Genes that have psychological and/or behavioral effects which promote the survival and reproduction of oneself and one's kin in a particular environment will, over time, tend to spread in that environment, while genes that detract from same will tend to die out.
Once again, I don't think that this proposition is seriously dubitable - and it's really all one needs to get evolutionary psychology off the ground. Obviously, it immediately raises all sorts of questions: what particular psychological and/or behavioral effects can be attributed to genetic causes, which such effects promote or detract from inclusive fitness in what particular environments, and so on. Answering such questions is, of course, what the field of evolutionary psychology is all about.
Now I think it's fairly clear that the sorts of psychological and/or behavioral variations that can be linked to genetic causes are going to be of a pretty general character: things like openness, conscientiousness, extraversion, agreeableness, general intelligence, and so on. But these are extremely interesting and important things. For example, differences in ancestral environment may well help to explain group differences in intellectual ability and criminality in ways that are wholly at odds with decades of public policy. If so, then that's a big deal.
Blanket complaints about the "poverty" of the whole field of inquiry because it can't offer us detailed explanations for every aspect of what was going on in Edison's mind when he invented the lightbulb or in Beethoven's when he composed the Grosse Fuge strike me as just silly. And complaining that it can't explain the origin of consciousness and solve the mind/body problem strike me as even sillier. Those are jobs for biographers and philosophers, respectively.