Biology 2900 Principles of Evolution and Systematics

1
Biology 2900Principles of Evolutionand
Systematics

• Dr. David Innes
• Jennifer Gosse
• Valerie Power

2
Announcements

• Lab 2 (Group 1) handout ?print from course web
  page
• Important Do the population genetics review
  before Lab.
• Readings for Lab. 2 (Futuyma)
• HWE Ch 9 (pp.
  190 - 197)
• Selection Ch 12 (pp.
  273 282)
• Genetic Drift Ch 10 (pp.
  226 231)
• http//www.mun.ca/biology/dinnes/B2900/B2900.html

3
Announcements

• Online Quiz https//online.mun.c
  a/
• Available Friday Feb 1, 1200 noon Monday
  Feb. 4, 1100 pm
• Midterm Test Thursday Feb.
  14, 2008

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Announcements
Midterm Test Thursday Feb. 14

• Format
• A. True / False
• B. Short Answer
• C. Matching terms
• D. Problem
• Read and interpret graphs,
  draw graphs
• Example questions Online Quiz

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Announcements

• Chapter References (Futuyma, 2005)
• Ch. 1. Evolutionary Biology p. 1 14
• Ch. 3. Evidence for Evolution (Box 3A p. 48 49)
• Ch. 9 Variation p. 189 222
• Ch. 10 Genetic Drift p. 225 235 Genetic
  differentiation p. 241 242
• Ch. 11 Natural Selection and Adaptation p. 247
  255
• Ch. 12 The Genetical Theory of Natural Selection
  p. 269 285 293 294
• Ch. 13 Evolution of Phenotypic Traits p. 297
  301 304 310 317 319
• Ch. 14 Conflict and Cooperation p. 325 329
  Sexual Selection p. 329 - 339
• Ch. 17 How to be fit Reproductive Success p. p.
  407 - 422

Midterm Test Thursday Feb. 14
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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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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

10
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.
• Determines degree of similarity between
  parents and
• offspring (basis for response to
  selection)

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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
3
1
2
3 components
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Response to Selection
R h2 S
Fig. 13.8 A, B, C
High h2 Higher S Higher R
Low h2 Low R
High h2 High R
Same S
20
Selection
Artificial Selection

Natural Selection
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Selection Relaxed
Drosophila abdominal bristle
number
Fig. 13.10
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Selection Relaxed
Drosophila Phototactic Behaviour
Phototactic Score
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Fig. 13.9 Artificial selection domestic breed of
pigeons
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Natural Selection Example
Alpine Skypilots (Polemonium viscosum)

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Natural Selection

• Pop. Pollinator Flower
  size
• Tundra bumble bees large (12
  )
• Timberline various small
• Question Is large flower size due to selection
• by bumble bees ?
• Tested on Timberline population

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Methods

• 1. Heritability (h2) of flower size
  Parent
• offspring regression
• 2. Selection differential (S) by bumble
  bees
• variation between flower size and fitness
• 3. Predicted response to selection
• R h2 S

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1. Heritability of flower size h2 0.20 to 1.0
offspring
Parent
28
2. Selection differential by bumble bees S
0.74 mm (5 increase) Relative fitness
6 yr. old offspring avg. 6 yr. old offspring
(Maternal)
Bees visit larger flowers ?more visits more
seed higher fitness
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Prediction

• h2 S
• Low R .2 x 0.05 0.01
• High R 1.0 x 0.05 0.05
• 1 5 increase in flower size

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Test Prediction

• Two treatments
• 1. Hand pollinate at random
• 2. Bumble bee pollinated
• collect seeds---gt grow offspring---gt flower size

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Random pollination
Bumblebee pollinated
9 larger
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Summary(Natural Selection)

• Flower size variable
• Variation heritable
• Larger flowers have a higher fitness
• (Bumble bee
  pollination)
• Timberline population can evolve
• larger flower
  size
• (directional selection)

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Modes of Selection
Fig. 12.1A
Favoured individuals
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Modes of Selection

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

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Example of Stabilizing Selection
Fig. 13.13 Gall-making Fly in goldenrod plant
Selection
Parasitism
Predation
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
38
Phenotypic Variation
Phenotype Genotype Environment
Further complication G x E interaction

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G x E interaction

• P G E G x E
• The phenotype that a genotype expresses can be
  modified by the environment

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Phenotype

• Norm of Reaction of a genotype
• - is the set of phenotypes expressed in
• different environments
• Phenotypic Plasticity
• genotype can express
• different phenotypes
• in different environments
• (adaptive)

spring summer
Fig. 13.20
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Genotype x Environment Interaction
Fig. 13.19
No G x E
Optimum phenotype

• G and E variation (no G x E)
• G x E interaction
• G x E interaction
• B and C variation in sensitivity to environment
  (degree of phenotypic plasticity)

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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
• (common environment)
• Results?

