An emancipation story that takes place mostly on the road and on the run, Natural Selection mixes elements of Transamerica and the recent Higher Ground to tell the story of how a God-fearing fortysomething woman found the greatest love of all. Linda (Rachael Harris) has been living a life of quiet deprivation with pious Abe (John Diehl), who holds fast to a refusal to have sex with his barren wife. Turns out Abe has been wearing himself out at a sperm-donor clinic all those years; when he has a stroke in mid-donation, Linda discovers the truth and sets out to recover one of his offspring. She locates Raymond (Matt O’Leary) in Tampa, where he’s living the extremely low life, and tries to convince him to come and meet his real father. Like Linda, writer/director Robbie Pickering’s feature debut starts out feeling a little mousy and undistinguished. The first creaks of the plot don’t bode well for the coming incongruous matchup shtick of would-be mother and son. But Pickering finds a more natural stride once Harris’s slightly nutty loneliness and O’Leary’s wonderfully convincing skid start digging into each other, first with aggression and then a different kind of heat.