Chapter 29: Evolutionary Genetics

The field integrates molecular data with classical population genetic principles to understand mechanisms of evolution at the genetic level. The chapter begins by reviewing fundamental concepts including allele frequencies, genetic drift, natural selection, and gene flow as primary forces shaping genetic composition in populations. Students learn how these mechanisms operate mathematically through Hardy-Weinberg equilibrium, which provides a null model for detecting when evolutionary forces are actively changing population genetics. The discussion progresses to explain how mutations generate new genetic variation, the raw material upon which selection and drift act. Natural selection is presented through the lens of differential reproductive success, where individuals carrying advantageous alleles leave more offspring, gradually increasing beneficial allele frequencies across generations. The chapter explores different modes of selection including directional, balancing, and disruptive selection, each producing distinct patterns in allele frequency change and population genetic structure. Genetic drift receives particular attention as a non-selective force causing random fluctuations in allele frequencies, particularly pronounced in small populations where chance events significantly impact evolutionary trajectories. Gene flow between populations is examined as a homogenizing force that can counteract local adaptation and maintain genetic similarity across geographic regions. The chapter then addresses molecular evolution, demonstrating how DNA and protein sequence comparisons reveal evolutionary relationships and rates of genetic change. Neutral theory is introduced to explain the high rate of molecular evolution in non-coding sequences relative to coding regions under selection. Students examine phylogenetic reconstruction methods that use genetic data to infer evolutionary histories and divergence times between species. The chapter concludes by connecting microevolutionary processes operating within populations to macroevolutionary patterns observed across species and higher taxonomic groups, demonstrating how small-scale genetic changes accumulate into major evolutionary transitions.