The easiest way to talk about evolution is talking about purpose and desires as if it were a conscious being.
Of course this is not true but there just aren't words in English for the subtle and specific concepts involved. Correctly communicating the concepts with our clumsy language requires stumbling around and turning concise ideas into difficult to understand monsters.
When you are talking about evolutionary purpose, you're actually talking about the set of cause and effect pressures that lead to the effect.
The human tailbone is less purposeful than the hair because we can reason fairly easily that the loss of hair would decrease fitness in some very believable situations. The loss of a tailbone we can't seem to think of any fitness changes.
We have both hair and a tailbone. There _is_ evolutionary pressure to keep hair, although not extreme pressure as there's quite a bit of variability. There is not much evolutionary pressure to have or get rid of a tailbone. In order for there to be a change there has to be a path of mutations that survives and grows in a population. There probably A) isn't an easy path of mutations to get rid of the tailbone and B) isn't a meaningful cost to grow and maintain one. Those two things put together make lack of purpose. It's there, but doesn't really make a difference and isn't easily changed.
>not a big one but bald women don't mate as successfully as women with nice hair do, on average
A trait being seen as attractive in a mate is another way that trait evolves. Hair isn't evolutionary advantageous because it attracts mates, it attracts mates because it is evolutionary advantageous. Sexual selection can get weird and select for things that are fairly far removed from what we might think as utility.
Part of hair's purpose is just as an indicator of health. If you're very unhealthy your hair will fall out or look particularly bad, if it's long and nice looking that means you've probably been healthy for a long time too. That's probably a good indicator at how good you are at taking care of yourself and likewise how good you will be at taking care of your offspring and how good they will be at taking care of themselves.
Sometimes sexual selection is just about how much energy you can spend on doing something to look attractive with no other purpose. Lots of birds exhibit this.
Not necessarily, the real suicides -- not those ones claimed to be but eventually not carried out -- terminate all evolution on the very specific subjects who perform that action.
Well, think of it like this: Evolution is a process, and the process is the purpose. And sometimes we end up with results that are "wrong". But this is never the end of the path.
