Randomness in Speciation and Darwin's Theory of Evolution

Since school days, it has been taught to us that natural selection is the primary reason for the origin of species. Only 'fit' individuals survive in a particular environment. That is, it means that that there are certain characteristics needed for survival in a particular environment. So, if natural selection had played its role well, there can be only one species in a given environment - one species of bacteria, one species of a plant, one species of a spider, one species of a mammal etc. But this doesn't seem to be the case. Often several species cohabit an environment. So, it becomes obvious that natural selection cannot explain the 'origin' of species. This has been a contentious issue among evolutionary biologists since long and a recent study probably has revealed the underlying 'fact'. See the nature article. Or, read this New Scientist article which explains the issue in layman terms. According to this study, the origin of species is a random event. A rare event triggers change and a group of individuals in a particular species become reproductively isolated. These isolated group of individuals then gradually change, adapt by natural selection and form the new species.

I think this completes the theory of evolution. A rare event triggers the formation of a new species. This species then adapts to the environment and forms a new species. It seems like randomness is an important aspect of life! Nothing can be predetermined as some claim.