Biology 2900 Principles of Evolution and Systematics

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Biology 2900Principles of Evolutionand
Systematics

• Dr. David Innes
• Jennifer Gosse
• Valerie Power

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Announcements

• Lab 2 (Group 2) 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

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Announcements

• Midterm test Thursday Feb 14, 2008
• First online quiz report ready by tomorrow
• Second online quiz?

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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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Phenotypic Variation
Phenotype Genotype Environment
Further complication G x E interaction

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G x E interaction

• VP VG VE VG x E
• Phenotypic variation due to
• - genetic variation
• - environmental variation
• - G x E interaction variation
• (effect of environmental differences on the
  phenotype differs among different genotypes)

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Phenotypic Variation

• Reaction Norm 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

• 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) (next slide-?)
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G x E interaction
Genotype
Environment
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Phenotypic Plasticity

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

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

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

light

cylinder
Daphnia
-
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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
Treatments
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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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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Principles of Evolution and Systematics

• Darwin Natural selection (Galapagos
  Finches )
• Population Quantitative genetics (Genes in
  populations)
• Natural selection Adaptation (Form and
  function)

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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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Studies in Evolution

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

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• 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 942,000,000
Praying mantis
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http//www.matings.co.uk/
Mating
Spawning
Pollination
Reproduction Sex
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• The Adaptive Significance of Sex
• Sexual reproduction is
• - complicated
• - costly
• - dangerous

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• 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 ?

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

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Alternative to Sex
asexual 469,000

• Asexual reproduction
• - Parthenogenesis
• - Apomixis

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

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• 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

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• 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)

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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 (evolutionary
  dead end)

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Conclusion

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

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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

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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.

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1. Do deleterious mutations accumulate?

• Theory
• Experimental Evidence

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

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

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

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Sex and recombination halts the ratchet

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

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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.

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• 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)

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Results
(Strength of selection)
Sexual wins
3 x Asexual wins
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• 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

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• 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

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

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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
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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

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Trematode parasite
Potamopyrgus antipodarum
Gonad
infected
normal
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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

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results

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• 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