Alleles are variants of the same gene. Different alleles can be associated with different traits (e.g. eye colour). The frequency of specific alleles in a population at two points in time can be used to determine the rate of genetic drift (Wang et al. 2016). Allele diversity can be used to identify genetic variants of interest for population persistence/fitness and can be sensitive to change on short timescales (Leroy et al. 2018).
In the absence of mutation, selection and migration, changes in allele frequencies reflect genetic drift, and can also be used to calculate Effective population size and rate of genetic drift. Typically based on a set of two samples (e.g. DNA extracted from faeces, hair, blood, animal/plant tissue) from multiple individuals within a population taken at least one or more generations apart (Wang et al. 2016).
As with calculation of Effective population size, estimation requires the collection and analysis of genetic data that requires a high level of technical expertise.
The desired allelic diversity will be species/project specific. Generally, a higher allelic diversity is better.