Can a jury convict a defendant based only on circumstancial evidence?

Yes. The law recognizes two kinds of evidence, direct and circumstantial. Direct evidence is testimony by a witness about what he or she personally observed, heard or experienced. Circumstantial evidence is indirect evidence. It allows the jury to infer, from other facts presented and proven, that a particular fact or circumstance exists, even though there has been no direct proof of it. The only limit on inferences drawn from proven facts is that they must be logical and reasonable, and not just speculation. Here's an easy example. Suppose when you wake up tomorrow morning, the sidewalk outside your house is wet. Did it rain? There's no direct evidence of that, since someone could have used a hose on the sidewalk. The law allows, but does not require you to find from the fact of the wet sidewalk that it rained. In deciding whether the wet sidewalk is proof that it rained, a court will tell you to consider all the evidence in light of reason, experience and common sense. When a jury retires to deliberate, it is instructed that there is no legal distinction between direct and circumstantial evidence so far as proof is concerned. It is up to them to determine how much weight to give any particular piece of evidence, whether it be direct or circumstantial.