Ecological speciation in lake organisms

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Ecological speciation in lake organisms

• Anisa Salmi
• 2008

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Outline

• What is a species?
• How does speciation work?
• Natural Selection
• Fundamental Theorem of Natural Selection
• What is ecological speciation?
• Environmental Differences
• Sexual Selection
• Ecological Interaction
• Examples
• 1. Cichlid diversity and background
• Case Study - Cichlid Fish Diversity Threatened by
  Eutrophication That Curbs Sexual Selection
• Stickleback diversity and background
• Case Study - Evidence for ecologys role in
  speciation

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What is a species?

• Ernst Mayers biological species concept
  Species are groups of interbreeding natural
  populations that are reproductively isolated from
  other such groups
• Distinct species
• 1. Genetic differences prevents species from
  living in the same area
• Can inhabit the same area but genetic
  differences prevent fertile hybrids

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What is a species?

• Species are maintained by isolating
  barriers, or those biological features of
  organisms that impede the exchange of genes with
  members of other populations

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What is a species?

• Isolating barriers
• Prezygotic Barriers
• Premating
• - habitat isolation, temporal isolation,
  behavioral isolation
• Postmating
• - mechanical isolation, gametic
  isolation
• Postzygotic Barriers
• - hybrid inviability, hybrid sterility,
   hybrid
• breakdown

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How does speciation work?

• Natural Selection
• Reproductive success in the long term
• Differential mortality there is competition
  among progeny for survival
• Differential Reproduction because there is
  competition some variants will be better adapted,
  compete more successfully and leave more
  offspring. These variants will comprise the next
  generation

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How does speciation work?

• The Fundamental Theorem of Natural Selection
• Some individuals will produce more offspring that
  survive and reach maturity than others
• These individuals are best adapted, they are
  best able to survive and reproduce an a given
  environment
• The genes of such individuals will predominate in
  the following generations

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How does speciation work?

• 4. When the environment changes the original
  types become less adapted and variant types, by
  chance may, be better adapted ? spread ? species
  change
• EVOLUTION
• 5. Therefore adaptation is evolution

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What is Non-Ecological Speciation?

• Reproductive isolation
• Genetic drift
• Founder effect
• Random fixation of alleles

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What is Ecological Speciation?

• ?The process by which barriers to gene flow
  evolve between populations as a result of
  ecologically-based divergent selection.
• Ecologically-based divergent selection
  interaction of individuals with their environment
  during resource acquisition
• ?Barriers to gene flow are ecological in nature

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What is Ecological Speciation?

• Ecological causes as a source of divergent
• selection
• Environmental differences
• 2. Sexual selection
• 3. Ecological interactions

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What is Ecological Speciation?

• Environmental differences
• Speciation is caused by adaptation to different
  environments
• Divergent selection between environments affects
  specific phenotypic traits causing reproductive
  isolation by means of sexual isolation

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What is Ecological Speciation?

• Environmental differences
• Allopatric speciation
• a population splits into two geographically
  isolated populations and then diverges
• Sympatric Speciation
• species diverge while
• inhabiting the same place

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What is Ecological Speciation?

• Sexual selection
• Acts on traits directly involved in mate
  recognition
• Differences in mate preferences divergent
  selection between environments
• Divergent selection between environments
• - spatial variation in natural
  selection on secondary sexual traits
• - mating or communication systems

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What is Ecological Speciation?

• Ecological interactions
• Interactions
• Interspecific competition
• Mutualism
• Facilitation
• Occur in sympatry
• Frequency dependent - individual fitness depends
  on the frequency of the various phenotypes

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Cichlid Fish Diversity in Lake Victoria

• Located on the border of Uganda, Kenya and
  Tanzania
• Lake Victoria is the worlds largest tropical lake
  and the second largest freshwater lake

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Cichlid Fish Diversity in Lake Victoria

• Cichlid fish species evolved rapidly in 12,400
  years
• Differ in the size and shape of their bodies,
  head morphology, male breeding coloration,
  trophic specialization and breeding behavior
• Mouth parts have evolved to suit different eating
  patterns

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Cichlid Fish Diversity in Lake Victoria

• Cichlid species of the lake Victoria basin are
  collectively referred to as thehaplochromines
• Common ancestor of the tribe Haplochromini in
  lake Tanganyika

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Cichlid Fish Diversity in Lake Victoria
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Cichlid Fish Diversity in Lake Victoria

