A population's variation is worth nothing until the environment changes, and cannot be acquired once it has.
Learning objective 7.11.A Explain how the genetic diversity of a species or population affects its ability to withstand environmental pressures.
The same five populations, in two different worlds
Five populations differ in one thing: how much heritable variation they carry for a trait. Each is put in a stable environment and then in one where the optimum moves by a small amount every generation. Switch between the two worlds and watch which of the five differences matter.
Heritable variation—
How far behind the optimum it ends—
Population at the end—
Fastest change it could survive—
All five, in both worlds
Heritable variation
Stable: population
Moving: lag at the end
Moving: population
Fastest survivable change
What variation is for
Where the last column comes from
That column was not measured from the simulation. It was worked out separately, from the model's equations, and then used to predict which rows would survive — and the prediction matched the simulation for every row. A model that only agrees with itself is not evidence of anything; this one makes a prediction that could have failed.
One field, and a pathogen that defeats one genotype
A field of plants, all of them healthy, divided among some number of resistance genotypes. A new strain of a pathogen arrives that gets past exactly one of them. The number of genotypes in the field is the only thing that decides what happens next.
Resistance genotypes in the field—
Plants lost to one new strain—
Share of the crop lost—
Lost after three strains arrive—
Three cases, and the third one is not what you expect
Case
Where the uniformity is
What happened
Why a monoculture is a single target
The same principle, pointed at us
What a crash costs, and how long it takes to buy back
Three real populations that went through a bottleneck. Heterozygosity — the fraction of loci at which a typical individual carries two different alleles — is the standard measure of how much diversity a population has left. Watch what each crash costs, and then watch how long the repair takes.
Founders—
Generations spent small—
Heterozygosity kept—
Lost—
The asymmetry
If the population recovers to
Diversity it can eventually hold
Generations to get there by mutation
Years, at a condor generation
Why numbers come back and diversity does not
The one fast remedy, and what it requires
Learning check
Answer these in your own words before you open the answers.
Explain how the genetic diversity of a population affects its ability to withstand an environmental pressure.
In the stable world, the population with the least variation did exactly as well as the one with the most. Explain why, and what that tells you about what genetic diversity is.
Explain why a population with low genetic diversity is at greater risk when the environment changes.
Explain why a crop grown as a single genotype can be destroyed entirely by one pathogen strain, while a mixed field is not.
Explain what a population bottleneck is and why it reduces genetic diversity.
The Illinois prairie chickens started their bottleneck with more founders than the condors or the ferrets, and came out of it with less diversity than either. Explain how.
A population recovers from a bottleneck and returns to its former numbers. Explain why its genetic diversity does not return with them.
Describe the one thing that can restore a population's genetic diversity quickly, and state what it requires.
Genetic diversity is usually described as good for a population. Explain how the same property makes a bacterial infection harder to cure.