One thing that can help:
The source of something is not the same as its effect.
Yes, evolution is responsible for human psychology, which has concepts such as love and morality. But this doesn't mean that evolution has concepts such as love or morality within it.
Do concepts such as "love" and "don't murder" help with human fitness? Yes. Would murder be right if it helped fitness? No.
Imagine somewhere in the universe, there is a species that murders its own members as a matter of course and they evolved to do so. They call murder something that vaguely translates to "good" if you had a translation program that translated one of their languages to English.
But murder-alien!good is not the same concept as human!good, and it is human!good that we are concerned with. It doesn't matter that murder is not written against in the rocks, it is written in us as a bad thing. And that's because of evolution, even though evolution does not contain some generalized moral principle within it.
As far as "truth" being "100% certainty" the obvious problem is most easily (without math) stated as "If you are 100% certain, that means that you cannot change your mind. There is nothing that could change it. So if you are even the slightest bit wrong when you are 100% certain, you can't fix it. So you wouldn't want that form of certainty, even if you could get it."
Finally, God makes sense to us not because this belief directly helps our survival. Rather, the ability to attribute events to Agents and understand Agents was really useful for early humans, far more so than the ability to attribute events to non-Agents. Because if Ulk stole your food, it was better to think in terms of "who stole my food?" than "my food vanished." We are social animals, and as such, Agent-based answers are highly useful to us. So when we started to explain the world, we typically attributed Agency to things, rather than non-Agents. But that doesn't mean that our Agent-thinking was good for figuring out about nature, it just means that we used a useful faculty in the wrong way. Moreover, Keller's comment doesn't distinguish between different religious interpretations. Ask your dad if he believes Thor causes lightning: it made sense to a lot of people once upon a time.
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!