Clearly, you're going at it from an anti-evolutionary standpoint. Here's how it works though.
Let's take the shark for example, aside from Megalodon disappearing, sharks in various forms have been around for over 400 million years. Some species died out while others carried on. What a creature does is....over time (hint: a very, very long time) change to adapt to its environment until its adaption fails and it dies out, or...it reaches a point at which what it has adapted to, is perfect for its environment and survival. In the case of the shark, it adapted until it reached its peak performance and thus ensured its survivability. Once that happens, it does not need to change.
If you would like a recent example of an evolutionary change in a short time, let us consider the "Eastern Fence-Lizard." It's a typical medium sized lizard found in the Eastern United States. It primarily sits on logs and rocks, sunning itself and not moving much, unless startled. Sometime between 1933 and 1945, Red Fire Ants were accidentally brought in through Mobile, Alabama via cargo ship. As the red ants spread throughout the southeast, they eventually encountered the Eastern Fence Lizard. Now, the Red Fire Ant will devour any critter it can, including lizards. The lizard mentioned would largely ignore the fire ants as they crawled over their bodies, until...the ants would burrow under their scales and begin eating them, once that began, the lizard would begin shaking and jumping, something it was not in the habit of doing. The young lizards were easily overcome and eaten alive, but those lizards that began shaking and jumping earlier, and retreated, survived. Over a short period of time, the offspring of those lizards that survived, were found to have been developing larger, longer rear legs, as a result of their new behavior. In short, their bodies were forced to change to adapt to the new threat. That is what is called, evolution. Evolutionary change to adapt in order to survive.
A more antiquated but good example of evolution in progress is the horse. Over time, it grew in size and its feet changed to hooves, we have ample evidence of the changes of their feet to hooves.