Progeny from genetically distant outcrossing exhibit lower fitness in parental environments. Outbreeding depression opposes inbreeding depression. First mechanism disrupts local adaptation in F1 hybrids. Second mechanism breaks co-adapted gene complexes in F2 or later. Interspecific hybrids like mules suffer sterility and fitness loss. Hybrid vigor masks depression in F1 but fails later. Plant F1 hybrids boost vigor but yield unpredictable weak offspring. Outbreeding depression worsens over generations without new gene complexes. Selection restores fitness only if populations persist long enough. Large hybrid populations rebound via diversity under pressure. Fitness decline lasts generations before potential recovery.