Many people seem hung up on large life forms as models of microbial evolution. The endless battle of survivals breading bigger teeth versus bigger horns makes many think the most competitive microorganism is the one that combines the most tough qualities and that a harsh environment like Mars is going to breed something so tough it will eat all Earth microbes for lunch. I've tried over and over to teach what I learned in gradschool microbiology about how microbes actually compete, but it keeps seeming to fall on deaf ears. I just came across an excellent example of why in the microbial world it is lean and mean that wins the day not an industructable juggernaut like D. radiodurans which only dominates where where it's extra metabolism is actually required. The example is Pelagibacter ubique. There are an estimated 20 billion billion billion Pelagibacter microbes scattered throughout the world's oceans accounting for a quarter of all organisms in the ocean according to this article yet it has the smallest most efficient genome known of any independent organism. It's simple folks - evolution favors more effective reproduction. Grow faster and reproduce more than your competitors and you win the battle of evolution. If your competitors are wasting energy on defensive structures and metabolic pathways that aren't needed they will be growing and reproducing slower.

Regards,
Mark