Natural Selection

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Natural Selection

• Or, how did we get here.

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(No Transcript)
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Natural Selection

• The Theory of Natural Selection is so simple that
  anyone can misunderstand it. (Anonymous)
• Charles Darwin (1809-1882) saw three problems in
  need of a solution.
• Darwin was not the only one to see these problems
  BTW
• Other Naturalists were struggling with the same
  issues

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Problem the First

• There is change over time in the flora and fauna
  of the Earth
• What we would commonly call evolution today
• The fossil record showed this to be pretty clear,
  even to people in the mid 1800s
• This was not controversial in Darwins time, and
  is not now.

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The Second Problem

• There is a taxonomic relationship among living
  things
• People were big into classifying stuff
• It was pretty obvious that there was a
  relationship between different species
• Different birds, different grasses, different
  cats etc

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The Third Problem

• Adaptation
• Different kinds of teeth for different animals,
  say carnivore ripping teeth and herbivore
  grinding teeth
• Different tissues within species
• Heart vs. eye etc.

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The Solution!

• Natural Selection provides a mechanistic account
  of how these things occurred and shows how they
  are intimately related.
• It is one of those oh man is that ever easy, why
  didnt I think of that? type things.

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Hows it work?

• There is competition among living things
• More are born or hatched or whatever, than
  survive and reproduce
• Reproduction occurs with variation
• This variation is heritable
• Remember, there was NO genetics back then, Chuck
  knew, he just knew.
• Realized that is wasnt blending

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Hows it Work?

• Selection Determines which individuals enter the
  adult breeding population
• This selection is done by the environment
• Those which are best suited reproduce
• They pass these well suited characteristics on to
  their young

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Hows it Work?

• REPRODUCTION is the key, not merely survival
• If you survive to be 128 but have no kids, you
  are not doing as well as I am
• I have reproduced
• Assuming the traits that made me successful will
  help them then I amore fit NOW than the 128 year
  old guy

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This lecture keeps evolving..

• Survival of the Fittest (which Chucky D NEVER
  said) means those who have the most offspring
  that reproduce
• So, the answer to the trilogy of problems is
• Descent with modification from a common
  ancestor, NOT random modification, but,
  modification shaped by natural selection

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Different types of selection

• Directional Selection
• What most of us think about when we think about
  selection
• An extreme value is selected for
• Human brain size is a nice example

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Different kinds of selection

• Stabilizing or Normalizing selection
• The middle is selected for
• Many examples here
• Symmetry
• Two eyes

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Different kinds of selection

• Disruptive selection
• Extremes are selected for
• Might be where the two sexes come from
• Trait was probably gamete sized
• Two mating types

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But

• Darwin was troubled by the preponderance of
  behaviour that seemed to be of no benefit to the
  actor
• Examples
• Sterile insect castes
• Bee stings
• Alarm calling
• Reproductive restraint

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Reproductive restraint

• Birds can produce many more eggs than they
  actually do!
• Wynne-Edwards beleied that selection also acted
  on the species level to stop massive
  overpopulation

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Reproductive Restraint?

• Lack argued that animals are doing what is best
  for the INDIVIDUAL
• Big clutch means possible exhaustion, death
• LIFETIME fitness, not just this season

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Lack

• Well, if clutch size depends on the environment,
  then clutch size should vary with quality of
  resources

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Hamilton figured it out

• Hamiltons Response was theoretical
• Look at behaviour from the genes point of view
• Inclusive fitness direct fitness effect on
  fitness of others

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Hamilton

• C lt rb
• Or r gt c / b
• Remember, r is relatedness, c is cost b is
  benefit
• Would you give your life for a brother?
• No, but maybe 2 brothers, or 8 cousins
• Two uncles and one mother.

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Mechanism

• For this to work there must be some sort of
  proximate mechanism
• Easy for mother/father and child
• How about everyone else?
• Green Beard hypothesis
• Allele produces some phenotype
• Also allows for recognition
• Tough for a single gene.

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The elusive mechanism

• Proximity
• Families
• Little dispersal
• So if close by, you are related
• Example
• Leaf eating ants
• Far nests ? more hostile
• Different plants ? more hostile

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More on mechanism

• Those crazy ants
• Split colony and put on different plants
• Hostile, but non injurious!
• Gene environment interaction baby!
• Vervet monkeys
• 2 year olds scream
• Mother comes
• Others watch the mother!

