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Testing adaptive hypotheses: a case study
Trinidad
Venezuela
John Endler
Poecilia reticulata
...the wild guppy
Poecilia reticulata
Lives in gravel-bottomed forest streams: headwaters to lowlands
Poecilia reticulata
Lives in gravel-bottomed forest streams: headwaters to lowlands
Great variation in coloration from population to population, even within the same stream
Poecilia reticulata
Lives in gravel-bottomed forest streams: headwaters to lowlands
Great variation in coloration from population to population, even within the same stream
Lots of natural predators; generally more and the most ferocious are at lower elevations (downstream)
Poecilia reticulata
Lives in gravel-bottomed forest streams: headwaters to lowlands
Great variation in coloration from population to population, even within the same stream
Up to 7 natural predators; generally more and the most ferocious are at lower elevations (downstream)
Brightest guppy populations tend to live upstream: populations downstream are duller in color
Poecilia reticulata
Lives in gravel-bottomed forest streams: headwaters to lowlands
Great variation in coloration from population to population, even within the same stream
Up to 7 natural predators; generally more and the most ferocious are at lower elevations (downstream)
Brightest guppy populations tend to live upstream: populations downstream are duller in color
Females are dull gray (and blend in with the stream bed) in all populations
Females (always look like this)
What are the necessary conditions for evolution by natural selection?
1. variation
2. heritability: offspring resemble parents
3. survival and reproduction are not random: consistent relationship between phenotype and fitness
Forming an adaptive hypothesis about wild guppies
What are the necessary conditions for evolution by natural selection?
1. variation
2. heritability: offspring resemble parents
3. SELECTION: consistent relationship between phenotype and survival and reproduction
What is an adaptation?
The phenotypic variant that results in the highest fitness in a given environment
Forming an adaptive hypothesis about wild guppies
Move and remove guppies and predators from streams
Maintain and breed guppies in a laboratory
Cross guppies from different populations
Determine paternity/maternity of individual guppies
Genetically engineer and clone guppies
Spend years of your life on this project (i.e. allow many, many guppy generations to pass
Anything else (within reason)
You can:
Endler’s experiments:
The phenotype of each population is adapted, and is the result of the local balance of sexual and predator selection
If the balance of sexual and predator selection is changed, the phenotype should evolve in response
Environment-dependent
Endler’s experiments: Greenhouse pool experiment
Sexual selection constant (always)
Vary predation pressure
Vary environment
Track phenotypes through time: number, size, location, color variation of spots
Set up guppy populations in pools in a greenhouse
Foundation population highly variable
Endler’s results:
“innocuous” predator
control
“voracious” predator
Foundation population
Treatments applied
Ponds with no predators (K) showed a steady increase in the # of spots
Little change in spot # (compared to the control) after addition of the innocuous predator (R)
Pools that received a voracious predator treatment (C) showed a marked decrease in the # of spots
Summary of greenhouse experiment:
Phenotypes evolved in response to changes in the balance of sexual and predation selection
Endler’s experiments: 2. Field transplant in Trinidad
Each population has an adapted phenotype, reflecting the local balance of sexual and predation selection.
If evolution was “replayed”, the same adapted phenotype would evolve.
Endler’s experiments: 2. Field transplant in Trinidad
No guppies: innocuous predator present
Each population has an adapted phenotype, reflecting the local balance of sexual and predation selection.
Trinidad stream: 3 sites...
1
2 3
2 km
Transplanted 200 guppies from site 3 to site 1, measured phenotypes 10 months later.
Population 1 should evolve the same phenotypes as population 2 (site 1 = site 2).
Guppies, innocuous predator and dangerous predator present
Both guppies and innocuous predator present
Endler’s field experiment: results
c = site 3 (source pop), x = site 1 (transplant site), r = site 2
In site 1 (x)....
Spot size, number and color diversity increased
Color pattern converged on that of fish in site 2 (r)
Endler’s work: Conclusions
Demonstration of natural selection in action
Evolutionary hypotheses are experimentally testable