Almost a year ago, I asked whether Blockbuster was the next Circuit City. About 66% of Foolish poll respondents said it was, and Blockbuster's recent bankruptcy filing ultimately proved them right. The former video-rental king's slow death may seem a bit anticlimactic, but it's a great illustration of the increasingly Darwinian landscape companies now face.

Blockbuster struggled with both formidable debt and a mighty, disruptive rival in Netflix (Nasdaq: NFLX). Some investors now wonder whether Netflix faces similarly disruptive forces from heavyweights like (Nasdaq: AMZN), Apple, and Google. But Netflix has a far superior track record of innovation than Blockbuster did, and unlike the bricks-and-mortar chain it drove into obsolescence, no one's catching Netflix resting on its laurels.

The retail landscape faces no shortage of challenges. Two years ago, I urged investors to give Borders Group (NYSE: BGP) a kiss goodbye, but it's still staggering around. Still, how bright can that stock's future possibly be? Borders still carries a staggering debt load (sound familiar?), and it's up against formidable competition in physical and digital books from, not to mention its more traditional rival Barnes & Noble (NYSE: BKS). Google and Apple are also encroaching here.

Other brands that once depended on baby boomers' lucrative largesse may now have to survive a leaner future. With many members of this demographic staring down the barrel of a fiscally difficult retirement, boomers are clamping down on their spending. As a result, boomer-beloved brands and retailers such as Harley-Davidson (NYSE: HOG), Ann Taylor (NYSE: ANN), and Chico's (NYSE: CHS) could face tough times ahead.

Blockbuster's sad, seemingly overdue outcome should remind investors to invest among the winners in the stock market's eternal game of "survival of the fittest." Go for the strongest brands with wide demographic appeal. Make sure the companies are well-run and have a solid track record of evolving and innovating with changing times, like Netflix and

Avoid companies with unwieldy debt, especially if they're also posting dwindling sales. Investors had plenty of time to evacuate Blockbuster before it sank, and similarly unfit companies could easily share the ailing video-rental company's fate.

Which stocks on the market today would leave Charles Darwin sadly shaking his head? Share your thoughts on the worst and weakest contenders in the comments box below.