Biology 2900 Principles of Evolution and Systematics

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Biology 2900Principles of Evolutionand
Systematics

• Dr. David Innes
• Dr. Ted Miller
• Jennifer Gosse
• Valerie Power

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Announcements

• Lab 2 (Group 2) handout ? print from course web
  page
• Do the population genetics review
  before Lab.
• Readings for Lab. 2 (Futuyma)
• HWE Ch 9
  (pp. 190 - 197)
• Genetic Drift Ch 10 (pp.
  226 231)
• Selection Ch 12
  (pp. 273 282)
• Gene Flow Ch 12 (pp.
  278 280)
• http//www.mun.ca/biology/dinnes/B2900/B2900.html
• Midterm Test Thursday Feb. 15

3
Announcements

• Lab 3 (Group 1) handout ? print from course web
  page
• http//www.mun.ca/biology/dinnes/B2900/B2900.html
• Midterm Test Thursday Feb. 15

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Evolution in the News
Science 2 February 2007Vol. 315. no. 5812, p.
597 Letters Darwin Not the First to Sketch a
Tree Charles Darwin's 1868 notebook sketch as
"the first known sketch of an evolutionary tree."
This is mistaken. Nearly 60 years earlier, in
1809, Jean Baptiste Lamarck presented an
evolutionary tree of the animals in Philosophie
Zoologique
Darwin 1868
Lamarck 1809
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Biology 2900Principles of Evolution and
Systematics

• Topics
• - the fact of evolution
• - natural selection
• - population genetics
• - natural selection and adaptation
• - speciation, systematics and
• phylogeny
• - the history of life

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Hardy-Weinberg TheoremChapter 9

• Null model
• Allele and genotype frequencies will not change
  across generations (equilibrium)
• Assuming - random mating
• - large population size
• - no selection
• - no migration
• - no mutation

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Hardy-Weinberg
p2 2pq q2 AA Aa aa

• Relax Assumptions
• ? - Mutation
• ? - Migration
• ? - Non-random mating
• ? - Finite population size
• ? - Selection - differential survival,
• fecundity etc. among genotypes

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Genetic Variation

• Loss of genetic variation
• - random genetic drift
• - inbreeding
• - migration
• - directional selection
• How can genetic variation be maintained ?

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Maintenance of Genetic Variation

• Balance of gain and loss of alleles
• Ö - balance of forward and reverse mutation
• Ö - selection - mutation balance
• Ö - selection - migration balance
• - heterozygote advantage
• - frequency-dependent selection

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Heterozygote Advantage

• Directional selection - one allele or other fixed
• Selection favours heterozygotes (heterosis
• 
  overdominance)
• A1A2 maintains both alleles
• A1A2 X A1A2 1A1A1 2 A1A2
  1A2A2

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Selection FavouringHeterozygotes

• Genotype A1A1 A1A2
  A2A2
• p2 2pq
  q2
• Fitness(w) w11 w12
  w22
• 1 - s 1
  1 - t
• w12 gt w11, w22 if s gt 0 and
  t gt 0

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Selection FavouringHeterozygotes

• Equilibrium
• q
  p
• if t 0 q 1.0 (A2
  dominant)
• if s 0 q 0.0 (A1
  dominant)

t
s
v
v
s t
s t
v
v
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Selection FavouringHeterozygotes

• Example
• t 0.80
• s 0.90
• Fitness

w11 w12 w22 1 - .90
1 1 - .80 0.10
1 0.20
14
v
w11 w12 w22 1 - .90
1 1 - .80
q 0.33
v
p .66
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stable equilibrium

-
p
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PopulationMean Fitness ( w )

• w p2 w11 2pq w12 q2
  w22

A1A1 1.0
Directional Selection
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Heterozygote advantage
p .66
18
, 0, -
gt 0
p
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Selection FavouringHeterozygotes

• Sickle-cell anemia
• hemoglobin gene w
• AA normal 0.9
• AS some sickle 1.0
• SS sickle 0.2
• favoured in the
• presence of malaria

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Sickle-cell allele frequency
Malaria Zone
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Selection FavouringHeterozygotes

• Further information
• http//en.wikipedia.org/wiki/OverdominanceHeteroz
  ygote_advantage_and_sickle-cell_anemia

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Frequency-DependentSelection

• Fitness of a genotype constant (w11 w12 w22)
• Genotype may have different fitness depending on
  the environment (food, pop density etc.)
• Genotype x Environment Interaction
• including the frequency of other genotypes