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Env. 1

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

Env. 2
Note ranking of genotype for height differs
between the two sites (G x E)
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Phenotypic Plasticity

• How to test same genotype in different
  environments ?
• Species that reproduce asexually (clone)
• - plants, invertebrates

46
Daphnia
Parthenogenesis genetically identical offspring
(clone) Experiment raise individuals with the
same genotype in different environments
47
Phenotypic plasticity

• Phenotypic plasticity in phototactic behaviour
• positive swims towards light
• negative swims away from light

light

cylinder
Daphnia
-
48
Methods

• Genetic variation for phototactic behaviour
• (among clone variation)
• Phototactic behaviour in the presence of fish
  chemicals (fish visual predators)

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Results
Habitat Fish few fish
no fish
Same clone
1. Genetic variation for 2. Phenotypic
plasticity phototactic behaviour 3.
Phenotypic plasticity has evolved
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Summary

• Most traits involve many loci
• Quantitative genetics can be used to
  analyze evolution
• in these traits
• Evolutionary response (R) can be predicted
  
• knowing h2 and strength of selection
  (S)
• Different patterns of selection
  (directional, stabilizing, disruptive)
• Importance of the environment ( G x E )

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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)
• Adaptation and Diversity
• (part I part II)

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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

55
Foraging Competition Hypothesis

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

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Giraffes Long Neck
Prefer to Forage at shoulder height
57
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)

Simmons, R. Scheepers, L. (1996). Winning By A
Neck Sexual Selection In The Evolution Of
Giraffe. The American Naturalist, 148, 772-786.
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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

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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

61
Studies in Evolution

• Reproductive success
• Sex and Sexual Selection
• Kin Selection and Social Behavior
• Life-history evolution

62

• Sexual Reproduction
• Reproduction an important adaptation
• A diversity of modes of reproduction
• dioecious
• hermaphroditic
• etc.

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Variation in Sexual Reproduction

• Separate sexes - dioecious (plants)
• - gonochoristic
  (animals)
• Co-sexual hermaphroditic
• (malefemale)

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Sex is Everywhere !
sex 347 x 106
Praying mantis
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http//www.matings.co.uk/
Mating
Spawning
Pollination
Reproduction Sex
66

• The Adaptive Significance of Sex
• Sexual reproduction is
• - complicated
• - costly
• - dangerous

67

• The Adaptive Significance of Sex
• Searching for a mate
• - takes time and energy
• - increases risk of predation
• Mating increases exposure to STDs
• Mate may be infertile
• Why not reproduce asexually ?

68

• The Adaptive Significance of Sex
• Many plant and animal species capable of both
  sexual and asexual reproduction
• (Aphids, Lizards, fish, Daphnia,
• plants)
• Parthenogenesis
• offspring develop from unfertilized eggs

69
Alternative to Sex
asexual 469,000

• Asexual reproduction
• - Parthenogenesis
• - Apomixis

70

• The Adaptive Significance of Sex
• Sexual and asexual reproduction in same
  population
• Will one mode replace the other ?
• Null model (John Maynard Smith)

71

• The Adaptive Significance of Sex
• Assumptions
• A females reproductive mode does not
• 1. affect the number of offspring produced
• 2. affect the probability that her offspring will
  survive

72

• The Adaptive Significance of Sex
• Mode of
• Reproduction Progeny
• Parthenogenetic female
  all female
• Sexual female
  ½ male ½ female

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• The Adaptive Significance of Sex
• Model
• Each female produces 4 offspring then dies
• Asexual female ? 4 females
• Sexual female ? 2 females and 2 males

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4/8
16/24

• Cost of males
• (Asexual has a 2 X advantage)

75
Each female produces 2 offspring and dies
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Paradox

• Sex more costly than asexual
• Despite costs, sexuality more widespread
• Asexuality can evolve from sexual species
• Asexuality is taxonomically and
• phylogenetically sporadic

77
Conclusion

• Sex is evolutionarily more successful than
  asexuality.
• But why ?

