This chapter begins by stating the core principle for any genetic analysis, the Hardy-Weinberg equilibrium. Any deviation from the model implicates that one or more evolutionary mechanisms are acting in the populations. We start with the first, random genetic drift. Genetic drift is the random fluctuation in allele frequencies due to sampling error in finite populations. All populations are finite; therefore genetic drift occur in every population continuously. However, the magnitude and speed of allele frequency changes throughout the genome over time are influenced by population size. Effective population size refers to the theoretical size of an idealized population that experiences the same amount of genetic drift as the actual population under consideration. Small or isolated populations are susceptible to genetic drift, and as a result, alleles may become lost over time, especially rare ones, increasing the genetic differentiation between populations. The neotropical region contains many endemic species that are found in specific habitats or geographic locations. Therefore, genetic drift can be particularly important due to the high levels of biological diversity and regional isolation of these populations. Understanding genetic drift and effective population size has important implications for conservation efforts, as loss of genetic diversity can reduce the adaptive potential of a population, making it less able to respond to environmental change.