Evolutions and species adaptions?
Genetic similarities are a direct indicator of how closely two species are related. As an example, something like 90% of all genes are inactive (they have no effect on the organism). Through random mutations over time the inactive genes will drift away from each other in two species that shared a common ancestor. The rate of drift is fairly constant and can be used to estimate how many generations back two species are related. Physical similarities are no guaruntee of relation. There can be cases of parallel evolution, or convergent evolution, where two unrelated species can have similar body plans because they can both fit an ecological niche. An example of this is the tasmanian wolf (now extinct), which despite looking and acting like a wolf, was actually a marsupial more closely related to the kangaroo. http://images.google.com/images?hl=en&q=tasmanian+wolf&gbv=2