"Gene flow is like... putting jellyfish DNA in a petri dish with E. Coli and stressing out the E. Coli and forcing them to take up the jellyfish DNA and glow. By putting stress on the organisms in their environment you require them to search for and apply new genetic traits. The E.Coli experiment isn't evolution or even gene flow, but it's the general idea. It's an integral part of evolution, but i think you're correct in saying it doesnt always help."