sexual dimorphism?
• wouldnt large size benefit both sexes so why
  do males tend to be bigger in vertebrates and
  females bigger in invertebrates?
• why do we see secondary sexual characters that
  seem to be maladaptive, e.g., peacock tail
  conspicuous dances and vocal displays?
• These not only cost a lot of energy to make and
  use, they also scream out to predators,
• here I am!

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Sexual selection natural selection for
reproductive success

• Features that enhance the ability to attract a
  mate, to successfully copulate, and to see to it
  that young have a good start in life should be
  selected for
• Longevity may not guarantee these
• Trade-off between selection for survival and
  selection for reproductive success (sexual
  selection)

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But why are males different from females?

• Males must experience selective forces different
  from females for maximizing reproductive success
• Energetics of egg vs. sperm production
• Females are more involved in rearing offspring
• Result is that females invest more
  energy/offspring (not necessarily more total
  energy in reproduction)

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Differential energy gt asymmetry in parental
investment gt differences in sexual selection
between ? ?

• ? maximize reproductive potential by investing
  lots of energy in small of offspring (limited
  by quality of mates)
• ? maximize reprod. potential by maxim-izing s
  because he can! (limited by access to females)
• This leads to females being choosy and males
  being highly competitive for mates

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Two forms of sexual selection

• Intrasexual main interaction is between the
  males in competing directly for access to
  females female role is minimal, she simply mates
  with the winner
• Intersexual main interaction is between males
  females males must advertise to attract females
  (songs, dances, displays) females play active
  role in choosing the male that is most impressive

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Intrasexual selection

• Males must eliminate each other to gain access to
  females. Selects for
• Harem the most dominant male mono-polizes the
  females
• Large body size these species often have the
  greatest size dimorphism (elephant seals, big
  horned sheep, etc.)
• Combat weapons antlers, crab claws, enlarged
  canines, walrus tusks, etc.

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Intrasexual selection (cont.)

• Body armor skull of big horned sheep belly
  skin in kangaroos
• Sperm competition large testes amplexus
  nuptial flights copulatory plugs pheromones
• Infanticide kill offspring from other male to
  eliminate his genes and make the females
  receptive to you as quickly as possible, e.g.,
  lions

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Intersexual selection

• Male cannot monopolize females even if dominant
  (cannot monopolize resources that the female
  needs) gt he has to convince her to choose him
  from among all the males available very common
  in territorial species. Selects for
• Advertisement of male quality gtdisplays, songs,
  dances, color patterns, etc.

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Intersexual selection (cont.)

• Advertisement gt species sex ID plus display of
  quality
• Bright colors, energetic dances songs tell
  female that he is strong, healthy, free of
  parasites I have best genes!

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How does intersexual selection differ among
species?

• Female mate choice exerts strong selective force
  on secondary sex characters of male, but these
  vary with the habits of the species
• The most elaborate displays, colors, dances, etc.
  are found in polygamous species males are
  needed only as sperm donors
• Females make quick choice of best male based on
  the obvious signs of quality supplied by these
  elaborate displays
• Male has no role in care of offspring
• Typically precocial young

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• Monogamous species typically have altricial
  (immature) young they need care by both parents
• Males must stick around to help raise young
• Males must still attract females (colors, songs,
  dances), but often more subtle features with less
  conspicuous secondary sex characters
• Mate choice takes place over long periods of
  courtship males help build nest, bring gifts!
• Male must demonstrate not only species sex
  recognition, but also that he will stay with
  female to help raise the young gt pair bond

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• In most examples we talk about females acting as
  agents of NS to determine secondary sex
  characters of male, but females are also subject
  to sexual selection
• Females must advertise receptivity
• Females must develop critical evaluation skills
  in order to discriminate among the males
• These skills will be best seen in species
  dominated by intersexual selection
• Females in species dominated by intrasexual
  selection also have to be willing to accept the
  winner of the male-male combat