Chapter 24: Evolutionary Genetics & Human Origins

The field matured with the rediscovery of Mendel's principles in 1900, which provided the crucial understanding of genetic transmission previously missing from Darwin's work. A central theme is the documentation of genetic variation in natural populations across four levels: phenotypes (polymorphisms like blood types or coat colors), chromosome structure (inversions observed in Drosophila showing temporal shifts), protein structure (detected by gel electrophoresis to measure heterozygosity), and nucleotide sequences (revealing SNPs and variation in coding vs. non-coding regions). The study of molecular evolution utilizes DNA and protein sequences as "documents of evolutionary history" to build phylogenetic trees using principles like parsimony. The concept of the molecular clock suggests amino acid or nucleotide substitutions occur at a relatively constant rate, though the actual rate varies dramatically among genes based on functional constraint—slowly evolving genes like histones are highly constrained, while pseudogenes evolve rapidly. The Neutral Theory of Molecular Evolution explains that most molecular changes are selectively neutral and fixed by random genetic drift at a rate equal to the neutral mutation rate. The chapter then details speciation, defined by the acquisition of reproductive isolation between populations via prezygotic or postzygotic mechanisms. Speciation occurs through allopatric (geographic separation, the prevalent mode) or sympatric (without geographic barriers) processes. Finally, human evolution is examined, noting the close genetic relationship between humans and great apes. Genetic data, particularly from mitochondrial DNA and Y chromosomes, supports the Out-of-Africa hypothesis, estimating the emergence of modern humans 100,000 to 200,000 years ago, based on the coalescent principle.