Having read a few books about evolution recently, it certainly seems like evolution is a bit lazy. Random mutation creates traits that may or may not be beneficial to the organism in the short run. The ones that are selected for are those that are beneficial in the short term (technically, those that improve reproductive fitness). Traits that help an organism in the short term (before reproductive maturity) and help an organism to survive till that point are selected for - even those that are detrimental down the line. Many genetic disorders such as (I can't remember - it's one of those that are commonly referred to in genetics books... Huntington's?) are adult-onset, usually during mid-life, after one has kids. Thus this disease is not selected for and the person survives to pass on the disease DNA. There has been research into whether these disease provide some moderate level of benefit to the organism, or whether they are just "piggyback diseases", similar to piggyback bills in Congress that just happen to make it through because they don't really matter and are passed along with another, more prominent one. IF this is the case, evolution is lazy - it conferred that particular benefit without regard for its detimental effects, or in another way of thinking about it, it was the "easiest" way to confer the benefit (by completely ignoring the massive downside). Of course, that's not true, but it's an amusing and cynical way of thinking about. In the same way it doesn't care about geriatrics - there is essentially no pressure against disease that appear late in life.
I have long thought about how humans have thwarted evolution in many senses - we don't necessarily mate according to fitness. The people who have kids are in no way representative of the most fit genetically, but rather that they just best followed the memes of society or socially matched up with someone enough to reproduce. We're living to ages way beyond our bodies were prepared for, and unlike any other species, mental fitness matters immensely over physical fitness. Of course, professional athletes do pretty well for themselves, but otherwise, as a species, we survive because of our minds. Our scientific/medical knowledge has brought up our whole society rather than preserving only the individuals who happen to have that knowledge, as would a beneficial physical trait.
For example, imagine a dinosaur that grows extra long and sharp claws because of some mutation in the claw-growth-stopping-gene. It's hard to imagine that trait would help all of that dinosaur's species. On the other hand, if I come up with a vaccine for HIV next week, it would help our whole species, diminishing the fitness-spread between me (the one with the trait that actually made the vaccine) and the rest of the population. Not that we shouldn't cooperate of course. Just musing.
