Scientists learn about the layers deep within the Earth's crust by studying how seismic waves travel through the Earth. If there is an earthquake somewhere, seismagraphs in many locations will record it.
Answer: the relative differences in arrival times of the S and P seismic waves at several recording stations tell scientists about the different speeds those waves were traveling at, which in turn gives information about the density of the material the layer is made of, and how thick the layer is in several directions.
Genetic drift is changes in allele frequencies of a gene pool due to chance or random events. This can occur in large or small populations. Genetic drift causes gene pools of two isolated populations to become dissimilar as some alleles are lost and other are fixed.
