1Why diversity gives resilience

Think of genetic diversity as a population's set of options. Natural selection can only choose among the phenotypes that exist. If the environment changes:

• in a diverse population, some individuals are likely to have the phenotype that survives; they reproduce, their alleles spread, and the population recovers, now adapted;
• in a uniform population, either every individual survives or every individual is affected the same way. If the pressure is severe, the whole population can crash.

Not every individual in a diverse population is susceptible to a disease, which is why a disease outbreak rarely wipes it out. The same logic explains antibiotic resistance from the bacteria's side: a diverse bacterial population almost always contains a few cells that survive treatment.

Genetic diversity does not help any one individual survive; it makes it more likely that some individuals in the population can survive whatever the environment does next.

2Alleles that help in one place and harm in another

An allele has no fixed value. It is adaptive or deleterious (harmful) only relative to the selective pressures of a particular environment.

• The sickle cell allele (topic 7.2) protects heterozygotes against malaria where malaria is common, and is only harmful where it is not.
• An allele for thick fur helps in a cold climate and causes overheating in a warm one.
• An insecticide-resistance allele (topic 7.8) is favoured in sprayed fields; in unsprayed fields it often carries a cost, such as slower development.

So removing "bad" alleles from a population is not always an improvement. Alleles that are deleterious today may be the ones that let the population survive tomorrow, and a population that has lost them has lost the ability to respond.

3Small populations: condors, ferrets and prairie chickens

The CED names three conservation cases. Each shows what happens when diversity is lost.

California condors. Numbers fell to fewer than thirty birds in the 1980s, and all were taken into captivity for breeding. Every condor alive today descends from those few birds, so the species carries only the alleles they happened to have. Breeders choose pairs carefully, using pedigrees and genetic testing, to keep as much of the remaining variation as possible and to avoid pairing carriers of harmful recessive alleles.

Black-footed ferrets. Thought to be extinct, they were rediscovered in Wyoming in 1981. Disease then reduced that last population further, and the survivors were taken into captivity. All living black-footed ferrets descend from a handful of animals, a severe bottleneck. Low genetic diversity leaves the population vulnerable to diseases and reduces breeding success.

Greater prairie chickens. In Illinois, the species declined as prairie was turned into farmland, until only a few small, isolated groups remained. As the population shrank, the proportion of eggs that hatched fell sharply, a sign of lost diversity and of harmful recessive alleles becoming common through inbreeding. When birds from larger populations in other states were released into the Illinois population, adding new alleles by gene flow, hatching success rose again.

The common thread: small size leads to drift and inbreeding, drift and inbreeding reduce diversity and expose harmful recessive alleles, and low diversity reduces survival and reproduction, which keeps the population small. Biologists call this a downward spiral; adding genetic diversity (gene flow from other populations) is one way out.

4Uniform crops: potato blight and crop diseases

Potato blight. In Ireland in the 1840s, much of the population depended on the potato. Potatoes are grown from tubers, which are clones, and much of the crop was a single variety, so plants across the country were genetically almost identical. When the water mould that causes potato blight arrived, nearly every plant was susceptible. The crop failed across the country year after year, and the famine that followed killed about a million people.

Rusts and other crop diseases. Cereal crops such as corn and wheat are often planted over huge areas as a few highly selected varieties (topic 7.3). A new form of a rust fungus or other pathogen that can attack one plant of the variety can attack them all, and outbreaks can then spread across whole regions. Plant breeders defend against this by keeping seed banks of traditional varieties and wild relatives, which hold alleles for resistance that modern varieties have lost.

5Making the argument: claim, evidence, reasoning

The CED's skill here is to back a claim by connecting evidence to biological theory. A full-credit answer has three parts.

An invented data set. Two populations of a wild grass were exposed to the same new fungal disease. Population A is large and has many alleles at the genes studied; population B descends from a few plants that colonised an island.

• Claim. Population A is more likely to persist after the outbreak than population B.
• Evidence. Population A has far more genetic diversity (7.4 alleles per gene against 1.6), and 38% of its plants survived the outbreak, against 3% of population B.
• Reasoning. A population with more alleles is more likely to contain individuals with alleles that give resistance to a new pathogen. Those individuals survive and reproduce, passing resistance on, so the population can recover. Population B, founded by a few plants, has lost most of its variation (founder effect), so few or no plants carry resistance, and the population may decline to extinction.

Notice that the reasoning names a mechanism from the theory (selection among existing variation; founder effect), not just "more diversity is better".