Neutral and Adaptive Markers

Neutral markers are genetic variations that do not confer any selective advantage or disadvantage to an organism. Instead, they evolve primarily under the influence of genetic drift and mutation. These markers are often used to study demographic processes, population history, and gene flow. For example, microsatellites, also known as simple sequence repeats (SSRs), are highly polymorphic neutral markers commonly used in molecular ecology studies. They consist of short tandem repeats of DNA sequences, such as CA or AT, repeated multiple times. Microsatellites evolve rapidly due to high mutation rates in the repeat regions, and mutations in these regions are generally neutral in terms of fitness.

Researchers studying the population genetics of a species of bird might use microsatellites to assess genetic diversity and population structure among different breeding populations. By genotyping individuals from multiple populations at several microsatellite loci, they can estimate gene flow, genetic differentiation, and demographic history without being biased by selection.

Adaptive markers, onthe other hand, are genetic variations that are directly or indirectly associated with fitness-related traits and are subject to natural selection. These markers often show signatures of positive selection or balancing selection, reflecting their role in adaptation to different environmental conditions. For example, single nucleotide polymorphisms (SNPs) are the most common type of genetic variation in the genome and can be both neutral and adaptive. In the case of high-altitude adaptation in humans, certain SNPs have been identified as adaptive markers associated with the physiological response to hypoxia (low oxygen levels) at high altitudes. For instance, researchers have found that populations native to high-altitude regions, such as the Tibetan Plateau, have higher frequencies of specific SNPs in genes related to oxygen transport and regulation, such as EPAS1 and EGLN1. These SNPs are thought to confer adaptive advantages, such as increased hemoglobin levels or improved efficiency of oxygen utilization, allowing individuals to thrive in low-oxygen environments. By comparing the frequency of these adaptive SNPs between high-altitude and low-altitude populations, researchers can infer the role of natural selection in shaping genetic variation associated with altitude adaptation.