Evolution does repeat itself after all
(PHYSorg.com Biology)

For every two species of mammal there is one species of cichlid fish, which goes to show that biodiversity is distributed rather unevenly among animals. But why? And to what extent can evolution be predicted? A variety of `internal` as well as ecological factors play a role. One decisive factor could be ecological conditions, i.e. the number of different habitats and the similarity of ecological niches available. That is one reason for why there are so many species in the tropics. The demographic history of a population can also influence biological diversity: Is the level of genetic variation in a population sufficient to allow it to adapt to ecological niches? Did the population have enough time to do so? Quantifying all potential factors that contribute to biological diversity, even for only one group of animals, is not easy, not to mention that comparing mammals with a group of fish would be like comparing apples and oranges.