You may not have known it, but Madison is both an epicenter of top-notch microbiology research AND zombie rights. The latest zombie lurch organized by COLD (Coalition of the Living Dead) inspired me to talk a little about what zombies can teach us about microbiology. What's on my BRAAAAIIIIN today are the concepts of fitness and selection.
In the latest zombie flick "Zombieland" the protagonist is a neurotic introvert, and he credits his success in a world overrun by zombies to his obsessive adherence to a set of survival rules and a lack of friends who could turn into zombies and devour him. His traits did not make him very popular or successful in the pre-zombie world.
A simple way to think of fitness is the capability to persist and continue your species. Based on this definition, our nerdy protagonist was not very fit, was he? He didn't have many resources and he certainly wasn't a hit with the ladies.
But then the world changes, and zombies kill off a huge portion of what was once considered a normal, healthy population. Under this different set of demands--under different selection--he is left to prosper.
In summary, different environments can select for different traits. As long the selection doesn't kill off ALL individuals, then those that possess the traits endure the selection and subsequently reproduce would be considered the most fit.
I think you'll find cinematic history is full heroes and heroines that had traits that helped them survive very unusual situations!
COMING SOON IN PART 2: In a world overrun by antibiotics, what are we selecting for?