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