Testing adaptive hypotheses: a
case study
John Endler
Trinidad
Poecilia reticulata
Venezuela
...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)
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. 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
You can:
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)
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
Set up guppy populations in pools in a greenhouse
Foundation population highly variable
Sexual selection constant (always)
Vary predation pressure
Vary environment
Track phenotypes through time: number, size, location,
color variation of spots
Endler’s results:
“innocuous” predator
control
“voracious” predator
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
Foundation
population
Treatments
applied
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
Each population has an adapted phenotype, reflecting the
local balance of sexual and predation selection.
Trinidad stream: 3 sites...
2 km
1
No guppies: innocuous
predator present
2
Guppies, innocuous
predator and dangerous
3 predator present
Both guppies and
innocuous predator present
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).
Endler’s field experiment: results
In site 1 (x)....
Spot size, number
and color diversity
increased
Color pattern
converged on that
of fish in site 2 (r)
c = site 3 (source pop), x = site 1 (transplant site), r = site 2
Endler’s work: Conclusions
Demonstration of natural selection in action
Evolutionary hypotheses are
experimentally testable