How does meiosis increase genetic variability

What is thought to have caused the mass extinction at the end of the cretaceous period?

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

The backbone of the double helix has an alternating molecules. what do you call these alternating molecule?