Ecological Speciation of Lake Organisms Jennifer Linton Outline

Ecological Speciation of Lake Organisms Jennifer Linton

Outline 1. Definitions 2. Ecological Causes of Divergent Selection 1. Ecological 2. Environmental Differences 3. Sexual Selection 3. Examples 1. Whitefish in the Yukon 2. Stickleback in Northern Hemisphere 3. Cichlids in African Rift Lakes 4. Implications of human impacts 1. Cichlids

What is a species? 1. Biological Species Concept • Related to reproductive compatibility 2. Morphological Species Concept • Based on anatomical differences 3. Recognition Species Concept • Characteristics that allow a species to recognize its mate (behavioural, morphological, molecular)

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

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

Modes of Speciation 1. Allopatric • (allos = other, patria = homeland) • Geographic barrier that physically isolates two populations blocking gene flow E. g. Mountain, river •

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

Modes of Speciation

Outline 1. Definitions 2. Ecological Causes of Divergent Selection 1. Ecological 2. Environmental Differences 3. Sexual Selection 3. Examples 1. Whitefish in the Yukon 2. Stickleback in Northern Hemisphere 3. Cichlids in African Rift Lakes 4. Implications of human impacts 1. Cichlids

Ecological Causes of Divergent Selection 1. Ecological • • • Divergence may arise between pop’n due to ecological interactions Similar to Sympatric Speciation – though could occur between allopatric pop’n E. g. interspecific competition Rundle & Nosil, 2005

Ecological Causes of Divergent Selection 2. Environmental Differences • Divergence due to differences in environments • • • Habitat structure Climate Resources Predators Competition Similar to both sympatric and allopatric speciation Rundle & Nosil, 2005

Ecological Causes of Divergent Selection 3. 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

Outline 1. Definitions 2. Ecological Causes of Divergent Selection 1. Ecological 2. Environmental Differences 3. Sexual Selection 3. Examples 1. Whitefish in the Yukon 2. Stickleback in British Columbia 3. Cichlids in African Rift Lakes 4. Implications of human impacts 1. Cichlids

Speciation of Lake Whitefish (Coregonus sp. ) 1. Dezadeash Lake 2. Squanga Lake 3. Little Teslin Lake Bernatchez et al. , 1996

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

3 Questions 1. Are there genetic differences between ecotypes? 2. Are the ecotypes monophyletic or polyphyletic?

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

3 Questions 1. 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?

Methods 1. Analyzed Mitochondrial DNA 2. Analyzed Nuclear Genes • Allozymes • Variants of enzymes with different genetic backgrounds

Results • • • > HGR whitefish in surface nets > LGR whitefish in bottom nets Little Teslin Lake – ~ Different spawning seasons • Dezadeash Lake – Different spawning seasons • Squanga Lake – Similar spawning season – Different spawning habitats

Conclusions 1. Genetic differences between ecotypes? • • Yes Reproductive isolation • • Highly significant difference in Mt. DNA for Little Teslin and Dezadeash Lake Significant (but weaker) difference for Squanga Lake

Conclusions 2. Monophyletic or Polyphyletic Evolutionary Events? • Polyphyletic • Based on Mt. DNA and Nuclear Gene Analysis Different ecotypes arose independently in each lake, more than once

Conclusions 3. Allopatric or Sympatric speciation? • • Sympatric and ~ Allopatric Mt. DNA data • • 2 monophyletic groups that evolved in allopatry during last glaciation Secondary contact in Little Teslin and Squanga

Squanga Creek Drainage Alsek River Drainage

Outline 1. Definitions 2. Ecological Causes of Divergent Selection 1. Ecological 2. Environmental Differences 3. Sexual Selection 3. Examples 1. Whitefish in the Yukon 2. Stickleback in Northern Hemisphere 3. Cichlids in African Rift Lakes 4. Implications of human impacts 1. Cichlids

Threespine Stickleback in B. C. (Gasterosteus sp. ) Mc. Kinnon et al. , 2004

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

Threespine Stickleback • Anadromous Stickleback – Geographically wide spread – Persistent – Larger • Stream Type Stickleback – Phenotypically similar in all regions – Evolved repeatedly from anadromous ecotype – Smaller

Methods 1. Collected individuals from allopatric populations 2. Laboratory experiments • • • Mating Expt’s with same ecotypes Mating Expt’s with different ecotypes Mating Expt’s within and between regions 3. Laboratory Experiment • • • Mating Expt’s with different sized ecotypes Small Anadromous Big Stream Type

• Greater success rate with same ecotype pairs Results Same Ecotype Different Ecotype • > 2 x within regions • ♀ preferred ♂ of same ecotype (within and between regions)

Results • Mating compatibility decreased with increasing body length difference • ♀ preferred a body size of ♂ that she was raised with

Conclusion Reproductive isolation brought on by adaptations to environment Reinforced by sexual selection

Outline 1. Definitions 2. Ecological Causes of Divergent Selection 1. Ecological 2. Environmental Differences 3. Sexual Selection 3. Examples 1. Whitefish in the Yukon 2. Stickleback in Northern Hemisphere 3. Cichlids in African Rift Lakes 4. Implications of human impacts 1. Cichlids

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 world’s ~25, 000 teleost species (~1750 sp. ) Salzburger et al. , 2005

Cichlids in African Rift Lakes Allopatric or Sympatric Speciation?

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

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

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

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

Cichlids in African Rift Lakes • Reproductively isolated groups form based on female mate colouration preference Sympatric Speciation Moyle and Cech, 2004

Cichlids in African Rift Lakes • Pharyngeal jaw apparatus is easily modified Sympatric Speciation – Phenotypically – Genotypically Liem, 1980

Outline 1. Definitions 2. Ecological Causes of Divergent Selection 1. Ecological 2. Environmental Differences 3. Sexual Selection 3. Examples 1. Whitefish in the Yukon 2. Stickleback in British Columbia 3. Cichlids in African Rift Lakes 4. Implications of human impacts 1. Cichlids

Human Impacts & Implications

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

Species Diversity • Clearer lakes have > # of species • 300 -2200 vs. 20 -130 • Considered lake size larger lakes were more turbid and had fewer species

Eutrophication & Turbidity • Eutrophication increases turbidity • Turbidity decreases secchi depth • Secchi Depth – 5. 5 -8 m in 1920’s – 1. 3 -3 m in 1990’s – Lake Victoria 3 m 1987 1. 5 m 1997

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)

Cichlid & Light Regime • > 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

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

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

Questions?