Suddenly, the world changes.

First, it's prosperity: Unlimited growth abruptly ends. Stark recession sets in. The stock market stops booming. Companies go bust.

Then, it's peace: An attack on the United States provokes a new and different kind of war. In a matter of a few months, the familiar world gets turned upside down, then inside out.

The challenge is to adapt -- or die. The sudden shifts in the economic and political environment only serve to underline the relevance of Charles Darwin's theory of natural selection -- a power "incessantly ready for action," as he wrote in The Origin of Species.

When the environment changes abruptly -- when the Ice Age arrives, say, or, more to the point, when war erupts -- adaptability becomes the only survival mechanism.

But how do you adapt? What does adaptive behavior look like? How are companies and their leaders practicing Darwin's lessons? Here are three versions of adaptability that suggest survival strategies for changing times.

JetBlue Airways: Starting With Fresh DNA

When David Neeleman launched JetBlue Airways in February 2000, he knew that he was entering into dangerous territory, a landscape that had claimed the lives of a number of ambitious new entrants: Kiwi International Airlines, People Express, and Tower Air, to name a few. He thought that he had figured out a unique way to fill a niche left empty by the big air carriers -- low-cost, high-touch service for harried travelers. But nothing could have prepared Neeleman for the events of September 11 or for their impact on the airline industry. Suddenly, he was operating on pure instinct.

Neeleman's first response was to buck the industry trend. He vowed not to lay off any of the airline's 2,000 employees, even though its planes, in the first weeks after they resumed service, were flying mostly empty to 17 cities from JetBlue's home base at John F. Kennedy International Airport. Neeleman's second big decision was about how to talk to his customers -- New Yorkers, who were the nation's most deeply affected fliers. In the wake of September 11, Neeleman knew that he had to adapt his marketing message. But what should JetBlue say?

In times of crisis, companies tend to fall back on their habitual patterns of behavior. Top-down hierarchies look for one decisive leader -- an approach that can be useful for managing certain operational matters, such as recovering airplanes that are stranded at airports around the country. JetBlue's ability to adapt is rooted in its small size and its focus on teamwork. In this crisis, Neeleman's team rallied together to locate the emotional center of its new marketing strategy.

Neeleman first tried drafting a personal letter that would run as a full-page ad. But he was vetoed by his team, who felt that the message was too self-conscious. Instead, JetBlue ran an ad that told its customers, We know you need time to heal. JetBlue will be here when you're ready to fly again.

Other carriers were urging Americans to shake off their fear of flying. "We really did not want to go down the path of, 'Get America moving again' or 'Get on board. It's important to show bin Laden we're not afraid to fly,' " says Amy Curtis-McIntyre, JetBlue's vice president of marketing. "That resonates in a lot of other parts of the country, but it just didn't feel right for New York. The worst thing that we can do right now is make a mistake. Being quiet is not necessarily a bad thing."

It was a lesson in restraint for Neeleman, a driven, hands-on entrepreneur who felt that the time was right to market JetBlue's low fares. "I'm being totally deferential and patient," Neeleman says, "because I think the situation demands it. I have to trust the instincts of the people around me."