How evolution works

Stephen Jay Gould has written extensively on the importance of evolved non-adaptive traits (what he calls 'spandrels'):

All organisms evolve as complex and interconnected wholes, not as loose alliances of separate parts, each independently optimized by natural selection. Any adaptive change must also generate, in addition, a set of spandrels, or nonadaptive by-products. These spandrels may later be "co-opted" for a secondary use. But we would make an egregious logical error if we argued that these secondary uses explain the existence of a spandrel.

(Care of Philosophy et cetera pixnaps.blogspot.com)

I would argue (as I did before this article existed) that the gene produces a protein (or more) NOT a trait. The protein then interacts with various other existing systems in the body and thus creates traits. A single gene will thus tend to be associated with multiple different things. One of which may well be dominant in its selection via natural selection many of the others may be detrimental.
But it is worse than that because evolution could occur by a sort of degradation of genes for example

Whether a gene is beneficial or not as other genes are added that are dependant on it and so it becomes fixed (i.e. if it changes the other genes fail). Thus a gene that is detrimental to survival may become a fundamental part of an organism by chance. It is difficult for natural selection to follow it all the way through BUT if natural selection has added a big disadvantage and is slowly covering up for it that gives much more latitude for experimentation.

This is a vital part of evolution because as creationists rightly point out - something that is beneficial in its final form (eg the human eye to use an often quoted example) may well be disadvantageous in its earlier forms.

It is not just random choice the experiments that are made because certain solutions will be more likely than others and certain old solutions may re-emerge by being activated from old genes.

In fact natural selection should lead to "dead ends" for example with bacteria that are perfectly evolved for breeding and surviving - why would it evolve into a "higher life form"? Any attempt to add a second cell would be likely to make a sacrifice somewhere else (eg speed) and thus be out competed. Every step it took in the direction of humans would make it less able to compete against its ancestors. The only reason is via a massive drop in the competitive pressures (like the birds on islands) OR spandrels.