Volume 38 - Issue 3 - 193 - 220

Genetic Footprints of Natural Selection and Drift in Human Evolution

Humans are genetically little diverged from their closest living relatives, chimpanzees. However, human anatomy, physiology and behavior show substantial divergence from those of chimpanzees and other primates. The evolutionary processes that generated this phenotypic divergence are still debated. Given the incomplete state of the hominid fossil record, genetic information is indispensible for studying human evolution. Recently, data from genome sequencing projects, gene expression profiling, and large-scale genotyping across multiple populations, has significantly improved our understanding of human origins, as well as the forces of selection, adaptation, and demographic forces that shaped human evolution. Recent multispecies comparative genomic studies indicate that random genetic drift, i.e. neutral evolution, is a leading evolutionary mechanism in shaping human protein coding sequences, as well as gene expression patterns. Still, there is also compelling evidence that a large number of human genes were affected by adaptive evolution. Genes involved in immunity, sensory perception, reproduction, and apoptosis appear among the most frequently positively selected classes. Likewise, comparative transcriptome studies indicate adaptive expression changes in human brain gene expression. Importantly, variants of both positively and negatively selected genes are frequently found to be responsible for human genetic diseases. Comparison of genetic diversity within and between human populations suggests that humans have been a relatively small, homogeneous species. Accordingly, patterns of genetic diversity are to large extent shaped by neutral, demographic processes. Meanwhile, genome scans and functional studies identify pathogens, diet, and environmental conditions as major selective forces driving genetic diversity in humans. These results portray human evolution as a complex process, simultaneously affected by forces of selection and drift. 

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