To those who have come to know and admire Greg Wasson, the just-announced Walgreens acquisition of Duane Reade comes as no surprise.

Indeed, in the year that has passed since Wasson was named chief executive officer of the Walgreen Co., he has thrown off surprises and excitement like a pinwheel. In acquiring Duane Reade, he leaves no doubt as to who runs Walgreens, while answering all questions about how capably he would perform in his new role.

Wasson has clearly been the right choice to run the drug chain. To his greater credit, he has become, in the space of just 12 months, one of the dominant CEOs in all of mass retailing.

Contrary to outward appearances, he hasn’t conquered this task overnight. Truth is, he’s been preparing for it during his entire 29-year career at Walgreens. In that time, he has come to understand and embrace the Walgreens culture, one that combines a belief in the Walgreens way of doing things with a commitment to recruiting and hiring the best young people available and promoting them to positions of increasing authority.

As the retailer’s chief operating officer during Jeff Rein’s tenure as Walgreens’ CEO, Wasson learned his way around health care in a retail setting. Entering his new job, he quickly mastered one lesson his predecessors were rarely forced to learn: Things don’t always go as planned, and change is frequently advisable — and sometimes necessary.

So it was that, early in his new assignment, Wasson encouraged his HR people to go outside the organization to find and recruit key people, the first time in the retailer’s 109-year history it has been compelled to do so on anything more than a random basis. So, too, he learned that opening 500 new stores each year is neither a guarantee of success nor the only way to run a retail business.

As well, he learned that the Walgreens drug store, long the industry standard, is not the only drug store — or necessarily the ideal drug store. Finally, he learned that internal communications must be more than a sometime thing requiring an occasional directive, that an effective chief executive must routinely communicate with and engage his employees.

Thus, as 2010 unfolds, Wasson finds himself running a drug chain that is vastly different from the one he took control of a year ago. It is a drug chain that has not hesitated to go outside to find talented people. It is a drug chain that has redefined its customer and its role in serving that customer. To that end, it is a drug chain that has adjusted its merchandise mix, tinkered with categories and deleted products, altering both the look of the store and the perspective of the shopper.

Wasson understands that, in changing, his company has sometimes confused its suppliers and occasionally annoyed or angered them. He knows as well that, in changing, he has had to reeducate customers who have sometimes been impatient with change, an education process that continues today.

Through it all, however, Greg Wasson has remained convinced of the need to change and, more to the point, of the wisdom of the changes he has orchestrated. If sales have sometimes suffered over the short term, customers have, over the longer period, come to generally approve of the retailer’s new directions.

On a personal level, Wasson has quickly come both to represent and reflect the drug chain he leads more openly and realistically than any CEO in the retailer’s history. Indeed, generations of Walgreens leaders have believed, perhaps correctly at the time, that the way to lead was to largely ignore the outside world and instead manipulate the internal strings that control and move a company. Wasson, on the other hand, correctly understands that, in today’s retail community, outside forces can influence a company’s performance almost as dramatically as its internal workings.

So it is that Wasson has emerged as the most visible and communicative leader Walgreens has yet employed. He correctly senses the need to participate in industry organizations and events. More than that, he has taken a leadership position within his industry, frequently offering opinions on his company, his industry and the health care community of which that industry is a vital part.

He correctly and perspicaciously comprehends the changing role of health care in the United States, and the potential role retailing can play in the health care community. He willingly shares his belief that health and wellness is becoming a retail focus, and that Walgreens needs to embrace health care as a concept that goes beyond pharmacy and the dispensing of prescription drugs.

Though his background has been largely operational, he has, at the same time, come to understand the importance of healthy retailer/supplier relationships and begun working behind the scenes at Walgreens to repair relationships that have frayed in the retailer’s rush to implement SKU-reduction and Customer-Centric Retailing programs.

He is moving quickly to grasp those issues that were not required learning in his prior assignments, specifically working to understand more effectively the advantages of a seamless and smoothly running supply chain function. At the same time, he has begun communicating more frequently — and effectively — with Walgreens’ staffers, specifically through the use of weekly webcasts.

Finally, more alert to outside criticism than perhaps any of his predecessors, he has determined that his company, in a rush to bring in senior-level people from outside the organization to fill critical roles, should not dilute the culture that has served the retailer so effectively for so long. Though eager to avoid this danger, he’s also not averse to integrating the thinking of new and longtime Walgreens staffers with the object of strengthening both — which the Duane Reade acquisition will challenge him to replicate — while emphasizing the core culture.

It is a characteristic of outstanding senior managers that they are not afraid of change. Nor are they content to stop learning and growing at a time when many of their counterparts no longer feel the need to master new thinking or reexamine old ideas. To Wasson’s credit, he understands that life is a learning experience, and that people who excel at what they do never stop growing.

If you need further proof, look no further than the stunning — and stunningly relevant — Duane Reade acquisition. It was orchestrated by a retailing executive who had grown up in a retail environment that strongly favored organic growth over growth by acquisition.

Indeed, it had been a core tenet at Walgreens that in acquiring another drug chain, Walgreens would merely be inheriting another retailer’s headaches, that it made more economic and cultural sense to build rather than buy. In a dramatic and event-filled year, Wasson has discarded that thinking, replacing it with a bolder, more exciting and potentially more rewarding approach to the retail business.

The Duane Reade announcement is further proof, if more evidence is needed, that Greg Wasson, in reenergizing a drug chain that had, in many ways, become old and tired, has changed, learned and grown more dramatically in the last year than in all the years that preceded it. That’s the way it often is with truly exceptional business leaders.