Building a Reference Map
Determining the Genetic Profile of a Population
For each elephant population, scientists collect 10 to 50 dung samples from individuals in different families, extract the DNA, amplify the sequences for each STR marker using the polymerase chain reaction, and determine allele sizes by gel electrophoresis. By counting the number of samples that have a particular allele, scientists can also determine the frequency of each allele in the population. Repeating the process for every marker and all populations provides a genetic map of allele frequencies for all 16 loci across the elephants' range.
As of May 2016, the genetic map of African elephants contains population profiles and allele frequencies for 81 elephant populations. In general, populations that are geographically closer to one another are more likely to have exchanged genetic information. As a result, they tend to have more similar genetic profiles and allele frequencies than populations that are farther apart. Scientists use this knowledge to build a statistical model that allows them to predict the genetic profiles of populations for which they do not have DNA samples.
The result of this work is a continuous landscape of genetic information, showing which alleles are present in which frequencies across the entire elephant range in Africa. By comparing the pattern of alleles of an ivory sample to this reference map, scientists can assign the origin of that ivory sample to within 300 kilometers of where it came from anywhere in Africa.