Imagine that scientists are studying a new infectious disease. They study the genomes of virus samples taken from different patients and build an evolutionary tree. The tree shows

that the samples from humans are all closely related to viruses from mice. They are more distantly related to viruses found in possums. However, the human samples do not form a

cluster, or clade. Instead they are spread out over the tree, and some are most closely related to other human viruses, but others are more closely related to mouse viruses than they

are to other human samples.

a. Based on this evidence, what do you think the source of the new disease is? Explain your reasoning.

b. Based on this evidence, how many times do you think the virus jumped to humans from another species? Explain your reasoning.

c. Based on this evidence, do you think the virus is spreading from human to human? Explain your reasoning.