This book describes and analyzes genetic and environmental factors that cause variation in individuals and populations. Data will be used to evaluate the processes by which variation is generated in organisms and how variation affects natural selection. Genetic factors include mutation, independent assortment, crossing over, and recombination. Environmental factors include gradients and differences in abiotic conditions. Genotype frequencies can be used to determine allele frequencies and this information can be used to determine whether a population is evolving at a genetic locus. The Hardy-Weinberg equilibrium will be applied as a null model to make this determination. Non-Mendelian genetics can affect the evolution of viruses and reassortment in viruses will be used to illustrate another mechanism that generates variation in organisms and how this mechanism relates to rapid evolution of viruses and the need for annual flu vaccines.