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Mechanisms, pheremones and bees

• Greenberg looked at bee relatedness and allowing
  bees in or not
• Probably genetically determined odour

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Conclusions about inclusive fitness

• Group selection is silly
• Individual selection is cool
• Gene level selection is VERY cool
• Hamilton is a genius
• Dont over use this
• Gives us some insight into some nasty human
  behaviour

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Games are fun

• Animals tend to behave in ways that maximize
  their inclusive fitness
• Usually pretty straightforward
• But, sometimes we must know what others are doing
  before we adopt a strategy
• What if your mating call is drowned out by
  others calls, what to do, ahh what to do

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Fitness and Strategies

• In certain cases payoffs, and hence fitness
  maximization, depend on what other populations
  are doing
• When the payoff to one individual depends on the
  behaviour of others we cannot use the principle
  of fitness maximization until we know
• What the alternatives are
• P(encountering alternatives)
• Consequences of encounter

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Game Theory

• Think of it like a game
• Each individuals behaviour is its strategy,
  payoffs are in units of fitness
• Players produce more players (offspring)
• Changes in fitness are directly proportional to
  payoffs
• An evolutionary Stable Strategy is one that, when
  adopted by enough individuals, maximizes payoff

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Pure Strategy

• One that cannot be replaced
• Food storing
• Recover your own seeds (Anderssen and Krebs,
  1978)
• If they recovered communally, a selfish hoarder
  would replace the communals damned quckly

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Mixed Strategies

• Hawks and Doves
• Not real hawks or doves, strategies
• Always fight, or always give up
• Look at the payoffs
• Look at the costs
• Determine what proportion should be hawks and
  should be doves

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Hawks and Doves

• Say its all Doves
• Hawk shows up, wins resource
• Spreads genes
• Now more hawks
• Oh oh, now you are fighting, P(injury) .5
• Now being a dove pays
• Either strategy good when rare, bad when common

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Doves and Hawks

• V Value of resource for winner
• W cost of a wound
• T cost of display (no fighting)
• (John Maynard Smith, 1978)

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Whoa, I know Kung Fu

• Set up a payoff Matrix
• Opponent in the contest
• Hawk Dove
• Payoff Hawk ½(V-W) V
• Received
• By Dove 0 ½V-T

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ESS as easy as 123

• If W gt V then there can be no pure ESS
• In a population of hawks, a small number of doves
  do better than hawks
• E(dove,hawk) gt E(hawk, hawk)
• E(dove, hawk) 0
• E(hawk, hawk) ½(V-W)
• W gt V, therefore ½(V-W) lt 0

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Pure Doves dont do it either

• Payoff to Hawk is V
• Payoff to doves is less than that
• (½W T)
• Hmmm
• So, what proportion of hawks and doves balances
  it out?

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What is theoretical population biologist to do?

• Find the proportion (p) of hawks of hawks such
  that the following equation balances
• p ½(V-W) (1-p) V p (0) (1-p) (½V T)
• Simply (?) solve for p
• p (V2T) / (W 2T)

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Apply it, sort of

• Say V 10
• W 20
• T 3
• Opponent in the contest
• Hawk Dove
• Payoff Hawk -5 10
• Received
• By Dove 0 2

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Now, sub that back into the formula

• P 16/26 or 8/13
• 8/13ths of the population, with these payoff
  values, must be hawks
• The values are not that important really, the
  point is that you can determine the point at
  which a strategy can coexist with another
  strategy as an ESS
• Could be percentage of population, or percentage
  of time each animal adopts a given strategy

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So?

• It is actually applicable thats so
• Toads looking for breeding grounds (Davies and
  Hallaway, 1979)
• Payoffs determined

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Another so

• Dungflies
• Should a male hang around poo as it gets older?

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Conclusions

• This is a very brief intro to game theory
• This stuff is way powerful
• You have to sit and think some about the payoffs
  and costs
• Dynamic programming models are becoming more
  popular

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Other Evolutionary Theories

• Lamarckism
• Inheritance of acquired characteristics
• E.g., giraffes really wanted leaves, so they
  stretched their necks and..
• Sounds crazy, but a lot of people think this way
• We will all have giant heads and tiny bodies
  someday
• Cave swelling fish dont use their eyes so they
  disappear
• We dont use our appendix so it is disappearing

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Silly incorrect evolutionary theories and ideas

• Orthogenesis
• There is some plan to evolution.
• NO WRONG INCORRECT, THANKS FOR PLAYING
• The idea of an evolutionary ladder fits in here
• It is wrong too

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Still another silly idea

• Intelligent Design
• Just Creationism with a fancy name
• God does not belong in a science class, any more
  than experiments belong in church
• NOT A SCIENTIFIC THEORY