Beta-Rays are a method of cloning Metroids. The Space Pirates have used this method on several Metroid specimens they have stolen from the Galactic Federation, including the samples recovered by the Space Research Vessel Marina, and the baby. Despite this, the Super Metroid PAL guide's backstory implies that the Federation cloned Metroids using Beta-Rays first, and then the Pirates used the same method.
Normally, Metroids reproduce via a Queen, but if they are exposed to extremely high Beta-Rays, Metroids are capable of asexual reproduction via division, in a process much like a cell undergoing mitosis. This method of Metroid cloning seems to be restricted to the Infant and adult stages of a Metroid; whether Metroids in the later stages of their natural life cycle can reproduce in the same manner is unknown.
Continued exposure to Beta-Rays has been known to have averse effects on Metroids. The baby's growth through the natural life cycle became stunted, and it increased in size rather than advancing to the Alpha stage. During Super Metroid, the Pirates attempted to clone Metroids using other methods, but they failed, and the result of those experiments, Mochtroids, were left to rot in Maridia.