Unrelated species may come to resemble one another over many generations due to

Answer 1

I would say "interbreeding" but different species are not supposed to be able to breed. However, there is a difference of opinion abolut this. See
http://www.newton.dep.anl.gov/askasci/mole00/mole00349.htm

I do not claim to be a biologist; my answer may be wrong.