Program mode eliminates the photographer's need to think about proper exposure. However, program mode only works in average conditions. The meter in your camera takes an average reading of the scene and exposes accordingly, but it can be fooled. You end up with underexposed pictures. Program mode also denies the photographer the freedom of creative exposures and aperture settings as the camera determines what these settings will be.
Aperture priority allows more creativity for the photographer than in program mode.
You can manually set aperture and the camera will automatically set the shutter speed. However, the same problems can arise as in program mode if the camera is pointed at a dark or light background. The solution is exactly the same. Switch to manual mode and meter off a neutral background to ensure proper exposure. If photographing wildlife is your thing, set the aperture so that it is wide open, usually F 2.8 or F 4 depending on your lens.