Ecological Speciation of Lake Organisms

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Ecological Speciation of Lake Organisms

• Jennifer Linton

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Outline

• Definitions
• Ecological Causes of Divergent Selection
• Ecological
• Environmental Differences
• Sexual Selection
• Examples
• Whitefish in the Yukon
• Stickleback in Northern Hemisphere
• Cichlids in African Rift Lakes
• Implications of human impacts
• Cichlids

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What is a species?

• Biological Species Concept
• Related to reproductive compatibility

• 2. Morphological Species Concept
• Based on anatomical differences

• Recognition Species Concept
• Characteristics that allow a species to recognize
  its mate (behavioural, morphological, molecular)

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What is a species?

• 4. Cohesion Species Concept
• Focuses on mechanisms that maintain discrete
  phenotypic entities
• e.g. reproductive barriers

• 5. Evolutionary Species Concept
• Related to a sequence of ancestral and descendent
  populations that evolve independently of other
  groups

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What is a species?

• 6. Ecological Species Concept
• Species defined by the unique role it plays or by
  a specific function or position in its
  environment
• Speciation by divergent natural selection
• Different members of the same species face
  different selective pressures
• Eventual reproductive isolation

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Modes of Speciation

• Allopatric
• (allos other, patria homeland)
• Geographic barrier that physically isolates two
  populations blocking gene flow
• E.g. Mountain, river

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Modes of Speciation

• 2. Sympatric
• Sym together
• Intrinsic factors that lead to reproductive
  isolation among individuals living in the same
  geographic area
• E.g. mate selection in animals

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Modes of Speciation
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Outline

• Definitions
• Ecological Causes of Divergent Selection
• Ecological
• Environmental Differences
• Sexual Selection
• Examples
• Whitefish in the Yukon
• Stickleback in Northern Hemisphere
• Cichlids in African Rift Lakes
• Implications of human impacts
• Cichlids

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Ecological Causes of Divergent Selection

• Ecological
• Divergence may arise between popn due to
  ecological interactions
• Similar to Sympatric Speciation though could
  occur between allopatric popn
• E.g. interspecific competition

Rundle Nosil, 2005
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Ecological Causes of Divergent Selection

• Environmental Differences
• Divergence due to differences in environments
• Habitat structure
• Climate
• Resources
• Predators
• Competition
• Similar to both sympatric and allopatric
  speciation

Rundle Nosil, 2005
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Ecological Causes of Divergent Selection

• Sexual Selection
• Animals who seek out a mate with a particularly
  desirable trait e.g. distinctive plumage,
  colouration
• Animals with those traits mate at a higher rate
• Desirable trait becomes more common, more
  exaggerated
• Sexual advantage to have trait
• Facilitates mate recognition

Rundle Nosil, 2005
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Outline

• Definitions
• Ecological Causes of Divergent Selection
• Ecological
• Environmental Differences
• Sexual Selection
• Examples
• Whitefish in the Yukon
• Stickleback in British Columbia
• Cichlids in African Rift Lakes
• Implications of human impacts
• Cichlids

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Speciation of Lake Whitefish (Coregonus sp.)

• Dezadeash Lake
• Squanga Lake
• Little Teslin Lake
• Bernatchez et al.,1996

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High Gill Raker Count (HGR)
Limnetic
Low Gill Raker Count (LGR)
Benthic
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3 Questions

• Are there genetic differences between ecotypes?

2. Are the ecotypes monophyletic or
polyphyletic?
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Monophyletic vs. Polyphyletic

• Monophyletic
• Descendants arising from a single common ancestor
  
• Polyphyletic
• Descendants arising from multiple ancestors

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3 Questions

• Are there genetic differences between ecotypes?

2. Are the ecotypes monophyletic or
polyphyletic?

• 3. If polyphyletic
• Multiple Allopatric Events?
• Incipient Sympatric Radiation with in each lake?

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Methods

• Analyzed Mitochondrial DNA
• Analyzed Nuclear Genes
• Allozymes
• Variants of enzymes with different genetic
  backgrounds

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Results

• gt HGR whitefish in surface nets
• gt LGR whitefish in bottom nets
• Little Teslin Lake
• Different spawning seasons
• Dezadeash Lake
• Different spawning seasons
• Squanga Lake
• Similar spawning season
• Different spawning habitats

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Conclusions

• Genetic differences between ecotypes?
• Yes
• Reproductive isolation
• Highly significant difference in MtDNA for Little
  Teslin and Dezadeash Lake
• Significant (but weaker) difference for Squanga
  Lake

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Conclusions

• Monophyletic or Polyphyletic Evolutionary Events?