• Three stages of Cichlid radiation
• a.Habitat divergence - evolution of distinct
  rock- and sand-dwelling clades
• Adaptation to different microhabitats
• b. Elaboration of morphologically distinct genera
  - differentiation of the feeding apparatus by
  natural selection
• Ecological selection on trophic morphology
• c. Color pattern
• Sexual selection on color pattern

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Cichlid Fish Diversity in Lake Victoria

• Evolutionary genomics
• Jaw shape
• Tooth shape
• Color pattern
• Visual sensitivity

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Cichlid Fish Diversity in Lake Victoria

• Selection Pressures
• Ecological selection
• different modes of feeding
• habitat preferences
• behavioral niche partitioning

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Cichlid Fish Diversity in Lake Victoria

• Selection Pressures
• Sexual Selection
• Females maternal mouth brooders
• Males contribute genes
• Asymmetric parenting strong sexual selection,
  sexual dimorphisms, life history and dispersal

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Cichlid Fish Diversity in Lake Victoria

• Selection Pressures
• Sexual Selection
• male traits evolve in response to female mate
  preferences
• a rapid co-evolution of male traits and female
  preferences
• Sexual selection can also lead to sympatric
  speciation

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Case Studies Cichlid Fish Diversity in Lake
Victoria

• Sympatric Speciation Models
• Allele for reversed female preference could
  invade and cause rapid sympatric speciation
  within small populations
• Male dispersal depends on mating success, and
  leads to linkage disequilibrium and an
  acceleration of Fishers process (runaway
  evolution of male traits and female mating
  preferences under sexual selection)
• Coupled sexual selection and niche differentiation

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Cichlid Fish Diversity in Lake Victoria

• Selection Pressures
• Genetic conflicts
• Selfish genetic elements, such as transposons,
• replicate to high copy numbers unless they
• are opposed by another form of selection
• The genes that are most often in conflict
• in different cellular compartments
• inherited asymmetrically

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Case Studies

• Cichlid Fish Diversity Threatened by
  Eutrophication That Curbs Sexual Selection
• Seehausen, et al. 1997

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Case Studies

• Hypothesis
• Dull Cichlid fish coloration,few color
  morphs, and low species diversity in turbid areas
  as a result of recent eutrophication
• of Lake Victoria

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Case Studies

• Eyes equipped with three retinal cone pigments -
  cover the light spectrum from blue to red
• Sympatric species - male coloration at opposite
• ends of the color spectrum
• Dichotomy among male color morphs matches the
  two absorbance peaks of retinal pigments

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Case Studies

• Is individual variation in color vision
• responsible for dichotomy in male coloration?
• Is variation in color vision responsible for
  color dichotomy maintenance?

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Case Studies

• Male red/blue coloration most intense when
  effect of red and blue color signals visually
  enhanced
• ? broad spectrumilluminated water
• ? sufficiently red and blue downwelling light
  contrasts against yellowish sidewelling light
• METHODS
• Female preference of sympatric red/blue sibling
  species under broad spectrum and monochromatic
  lights

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Case Studies

• RESULTS
• Horizontal line equal reflectance of red and
• blue
• Above line red more reflected than blue
• Below line blue is more reflected than red

Nyererei Neochromis
Ratio of Reflectance
Width of Transmission Spectrum (nm)
Hue of body colors as a function of light
transmission
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Case Studies

• RESULTS
• Broad spectrum illumination
• conspecific (same species) over
  heterospecific (different species) males
• Monochromatic light (color differences masked)
• mated indiscriminately

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Case Studies

• CONCLUSION
• Haplochromine cichlids choose mates on the
• basis of coloration
• ? Sexual isolation maintained by color
  preference

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Case Studies

• Is male coloration determined by sexual
  selection?
• More brightly colored males in highly transparent
  water
• Easier target for predators
• - Suggests against natural
  selection
• hypothesis
• Therefore male coloration is most likely
  determined by sexual selection
• Light conditions constrain this choice
• then light should limit number of sexually
  isolated species and the number of color morphs

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Case Studies

• Does increasing turbidity, curb the impact of
  sexual selection on sexual isolation ?
• Is that responsible for the decline in cichlid
  diversity?

