No reason? No reason? So you are implying evolution is intellegently directed.
No reason = no pressure on it in the form of scarce resources, competition for resources, predators, or environmental changes that influence its development. Meanwhile, mutations that have also found their own niches have evolved into present day forms of sharks, eels, etc.
No. Absent any environmental factor pushing evolution in any direction, each generation of an organism will have roughly the same allele frequencies as its parent. Obviously there will be some genetic drift over time, but if the organism has a successful niche, then there's no ecological advantage to evolving.