Survival of the fittest? That is a catch phrase I have heard a million times. According to the Darwinian theory, traits that are successful in a given environment are passed on genetically to subsequent generations.
However, every environment has a variety of species. Often times there is at least one species that resembles the common ancestor, perhaps an intermediate species, and an end species.
So my question is this: Why did the common ancestor survive along with the end species? If it is an advantage for reptiles to evolve into birds and mamals, why are there any reptiles left? Obviously a bird/reptile was not more suited to the environment than just a plain old reptile, or the bird/reptile would have survived and the reptile would be extinct--and given enough time, all species would resemble the end species: birds and mamals--even humans--and the common ancestors would fall to the way side. Yet, the single cells we evolved from are still around and living in our environment, and they are well suited, perhaps better suited. So how does the Darwinian theory account for this? Or is there a more modern theory that accounts for it?