Because viruses are rapidly replicated, it is possible to observe hundreds of generations in a single host (infected organism). Some viruses (such as influenza and HIV) evolve rapidly, and others (such as rabies virus and poliovirus) are relatively stable. Two of the selective forces that influence the evolution of viruses are the immune system, which works to destroy the virus, and the behavior of the host in assisting transmission of the virus. How could the observation that some viruses evolve slowly and others rapidly be interpreted in terms of these selective forces?
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DNA evidence shows that the closest living relatives of elephants are manatees, aquatic mammals found in the Atlantic Ocean. Two possibilities exist: manatees evolved from elephants (or their immediate ancestors), or elephants evolved from manatees (or their immediate ancestors). A study of embryonic and adult anatomy of manatees and elephants might reveal structures that would help to more firmly establish the evolutionary relationship between these two animals. Hypothesize what kind of structures these might be.
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