What is common descent? why is it important to the theory of evolution?

Which of the following factors would not contribute to allopatric speciation? a) the isolated population is exposed to different selection pressures than the ancestral population.b) a population becomes geographically isolated from the parent population. c) gene flow between the two populations is extensive.d) the separated population is small, and genetic drift occurs. e) different mutations begin to distinguish the gene pools of the separated populations.