78
Maintenance of Sex

• Short-term advantage of sex?
• Differences
• 
  Sexual Asexual
• Recombination Yes
  No
• Offspring genetically diverse
  uniform
• Multi-locus genotypes breaks up
  preserves
• AaBb Aabb
  AaBb

79
Advantages of Sex

• Two main theories
• 1. Sex prevents the accumulation of deleterious
  mutations
• (or slows)
• 2. Sex (recombination) produces new genotype
• combinations favoured in a changing
  environment.

80
1. Do deleterious mutations accumulate?

• Theory
• Experimental Evidence

81

• TheoryMullers Ratchet
• accumulation of mutations in an asexual
  population genetic load
• (mutation and drift)

82
Salmonella 444 cultures Periodic bottlenecks
genetic drift 1,700 generations 5/444 had lower
growth (fitness) None higher
83
Evidence for the accumulation of deleterious
mutations
E. coli
Others Flies Worms Chromosomes
84
Mutational Meltdown

• Mullers Ratchet
• Accumulation of mutations
• Decreased population size
• Increased rate of mutation accumulation
• Feedback ? extinction

85
Sex and recombination halts the ratchet

• Sexual
  Asexual
• AABb x AaBB AABb AaBB
• AABB AABb
  AaBB
• (a and b deleterious alleles)

86
Advantages of Sex

• Two main theories
• 1. Sex prevents the accumulation of deleterious
  mutations
• (or slows)
• 2. Sex (recombination) produces new genotype
• combinations favoured in a changing
  environment.

87

• The Adaptive Significance of Sex
• (Dunbrack, Coffin Howe 1995)
• Experiment (30 generations 2 years)
• Tribolium beetle compared
• 1. Sexual (evolving)
• 2. Asexual (nonevolving) 3X advantage
• Evolve resistance to an insecticide (Malathion)

88
Results
Sexual wins
3 x Asexual wins
89

• The Adaptive Significance of Sex
• Assumptions
• A females reproductive mode does not
• 1. affect the number of offspring produced
• 2. affect the probability that her offspring will
  survive

90

• The Adaptive Significance of Sex
• Interpretation
• Evolving sexual eliminated non-evolving
  asexual despite the 3 x advantage
• Assumption 2 incorrect
• Progeny from sexual females had a higher
  probability of survival
• Why ?
• Sexual progeny genetically variable
• Asexual progeny genetically identical

91

• The Adaptive Significance of Sex
• Sex beneficial in a changing environment
• (genetic variation ? natural selection)
• Red Queen Hypothesis
• change in the biotic environment

92
Red Queen
At the top of the hill, the Red Queen begins to
run, faster and faster. Alice runs after the Red
Queen, but is further perplexed to find that
neither one seems to be moving. When they stop
running, they are in exactly the same place.
Alice remarks on this, to which the Red Queen
responds "Now, here, you see, it takes all the
running you can do to keep in the same place".
Evolutionary Theory The biotic environment is
constantly changing due to the evolution of
predators, parasites, disease organisms and
competitors. Need to evolve to avoid extinction.
Result Evolutionary arms
race
93
Red Queen
Host Evolve resistant genotypes Parasite Evolve
to overcome resistant genotypes
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Example

• Freshwater Snail
• - sexual form (males and females)
• - parthenogenetic form (female)
• - trematode parasite (infects gonads)
• 
  ?sterilizes

95
Trematode parasite
Potamopyrgus antipodarum
Gonad
infected
normal
96
Males (sexual)
Sexuals more common in populations with higher
trematode infection rates
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More Theory

• Advantages of sex
• Remove deleterious mutations
• Genetic diversity in a changing environment
• But, simulations show the 2 advantages
  individually not sufficient to maintain sex

98
results

99

• Sex
• Search for the adaptive significance of sex
  continues
• A diversity of theories exist for the
  predominance of sexual reproduction
• Much interest in the adaptive significance
  of variation in sexual reproduction