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Frequency-DependentSelection

• Polymorphism 2 or more types

Predator search image selects most
common (protects rare form)
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Rare morph protected
in population
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Frequency-DependentSelection

• Cepaea snail shell-colour polymorphism and
  predation by thrushes
• - frequency in
  population
• - frequency taken

Lab. 1
Variation maintained by crypsis
frequency-dependent selection
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Aquatic bug and fish predator
Increased mortality when common
Decreased mortality when rare
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Maintenance of Genetic Variation

• Balance of gain and loss of alleles
• balance of forward and reverse mutation
• selection - mutation balance
• selection - migration balance
• heterozygote advantage
• frequency-dependent selection

Ö
Ö
Ö
Balancing Selection
Ö
Ö
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Population Genetics Summary

• Synthesis of Mendelian genetics and Darwinian
  evolution
• Hardy-Weinberg Null model
• Genetic variation is present in natural
  populations
• Maintenance of genetic variation a dynamic
  process

Is natural selection the only mechanism of
evolution ?
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Natural Selection(Revisited)

• 1. Phenotypic Variation
• 2. Variation heritable
• 3. Individuals vary in their success at
  surviving or reproducing
• 4. Survival and reproduction not random (fitness
  differences)

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Phenotypic Variation

• Population genetics theory
• Single-gene polymorphisms
• AA, Aa, aa allele freq. p
  q
• Phenotype classes discrete
• Genotype Phenotype
• little influence of the environment

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Phenotypic Variation
Most traits controlled by many loci Phenotype
Genotype Environment Continuous
variation Selection acts on the whole phenotype

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Continuous phenotypic variation
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Quantitative Genetics(Chapter 13 299 300, 304
308)
Estimates heritable variation VP
VG VE H2
(Heritability)

VG
VG
VP
VG VE
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Estimating Heritability
Control environment Compare individuals
with known genetic relationship
Parents - offspring (
Galapagos Finch Example)

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Narrow- SenseHeritability h2 or hN2
VG VA VD VA additive genetic
variation VD dominance genetic variation h2

VA
VA
VP
VA VD VE
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Additive Genetic Variance

• Can be used to predict the response to selection.
  Why ?
• Closer correlation between phenotype and genotype.

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1
0
0.5
h2
Common environment ?
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Heritability (h2)
h2 predicts response to selection
R h2 S R response
S selection differential

Selected group
S 1.6 SD units S 2.8 SD units
(50 ) (20 )
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Response to Selection
R h2 S S selection
differential Xs - Xpop R response t
generation Xt1 - Xt

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Response to Selection
Fig. 13.8 A
R h2 S
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Response to Selection
Fig. 13.8 A, B, C
High h2 Higher S Higher R
Low h2 Low R
High h2 High R
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Selection
Artificial Selection

Natural Selection
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Selection Relaxed
Drosophila abdominal bristle number
Fig. 13.10
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Fig. 13.9 Artificial selection domestic breed of
pigeons
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Modes of Selection

• Mean Variance
• Directional , - -
• Stabilizing 0 -
• Disruptive 0

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Modes of Selection
Fig. 12.1A
Favoured individuals
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Stabilizing Selection
Fig. 13.13 Gall-making Fly in goldenrod plant
Predation
Parasitism
Gall diameter
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Fig. 13.12 Stabilizing Selection for birth weight
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Disruptive Selection
Black-bellied seed cracker (African finch)
survival to adulthood Beak size polymorphism
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G x E interaction

• The phenotype that a genotype expresses can be
  modified by the environment
• Reaction Norm of a genotype is the set of
  phenotypes expressed in different environments

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Experiment

• Perennial wildflower
• Achillea millefolium
• (Yarrow)
• Clausen, Keck and Hiesey (1948)

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Experiment

• cuttings from seven plants (genotypes)
• transplanted same genotype to two environments
• (Mather mountain Stanford coastal)
• elevation high low
• Common Garden Experiment
• Results?

