Between Easter’s religious reminders and a molecular evolution class overdose of population genetics, I shouldn’t have been surprised to wake up yesterday from an unsettling dream about taking my midterm exam on Noah’s Ark. The ocean was rising, Noah was hustling animals aboard, and I was battling asthma (thanks, furry animal allergies). But what bothered me most about all this wasn’t that I’d forgotten the formula for heterozygosity. It was that there were only two animals of every kind.

Religious beliefs aside, today’s scientific consensus is that you need more than two individuals to save a species.

That’s because species survival isn’t just about baby-making potential. It also depends on genetic diversity: the number of traits (in humans, things like skin color, height or even intellectual aptitude) present in the breeding population. This diversity may be obvious, it may be subtle or it may be invisible, detectable only by extracting and sequencing samples of DNA.

The more genetic diversity a population has, the more resilient it tends to be. When the environment changes, the population faces new adaptive pressures. In a diverse population, it’s more likely that someone will have a set of traits well-suited for the new conditions. The idea also suits a well-developed NFL defense: the team practices a variety of plays so that it’s prepared for any offensive strategy.

As you might expect, the smaller your team (or your population), the slimmer your adaptive playbook. So unless they got really lucky, the pair of animals you picked for your modern-day ark probably wouldn’t be able to re-establish their species.

In genetics, we use the phrase “genetic bottleneck” to describe diversity loss in a shrinking population. If the breeding population gets too small, the negative effects of inbreeding may run rampant. Think of the Romanov family and hemophilia, for example, and you’ll begin to see some biological reasons for the social stigma surrounding “incest.”

If the population isn’t fraught with lethal genetic disorders, it may rebound to a substantial size — but that doesn’t mean it’s safe. It takes millennia for genetic diversity to develop (through the slow accumulation of changes to the DNA sequence), so even a large population may still bear the low-diversity signature of bottlenecks past.

That’s a major reason that cheetahs, for example, are hovering on the brink of extinction. Of course, cheetahs face the same human threats (habitat loss, poaching, etc.) that most African wildlife does. But while other species are recovering slowly under watchful conservation eyes, the cheetah isn’t sprinting back. The secret is written all over its DNA.

Where most mammals share about 80 percent of their genes with other members of their species, cheetahs share 99 percent — more than you or I have in common with even our closest relatives (save for identical twins). So the miraculous genetic reshuffling of sexual reproduction — which evolved to produce varied offspring to meet a variable world — can’t help cheetahs claw back into synchrony with a changing environment.

The original cheetah bottleneck probably happened about 10,000 years ago, but other bottlenecks have been much more recent. The European bison, or wisent, population dwindled to 12 in the 1920s; California’s sea otters trace their ancestry to only 50 individuals alive in 1938. We may yet see the legacy of those bottlenecks in our conservation efforts.

But while most of us love animals, all of us depend on food. And the most troubling bottlenecks of all are those of our key crops, cut off from wild populations through domestication and further winnowed by breeding and (lately) genetic modification to just a few varieties.

In a stable environment (like the one we create with irrigation, fertilizers and pesticides), it makes sense to plant only the highest-yielding variety. But monocultures are risky: The Irish Potato Famine killed one million people because single-variety crops were entirely vulnerable to disease.

Today, you can see the importance of crop diversity in eastern Ethiopia, where a hard year means a hard life (not just an expensive one). There, Romina Cavatassi and colleagues from the UN’s Food and Agriculture Organization found that growers who’d switched to modern monocultures suffered more crop failures than their native-mixture-growing neighbors — even though, in a good year, modern seeds performed better.

Worldwide, though, agriculture is shifting from diverse crops to industrial monocultures. We’re losing the very varieties that spare Ethiopian fields as a result. At a time when climate change and fuel shortages loom on the horizon, it’s never been more important to save that diversity. Who knows which variety holds the key to drought tolerance? Or will resist the sweep of the next major pest?

While squirreling DNA away in freezers and seed banks has won favor amid desperate conservation efforts, in reality the best way to preserve genetic diversity is to preserve it in situ. Today, we award huge grants to jet-setting scientists, who stock seed banks with 70-percent redundant collections. Instead, we should reward the farmer for planting his family’s heirloom varieties instead of Monsanto’s globalized products. And we should expand “biodiversity” to mean “genetic diversity,” and protect our wildlife (and our crops) from future genetic bottlenecks. Because a genetic bottleneck, like a real one, is nearly impossible to break out of.