Which of the following are true about the logic we use to identify alleles under positive selection at a given locus?
a. Alleles under positive selection should occur at relatively high frequency.
b. An allele that occurs at relatively high frequency because of genetic drift is likely to be an "old allele," which means it arose by mutation relatively recently.
c. An allele that occurs at relatively high frequency because of positive selection is likely to be a "young allele," which means it arose by mutation relatively recently.
d. "Old alleles" are likely to be in linkage equilibrium with nearby loci.
e. "Young alleles" are likely to be in linkage disequilibrium with nearby loci.