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• Common environment, variation within each site
  genetic
• Stanford plants taller (avg)
• Height of each plant a function of both genotype
  and environment

Note ranking of genotype for height differs
between the two sites (G x E)
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Genotype x Environment Interaction
Fig. 13.19
No G x E
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Summary

• Most traits involve many loci
• Quantitative genetics can be used to
  analyze evolution
• in these traits
• Evolutionary response can be predicted
• ( knowing h2 and strength of selection)
• Different patterns of selection
• Importance of the environment

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Principles of Evolution and Systematics

• Darwin Natural selection (Galapagos Finches )
• Population Quantitative genetics (Genes in
  populations)
• Natural selection Adaptation (Form and
  function
• 
  Sexual
  selection)
• Adaptation and Diversity
• (part I part II)

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Studies in Evolution

• Methods of Evolutionary Analysis
• Adaptation (Form and Function)
• Sex and Sexual Selection
• Life-history evolution
• (Kin Selection and Social Behavior, Ch. 14)

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Kin Selection and Social Behaviour

• Evolution of
• - Cooperation (social animals)
• - Altruism (alarm calling)
• Actor benefits
  Actor harmed
• Recipient benefits Cooperation Altruism
• Recipient harmed ?
  ?

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Kin Selection and Social Behaviour

• (W.D. Hamilton, Robert Trivers,
• Richard Dawkins)
• Kin Selection
• Inclusive Fitness direct fitness indirect
  fitness
• (personal)
  (relatives)

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Kin Selection and Social Behaviour

• Allele for altruistic behaviour will spread if
• B r C gt 0
• B benefit to recipient
• r relationship between actor and recipient
• C cost to actor
• (B C units of surviving
  offspring)
• JBS Haldane said that he would
  cheerfully
• sacrifice his life for two
  brothers or eight cousins

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Calculating Relationship (r)
sibs
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Kin Selection
B 10
1.25
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Kin Selection and Social Behaviour

• Evolution of social behaviour
• - game theory
• - reciprocal altruism
• Evolutionary explanation for
• - helping behaviour
• - alarm calls

Beldings ground squirrel
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Kin Selection and Social Behaviour

• Research Areas
• kin recognition (MHC- major histocompatibility
  complex)
• eusociality (ants, bees etc.)
• - overlap between parent offspring
• - cooperative brood care
• - specialized non-reproductives
  (workers)

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Kin Selection and Social Behaviour

• Eusociality Evolution of non-reproductives ?
• Sex determination Haplodiploidy
• 
  females ? diploid
• males
  ? haploid
• Coefficient of Relationship
• among sisters 3/4 gt female
  and offspring 1/2
• Therefore, invest in the
  production of sisters by being workers and not
  reproducing themselves

AB x C ½ AC ½ BC
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(½ x ½ ) ( ½ x 1)
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Kin Selection and Social Behaviour

• Parent offspring conflict maximizing fitness
• Parent?invest in all offspring equally
• Offspring? attempt to receive more than
• siblings

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Kin Selection and Social Behaviour

• Reciprocal Altruism (unrelated individuals)
• Conditions
• - cost to actor lt benefit to recipient
• - recipient that fails to reciprocate
  punished
• (Difficult to study)
• Example blood-sharing in vampire bats ?

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Methods of Evolutionary Analysis

• Ask interesting questions
• Answer with
• Observations from nature
• Controlled Experiments
• - laboratory
• - nature

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Adaptations

• Adaptation a trait, or suite of traits, that
  increases the fitness of its possessor
• Evolutionary Biology demonstrate the evolution
  of adaptation through natural selection

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Adaptations

• Adaptive significance of some traits obvious
• Other traits less obvious
• (understanding the adaptive significance
• requires more effort)
• No adaptive explanation should be accepted
  because it is plausible and charming

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Testing Hypotheses for Adaptations

• Evolution of long necks in giraffes
• 1. Giraffe ancestors competed for access to food
• 2. Giraffes with the longer neck got more food
• and consequently produced more offspring
• Hypothesis to be tested

73
Foraging Competition Hypothesis

• Prediction
• When food is scare, giraffes should forage
  above the reach of their competitors
• What is observed ?

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Giraffes Long Neck
Fig. 9.2
Prefer to Forage at shoulder height
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Alternative Hypothesis (1996)

• Observation
• Bulls fight using necks and heads as clubs
• Alternative hypothesis
• Neck evolved as a weapon in male-male
  competition for mates
• (Sexual Selection)

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Alternative Hypothesis

• Observations consistent with hypothesis
• 1. Male necks longer than females
• 2. Males have heavier heads with more armor
• 3. Social interaction
• - dominant males have longer necks
• and heavier heads
• - females choose males with longer
  necks

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Adaptations

• Moral of the story
• Alternative explanations must be
  considered
• Also, - differences not always adaptive
• - not every trait an adaptation
• - not every adaptation is perfect