Eons of Evolutionary History
The same factors that drive insects to branch out into species today have also driven large-scale changes in the distant past. These include mutation, natural selection, migration, isolation and genetic drift, the tendency of some offspring, and their genes, to survive due to random forces not related to adaptation. However, the rise of such new clades, or groups of species, like bees or beetles, requires far longer stretches of evolutionary time [source: UC Berkeley].
Insects first appeared at least 450-500 million years ago. Their ancestors evolved from crustaceans, and the earliest creatures that we'd recognize probably looked like modern silverfish (Lepisma saccharina). A major flowering of species 100-150 million years later produced grasshoppers and cockroaches. Insect groups like flies, wasps and beetles made the scene around 200 million years ago [sources: Misof et al., Yeates].
Each group represents specializations made possible by large spans of time. Over eons, the clades radiated into species adapted to numerous land and freshwater niches [source: Misof et al.]. This process profited not only from time, but also from the survival value of high fertility rates and large broods in many species.