Outbreeding depression reduces fitness from crosses between genetically distant groups. It contrasts with inbreeding depression and affects progeny performance. Chromosomal differences like fixed inversions cause sterility or low fitness in hybrids. Loss of coadapted gene complexes via recombination reduces F2+ fitness. Ecological mismatch between hybrid traits and environments lowers survival. Effects appear in F1 or later generations after recombination. Hybrid vigor in F1 can mask initial depression. Plant breeders avoid F2 hybrids due to unpredictable low fitness. Selection can restore fitness over generations if populations persist. Mules show extreme outbreeding depression from interspecific crosses. Stickleback forms from different lakes have low spawning when crossed. Drosophila and leaf beetles produce intermediate low-fitness progeny from distant crosses. Poison dart frogs, anole lizards, and cichlid fish exhibit adaptation loss in hybrids. Ipomopsis plants show higher depression from farther spatial crosses. Environment influences outbreeding depression in angiosperms.