7.11 Variations in Populations

The amount of genetic variation in a population affects how it responds to change. If individuals differ in alleles for disease resistance, heat tolerance or diet, then when a new pressure arrives, some individuals are likely to survive it and reproduce, so the population can persist and adapt. A genetically diverse population is more resilient to environmental disturbance.

A population with little diversity has few options. If one pathogen can infect one individual, it can probably infect them all. Low diversity also tends to come with inbreeding, which makes harmful recessive alleles more likely to show up in homozygous offspring and lowers fertility and survival.

• California condors: the total population fell to just 27 birds in 1987, when the last wild birds were captured for breeding. Captive breeding has rebuilt the population to hundreds, but all descend from those few birds, so genetic diversity is low and managers track family lines carefully.
• Black-footed ferrets: thought extinct, then rediscovered in Wyoming in 1981. After disease struck that colony, 18 survivors were captured in the mid-1980s for captive breeding. Today's ferrets all descend from a handful of founders.
• Prairie chickens in Illinois: as grassland habitat disappeared, the population shrank to a few dozen birds and lost genetic diversity. Egg hatching success dropped. When birds from larger populations in other states were brought in, adding new alleles (gene flow), hatching success recovered. This is called genetic rescue.

Farmers often plant genetically uniform crops because they're predictable, but uniformity is risky. In the late 1840s, potato blight, caused by a fungus-like water mold, swept through Ireland. Potatoes are usually grown from pieces of tubers, so whole fields were clones, and most Irish farmers relied on very few varieties that were all susceptible. Harvests failed repeatedly, leading to the Irish Potato Famine.

Corn has the same problem. Fungal diseases such as rusts and blights can spread rapidly through fields planted with a single genetic type. In 1970, a leaf blight hit U.S. corn hard because most hybrid corn shared a genetic trait that made it susceptible. Planting more varied crops spreads the risk.

Whether an allele helps or hurts depends on the selective pressures around it. The sickle-cell allele protects heterozygotes against malaria but causes disease in homozygotes and has no benefit where malaria is absent. Dark fur helps pocket mice on dark lava but exposes them on light sand. Antibiotic resistance helps bacteria when the drug is present but can slow their growth without it. Keeping a range of alleles in a population means it's ready for whichever conditions come next.

Diversity also explains why outbreaks rarely wipe out a whole diverse population: not every individual is susceptible.