Transcript Natural Selection

Natural Selection
Or, how did we get here….
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
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 Darwin’s time, and is
not now.
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
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.
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 didn’t I think of that?’ type things.
How’s 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 wasn’t ‘blending’
How’s 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
How’s 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
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’
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
Different kinds of selection
• Stabilizing or
Normalizing selection
• The middle is selected
for
• Many examples here
– Symmetry
– Two eyes
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’
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
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
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
Lack
• Well, if clutch size
depends on the
environment, then
clutch size should vary
with quality of
resources
Hamilton figured it out
• Hamilton’s Response
was theoretical
• Look at behaviour
from the gene’s point
of view
• Inclusive fitness =
direct fitness + effect
on fitness of others
Hamilton
• C < rb
• Or r > 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….
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….
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
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!
Mechanisms, pheremones and bees
• Greenberg looked at
bee relatedness and
allowing bees in or not
• Probably genetically
determined odour
Conclusions about inclusive fitness
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Group selection is silly
Individual selection is cool
Gene level selection is VERY cool
Hamilton is a genius
Don’t over use this
Gives us some insight into some nasty human
behaviour
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…
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
Game Theory
• Think of it like a game
• Each individual’s 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
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
Mixed Strategies
• Hawks and Doves
– Not real hawks or doves, strategies
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Always fight, or always give up
Look at the payoffs
Look at the costs
Determine what proportion should be hawks
and should be doves
Hawks and Doves
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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
Doves and Hawks
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V = Value of resource for winner
W = cost of a wound
T = cost of display (no fighting)
(John Maynard Smith, 1978)
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
ESS as easy as 123
• If W > 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) > E(hawk, hawk)
• E(dove, hawk) = 0
• E(hawk, hawk) = ½(V-W)
– W > V, therefore ½(V-W) < 0
Pure Doves don’t 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?
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 = (V+2T) / (W+ 2T)
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
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
So?
• It is actually applicable
that’s so
• Toads looking for
breeding grounds
(Davies and Hallaway,
1979)
• Payoffs determined
Another so
• Dungflies
• Should a male hang around poo as it gets older?
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
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 don’t use their eyes so they disappear’
• ‘We don’t use our appendix so it is disappearing’
Silly incorrect evolutionary theories and
ideas
• Orthogenesis
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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……
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