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[QUOTE]Originally posted by zarahan- aka Enrique Cardova: [QB] [b]Most of the genetic change in human populations is not from natural selection but genetic drift. [/b] [i]"It may appear counterintuitive, but a large part, if not the majority, of genetic change in human populations is not thought to be due to natural selection but rather due to the play of chance (genetic drift; Harris and Meyer, 2006; Li et al., 2008; see Table 2 for a glossary of terms frequently used in population genetics). Many opportunities for chance can occur in the transmission of alleles from parents to offspring, and evidently did occur as part of the demographic process of dispersal out of Africa. Thus, finding differences in the frequency of alleles at a particular locus between populations is not an evidence of natural selection per se. The default position is that of neutral theory, whereby chance events account for most patterns of genetic diversity (Harris and Meyer, 2006). Of course, deleterious mutations will be selected against (purifying selection) and beneficial mutations may increase in frequency to fixation, but overall these events will contribute little to explaining the presence of most polymorphisms." [/i] --J. Rees and R. harding 2011. Understanding the Evolution of Human Pigmentation: Recent Contributions from Population Genetics. Journal of Investigative Dermatology. 358 [IMG]http://ww1.prweb.com/prfiles/2011/05/17/8433243/alondra.jpg[/IMG] [b]NATURAL SELECTION LIMITATIONS[/b] [i]"How do we know for certain whether a trait evolved through natural selection or instead, through a neutral evolutionary mechanism of genetic drift? This question (as well as any answers we may offer) is fundamental to understanding the adaptive significance, if any, of population-based (or race-based) differences in functional traits. Many experts maintain that although natural selection plays a critical role in the evolutionary origin of many traits, it is not the driving force behind all biological phenomena. In fact, according to some evolutionary biologists who conduct empirical field research, genetic drift is typically assumed by default to account for most traits. Proving that natural selection is involved in the origin of a particular trait is a complicated process. Given the complexity of natural selection, it is not surprising that biologists cannot ascertain if there are long term differences in traits that have evolved through natural selection versus those that emerged through neutral selection. 3 There are other enigmas that must be sorted out as well if we are to identify the features that distinguish natural selection from neutral selection. For instance, genetic drift tends to be more influential in small populations while natural selection is more powerful in large populations. The microevolution of human races that occurred over the past 15,000 to 30,000 years affected smaller human populations. At the same time, however, natural selection had a momentous impact on the evolution of certain anatomical and physiological traits in larger geographical populations. Both genetic drift and selection could have operated in tandem to initiate the emergence of different traits in the same populations, or in clusters of geographical populations. Positive natural selection increases fitness, which is measured in terms of survival and reproduction. However, natural selection may act on different levels of biological organization, even simultaneously at times. Classic bioevolutionary studies emphasize the influence of natural selection on individual organisms, populations, and even species. Yet, selection can also act at the level of the genome, chromosomes, and genes (DNA sequences). "[/i] The Adaptionist Yardstick: Rethinking the Social Implications of Sarich’s and Miele’s Fast-Track Micro-Evolution A REVIEW BY ALONDRA OUBRÉ, Ph.d. Medical Anthropologist [/QB][/QUOTE]
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