I think this is the best piece of advice overall. You are likely not going to convince your father, whose opinions probably even predate your birth. The real thing at stake here isn't scientific truth, and trying to convince him is to fight the wrong battle.
People have a lot of beliefs they don't feel the need to constantly justify to others, and I think it's an accepted social convention to seek shelter in that principle. Being evasive and using relativism can help : admitting you can't be sure about science and evolution is an acceptable compromise if you can trade it for the opposite fact that your father and people like him can't be sure about God either.
Once swamped in such a position on both sides, you can also simply chose not to pursue the argument further, and believe your stuff "just because I feel this is the truth", the same way they do for their own stuff, at which point it would be hypocritical for them to still try to convince you they're right. This may not seem like a "truth seeker" thing to do, but you're playing by his rules already anyway, and in his world, truth may not have the same meaning as it does in ours.
Also, trying to justify atheism like this feels like you're playing a game where you've both already agreed tacitly that atheism is the side of the argument that has the burden of proof, which it hasn't. Once again, don't fight on your father's ground with weapons adapted to your own. Either use other tools adapted to this battle ground (where truth is relative and backhanded arguments will be used), or move to another battle ground adapted to your tools.
So, a little background- I've just come out as an atheist to my dad, a Christian pastor, who's convinced he can "fix" my thinking and is bombarding me with a number of flimsy arguments that I'm having trouble articulating a response to, and need help shutting down. The particular issue at the moment deals with non-theistic explanations for human psychology and things like love, morality, and beauty. After attempting to communicate explanations from evolutionary psychology, I was met with amused dismissal of the subject as "speculation".
There's one book in particular he's having me read- The Reason for God by Timothy Keller. In the book, he brings up evolutionary psychology as an alternative to theistic explanations, and immediately dismisses it as apparently self-defeating.
"Evolutionists say that if God makes sense to us, it is not because he is really there, it's only because that belief helped us survive and so we are hardwired for it. However, if we can't trust our belief-forming faculties to tell us the truth about God, why should we trust them to tell us the truth about anything, including evolutionary science? If our cognitive faculties only tell us what we need to survive, not what is true, why trust them about anything at all?" -Timothy Keller
The obvious answer is that knowing the truth about things is generally advantageous to survival- but it hardly addresses the underlying assertion- that without [incredibly specific collection of god-beliefs and assorted dogmas], human brains can't arrive at truth because they weren't designed for it. And of course, I'm talking to a guy with an especially exacting definition of "truth" (100% certainty about the territory)- I could use an LW post that succinctly discusses the role and definition of truth, there.
Another thing Dad likes to do is back me into a corner WRT morality and moral relativism- "Oh, but can you really believe that the act of rape doesn't have an inherent [wrongness]? Are you saying it was justified for [insert historical monster] to do [atrocity] because it would make him reproductively successful?" Armed only with evolutionary explanations for their behavior, I couldn't really respond- possibly my fault, since I haven't read the Morality sequence on account of I got stuck in the Quantum Physics ultrasequence, and knowing that reality is composed of complex amplitudes flowing between explicit configurations or aaasasdjgasjdga whatever the frig even (I CAN'T) has proven to be staggeringly unhelpful in this situation.
In addition to particular arguments WRT the question posed, I could also use recommendations for good, well-argued and accessible books on the subject of evolutionary psychology, with a focus on practical experimental results and application- the guy can't be given a book and not read it, so I'm hoping to at least get him to not dismiss the science as "speculation" or a joke. It's likely he's aware that the field evolutionary psychology is really prone to hindsight bias and thus ignores it completely, so along with the book, a good article or study demonstrating the accuracy and predictive power of the evolutionary psychological model would be appreciated.
Thanks!