• Polyphyletic
• Based on MtDNA and Nuclear Gene Analysis

Different ecotypes arose independently in each
lake, more than once
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Conclusions

• Allopatric or Sympatric speciation?
• Sympatric and Allopatric
• MtDNA data
• 2 monophyletic groups that evolved in allopatry
  during last glaciation
• Secondary contact in Little Teslin and Squanga

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Squanga Creek Drainage
Alsek River Drainage
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LGR
LGR
HGR
HGR
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Outline

• Definitions
• Ecological Causes of Divergent Selection
• Ecological
• Environmental Differences
• Sexual Selection
• Examples
• Whitefish in the Yukon
• Stickleback in Northern Hemisphere
• Cichlids in African Rift Lakes
• Implications of human impacts
• Cichlids

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Threespine Stickleback in B.C.(Gasterosteus sp.)
McKinnon et al., 2004
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Hypothesis

• Reproductive isolation may be accounted for by
  non-random mating based on
• Body Size
• Speciation by ecological differences and
  divergent selection

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Threespine Stickleback

• Anadromous Stickleback
• Geographically wide spread
• Persistent
• Larger

• Stream Type Stickleback
• Phenotypically similar in all regions
• Evolved repeatedly from anadromous ecotype
• Smaller

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Methods

• 1. Collected individuals from allopatric
  populations

• 2. Laboratory experiments
• Mating Expts with same ecotypes
• Mating Expts with different ecotypes
• Mating Expts within and between regions

• 3. Laboratory Experiment
• Mating Expts with different sized ecotypes
• Small Anadromous
• Big Stream Type

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Results

• Greater success rate with same ecotype pairs

Same Ecotype
Different Ecotype

• gt 2x within regions

• ? preferred ? of same ecotype (within and between
  regions)

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Results

• Mating compatibility decreased with increasing
  body length difference

• ? preferred a body size of ? that she was raised
  with

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Conclusion

• Reproductive isolation brought on by adaptations
  to environment

Reinforced by sexual selection
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Outline

• Definitions
• Ecological Causes of Divergent Selection
• Ecological
• Environmental Differences
• Sexual Selection
• Examples
• Whitefish in the Yukon
• Stickleback in Northern Hemisphere
• Cichlids in African Rift Lakes
• Implications of human impacts
• Cichlids

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Cichlids in African Rift Lakes

• 80 of 2,500 cichlid species from East Africa
  Haplochromis genus (2000 sp.)
• All haplochromines ultimately derived from Lake
  Tanganyika endemics
• Most have evolved in a short period of time
• e.g. Lake Victoria 15-250,000 years
• e.g. Lake Malawi dry about 200 y.a. with
  current endemics
• 4 defined lineages
• 3 ancestral lineages are species poor
• 1 modern lineage 7 of worlds 25,000
  teleost species (1750 sp.)

Salzburger et al., 2005
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Cichlids in African Rift Lakes

• Allopatric or Sympatric Speciation?

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Cichlids in African Rift Lakes

• Shorelines of lakes tend to be broken up into
  small isolated areas
• Rocky points
• Sandy beaches
• Swampy areas
• Some species reluctant to move across habitat
  barriers
• e.g mbuna cichlids

Fryer and Illes, 1972
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Cichlids in African Rift Lakes

• Lake level changes can also create microhabitats
• Sandbar separated Lake Nabugabo from Lake
  Victoria 4,000 y.a.
• Lake Nabugabo formed its own endemic species

Allopatric Speciation
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Cichlids in African Rift Lakes

• Molecular data species flocks in each lake are
  monophyletic
• Evolved after a single colonization event
• Sympatric Speciation

Joyce et al., 2005
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Cichlids in African Rift Lakes

• ? prefer ? that are brightly coloured

• Brightly coloured ? cost
• More susceptible to predation

• If they are able to avoid predation superior
  fitness compared to other ?

• ? have genetically based colour preference

Seehausen et al., 1997 Moyle and Cech, 2004
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Cichlids in African Rift Lakes

• Reproductively isolated groups form based on
  female mate colouration preference

Sympatric Speciation
Moyle and Cech, 2004
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Cichlids in African Rift Lakes

• Pharyngeal jaw apparatus is easily modified
• Phenotypically
• Genotypically

Sympatric Speciation
Liem, 1980
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Outline

• Definitions
• Ecological Causes of Divergent Selection
• Ecological
• Environmental Differences
• Sexual Selection
• Examples
• Whitefish in the Yukon
• Stickleback in British Columbia
• Cichlids in African Rift Lakes
• Implications of human impacts
• Cichlids

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Human Impacts Implications
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Cichlid Fish Diversity Threatened by
Eutrophication

• Cichlids chose mates based on coloration

• Eyes are equipped with 3 retinal cone pigments

• Cover spectrum from blue to red

Seehausen et al., 1997
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Species Diversity

• Clearer lakes have gt of species
• 300-2200 vs. 20-130
• Considered lake size ? larger lakes were more
  turbid and had fewer species

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Eutrophication Turbidity

• Eutrophication increases turbidity

• Turbidity decreases secchi depth

• Secchi Depth
• 5.5-8m in 1920s
• 1.3-3m in 1990s
• Lake Victoria 3m 1987 ? 1.5m 1997

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Turbidity and Light

• Light conditions constrain mate choice

• Light cannot penetrate as deep when turbidity
  increases

• Narrows light spectrum
• Strong loss of shortwave light (reds)

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Cichlid Light Regime

• gt part of interpopulation variation in male
  nuptial hue explained by aquatic light regime

• Males more red or blue when light conditions
  enhance effect

• Clear broad spectrum-illuminated water
• sufficient red and blue down-welling light to
  contrast against yellowish side-welling light

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Nyererei Neochromis
Ratio of Reflectance
Width of Transmission Spectrum (nm)
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Cichlid Diversity

• Turbidity is increasing due to agricultural
  runoff from surrounding land
• Decreasing effectiveness of colour signals
• ? cannot find appropriate ? to mate with

Rapid Loss of Diversity
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Questions?