Organs can now be grown!

[Occhidiangela]

Evolution, at its core, is only a process to force variety with the idea that the more variety (in a genetic sense) the more chances that the species will be able to overcome environmental obstacles.  there is no gaurantee that evolutionary adaptations will be helpful to a species and in many cases what was originaly an adaptation later becomes maladaptive.  There is a higher number of cases of sickle cell anemia in people of african decent, this occurance has been shown to derive from a particular genetic attribute that was originaly an adaptation to resist malaria.

Survival of the fittest is not cumulative.  In other words: the "fit" of one generation having offspring will not necisarily create a more "fit" second generation.  At it's heart Survival of the Fittest only means that the "fit" are those that live long enough to create self reliable offspring, nothing more.  Thier genetic qualities are not necissarily better or worse than any other.

The only way that Humans, or any other species, could be getting genetically weaker is if evolution stopped creating variety. 
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OK, genetic diversity is a "hedging of bets" at the species level. I am with you.

While you make some very good points, we should not forget the synergistic effect of nature AND nurture. Developing an advanced brain means nothing if it isn't used. Adapting sound survival strategies can overcome any number of purely genetic obstacles. Put a different way, some of the environmental obstacles are within us. As another example of non optimal evolutionary development, the appendix, to go with your sickle cell example.

Occhi