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Case Studies

• METHODS
• Examined spectral bandwidth at 2 m water
  depth and the number of coexisting haplochromine
  cichlid species

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Case Studies

• RESULTS
• Clearer water broader spectrum of
  transmitted light more color morphs of a
  species more species of a genus

Relation between bandwidth of the transmission
spectrum and cichlid diversity
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Case Studies

• CONCLUSION
• Turbid waters monochromatic light conditions
  only one drab-colored species breakdown of
  reproductive barriers decrease in diversity

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Case Studies

• Eutrophication caused by
• Deforestation
• Agricultural practices
• Predation of Nile perch
• Industrialization and urbanization

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Stickleback Diversity

• Restricted to Northern Hemisphere and margins of
  Atlantic and Pacific Oceans
• Very phenotypically diverse
• ? marine, anadromous and freshwater populations
• Broad geographical and ecological distribution

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Stickleback Diversity

• Anadromous, Marine
• Changed little in the past 10 million years
• Limited geographical variation

• Stream Resident
• Founded repeatedly from marine and anadromous
  populations
• Post-glacial isolates ? huge adaptive
• radiation

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Stickleback Diversity

• No sustained evolutionary divergence
• no widespread, phenotypically distinct
  species
• Highly divergent populations selective
  extinction
• ? endemic specialized for small, isolated
  habitats that quickly dry up

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Case Studies

• Evidence for ecologys role in speciation
• McKinnon et al. 2004

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Case Studies

• Stream-resident threespine stickleback
• Stream populations from different regions are
  phenotypically similar
• Smaller in size

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Case Studies

• Anadromous threespine stickleback
• Ancestral
• Persistent
• Geographically widespread marine habitats
• Larger in size

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Case Studies

• Have stream ecotypes evolved repeatedly from
  anadromous ecotypes?
• Microsatellite data closer relationships
  between geographically adjacent populations than
  same ecotype
• ? stream populations evolved
• repeatedly from anadromous
  populations
• Allozymes and mitochondrial DNA sequences
• ? replicated origins of freshwater
  populations from
• anadromous ancestors

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Case Studies

• Hypothesis
• Assortative mating based on body size in
  stickleback populations evolved according to the
  environment
• Ecological differences divergent selection on
  small number of phenotypic traits ? by product
  speciation

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Case Studies

• METHODS
• 1.Collected individuals for mating trials from
  geographically distant regions (Alaska, British
  Columbia, Iceland (stream only), Scotland, Norway
  (stream only) and Japan)
• ? assess reproductive compatibility or isolation
• Mating of same ecotypes
• Mating of different ecotypes
• Mating within and between regions

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Case Studies

• METHODS
• 2.Experimental manipulation of body size
• Raised large/small females of each ecotype
• by altering growing periods
• Tested mating patterns using field collected
  males
• ? Mating with different sized ecotypes

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Case Studies

• RESULTS
• Courtship gt twice as successful between pairs of
  the same ecotype
• Female stream and anadromous sticklebacks
  preferred males of own ecotype

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Case Studies

• RESULTS
• Mating compatibility negative relationship with
  mean length difference
• Preference of male ecotype depends on female size
• Females retained weak preference for males of
  own ecotype

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Case Studies

• CONCLUSION
• Adaptation to different environments
  reproductive isolation
• Confirmed connection between size divergence and
  the build-up of reproductive isolation
• Traits other than size make a secondary
  contribution to reproductive isolation

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Questions?
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References

• Bell, M.A. (2001).Lateral plate evolution in the
  threespine stickleback getting nowhere fast.
  Genetica 112113 445461
• Kocher T.D. (2004). Adaptive evolution and
  explosive speciation the Cichlid fish model.
  Nature, 5288-298
• Krebs C.J. (2001). Ecology. Benjamin
  Cummings.ch.2.
• McKinnon, J. S., Mori, S., Blackman, B. K..
  David, L.. Kingsley, D. M., Jamieson, L., Chou,
  J.. Schluter, D.(2004) Evidence for ecology's
  role in speciation. Nature, 429, 294-298
• Rundlel H.D. and Nosil P. (2005). Ecological
  speciation. Ecology, 8 336352
• Schluter D. (1995). Ecological speciation in
  postglacial fishes. Phil. Trans. R. Soc. Lond.
  351807-8-14
• Seehausen, O., van Alphen, J.J.M., Witte, F.
  (1997). Cichlid fish diversity threatened by
  eutrophication that curbs sexual selection.
  Nature 2771808-1811