The concept of transitional species is an important and complex notion in evolutionary biology. To begin with, there are no transition species since all living beings have always evolved in the past, without stopping at one stage or another, and will continue to evolve in the future. In terms of evolutionary biology, we use the concept of transitional species as a way to mitigate ambiguity. Much like using the Linnaean species taxonomic system, we develop concepts such as transitional species to organize and classify species in order to understand their evolutionary roots and how those species have changed throughout the history of life to become what they are today . “In the same way that the species concept may be provisionally meaningful for describing organisms at a single moment in time, the transitional species concept may be provisionally meaningful for describing organisms over a period of time, usually quite long, such as example hundreds of thousands or millions of years” (111). While it can be difficult to distinguish what may be considered one ancestral species from another, the fossil record can show us how species change over time as they develop ways to adapt to the stresses present in their environments. “In the modern sense, organisms or fossils that show intermediate stages between the ancestral and the present are called transitional species” (222). The concept of transitional species is, in essence, quite simple. This article will outline the concept of transitional (or sometimes called intermediate) species and their role in evolutionary biology, as well as delve into several common transitional species: Tiktaalik, an animal at the cusp of life in the water and… .... middle of paper ...... as stated previously, Archeopteryx's ability to fly is unclear. Several models have been proposed to explain why Archeopteryx or its descendants might develop the ability to fly. The “pouncing proavis” or “trees-down” model was proposed by J.P. Garner and colleagues in 1999. They theorize that birds evolved the ability to fly by first living in trees and then gliding down to gain a foothold. ambush on prey. Natural selection favored individuals who could glide farthest to catch prey and ultimately led to the origin of flight. Garner and colleagues (1999) believed that this theory explained three aspects of early flight: the model corresponds to the secession observed in the evolution of flight based on fossil records, predicts that a primitive bird-like animal had few adaptations to flapping but very complex aerodynamic feathers, and explains the origin of the rachis in the feathers.