Adaptation is considered a characteristic that increases the rate of survival or reproduction of an organism. These traits or characteristics could be examined as part of natural selection or as a characteristic best suited to survive during evolution. An example of adaptation could be seen in rainbow trout. They are known to use hormonal regulation, migration and specific mechanisms to help them survive. Such characteristics are known to increase their ability to survive and adapt while migrating back and forth under different environmental conditions. This article will review some of these mechanisms/methods. Rainbow trout are also known as steelhead trout and have been classified as a member of the Oncorhynchus genus (Oncorhynchus mykiss). This genus of species is known for migrating to the sea as a juvenile and returning to freshwater as an adult when ready to spawn. Their ability to survive has allowed biologists to observe, examine, and explain the function of these physiological structures and their role in adaptation, but first their phylogeny will be examined. Rainbow trout are known to not die after spawning and to survive for up to 8 years – compared to Pacific salmon and other Atlantic salmon. However, many biologists wanted to know how they managed to survive and what kind of species they were closely related to. After several sequencing and genetic studies, it was determined that they came from Pacific salmon. Based on the results, it was discovered that they had similar genes for growth hormone. The two types of growth hormone genes were isolated and sequenced from sockeye salmon (Oncorhynchus nerka). The genes encoded proteins of 210 amino acids and showed a remarkable resemblance to the ocean. This breeding process is considered a very vital stage of salmon development and it is from here that it can be determined whether they will survive. However, larger trout are likely to show aggression towards younger or smaller trout (competition/predation). Rainbow trout are considered notable species. Evidence of their ability to adapt and use their characteristics has evidently been beneficial, increasing their ability to survive during development and migration. It is clear that they are unable to reproduce without laying eggs, and without migration they cannot lay eggs. This concludes that as a result of evolution, their environmental instincts and physical maturation, they were able to adapt. Regardless of natural selection and predators, it is clear that thanks to their characteristics they have managed to survive longer.