Magikarp, as any evolutionary biologist will tell you, should no longer exist. Not because they have few living ancestors and most of their relatives died when a meteor hit the planet countless million years ago, but because they are so poorly adapted to their environment.
Although their rough skin would seem to make them a poor choice as food, they are not without nutritional value, and many pokemon have fangs capable of piercing Magikarp skin. Gyarados are powerful, to be sure, but most Magikarp are born of other Magikarp, and Gyarados lay eggs: they do not parent. The ancient ancestors of Magikarp were large and powerful fish capable of competing for food: the modern kind seem to survive only because of large litters, but even this is not a satisfying explanation, for rarely do even two of the thousand eggs (at least among observed Magikarp) hatch and survive to reach adulthood. Typically, the rest are defeated in battle by plankton, or killed by carnivores as food or herbivores as competition. Yet their population is not only large but growing, and the world remains baffled.
Current research is aimed primarily at their splash attack, a pointless ritual and the other mystery of this pokemon. It is believed that they are connected, but only because neither of them are understood at all.