User talk:Professor marginalia/scratch4

"The word “race” is rarely used in the modern, nonhuman evolutionary literature because its meaning is so ambiguous. When it is used, it is generally used as a synonym for subspecies (Futuyma, 1986), but this concept also has no precise definition. The traditional meaning of a subspecies is that of a geographically circumscribed, sharply genetically differentiated population (Smith, Chiszar, & Montanucci, 1997). The problem with this definition from an evolutionary genetic perspective is that many traits and their underlying polymorphic genes show different patterns of geographical variation (Futuyma, 1986). As a result, some combination of characters will distinguish virtually every population from all others. There is no clear limit to the number of races that can be recognized under this concept, and indeed this notion of subspecies quickly becomes indistinguishable from that of a local population. One way around this difficulty is to place minimal quantitative thresholds on the amount of genetic differentiation that is required to recognize subspecies (Smith et al., 1997). A major difficulty with this solution is that there is no objective criterion for defining the threshold. A second solution is to allow races or subspecies to be defined only by the geographical patterns found for particular racial traits or characters. A similar problem is faced in defining species. For example, the biological species concept focuses attention on characters related to reproductive incompatibility as those important in defining a species. These reproductive traits have priority in defining a species when in conflict with other traits, such as morphology (Mayr, 1970). Unfortunately, there is no such guidance at the subspecies level, although in practice easily observed morphological traits (the very ones deemed not important under the biological species concept) are used. There is no evolutionary justification for this dominance of easily observed morphological traits; indeed, it merely arises from the sensory constraints of our own species. Therefore, most evolutionary biologists reject the notion that there are special racial traits. Indeed, as discussed later, using morphological traits to define races can be an impediment to understanding the evolutionary significance of morphological variation.

Because of these difficulties, the modern evolutionary perspective of a subspecies is that of a distinct evolutionary lineage within a species (Shaffer and McKnight, 1996). (However, one should note that many current evolutionary biologists completely deny the existence of any meaningful definition of subspecies, as argued originally by Wilson & Brown, 1953.) The Endangered Species Act requires preservation of vertebrate subspecies (Pennock & Dimmick, 1997), and the distinct evolutionary lineage definition has become the de facto definition of a subspecies in much of conservation biology (Amato & Gatesy, 1994; Brownlow, 1996; Legge, Roush, Desalle, Vogler, & May, 1996; Miththapala, Seidensticker, & O'Brien, 1996; Pennock & Dimmick, 1997; Vogler, 1994). This definition requires that a subspecies be genetically differentiated due to barriers to genetic exchange that have persisted for sufficient periods of time to have detectable genetic consequences; that is, the subspecies must have historical continuity in addition to current genetic differentiation. It cannot be emphasized enough that under this perspective genetic differentiation alone is insufficient to define a subspecies. The additional requirement of historical continuity is particularly important because many traits should reflect the common evolutionary history of the subspecies, and therefore in theory there is no need to prioritize the informative traits in defining subspecies. Indeed, the best traits for identifying subspecies are simply those with the best phylogenetic resolution. In this regard, advances in molecular genetics have greatly augmented our ability to resolve genetic variation and provide the best current resolution of recent evolutionary histories (Avise, 1994), thereby allowing the identification of evolutionary lineages in an objective, explicit fashion (Templeton, 1994, 1998c; Templeton, Routman, & Phillips, 1995).

The nonhuman literature therefore uses two definitions of race: (a) races as geographically circumscribed, genetically differentiated populations (the traditional definition) but now coupled with a minimum threshold of genetic differentiation that can be quantified with modern molecular genetic techniques;   and (b) races as distinct evolutionary lineages within a species (the more modern definition). The fundamental question is whether human populations are genetically differentiated from one another in such a fashion as to constitute either sharply genetically differentiated geographical populations or distinct evolutionary lineages of humanity. These questions are answered with molecular genetic data and through the application of the same, explicit criteria used for the analyses of nonhuman organisms. The fundamental question is whether human populations are genetically differentiated from one another in such a fashion as to constitute either sharply genetically differentiated geographical populations or distinct evolutionary lineages of humanity. These questions are answered with molecular genetic data and through the application of the same, explicit criteria used for the analyses of nonhuman organisms."

(In analyzing criteria *a) "The validity of the traditional subspecies definition of human races can now be addressed by examining the quantitative patterns and amount of genetic diversity found within and among human populations. A standard criterion for a subspecies or race in the nonhuman literature under the traditional definition of a subspecies as a geographically circumscribed, sharply differentiated population is to have F st   values of at least 0.25 to 0.30 (Smith et al., 1997). "  (Human F  st   values are about .15.)

(In analyzing criteria *b) "Because the F st   statistic is incapable by itself of discriminating between these two (continuous and recurring gene flow vs populations split and no gene flow) interpretations of genetic differentiation (Templeton, 1998b), most studies that portray human races as isolates or evolutionary lineages use pairwise genetic distance ... If human races can truly be represented as branches on an evolutionary tree, then the resulting genetic distances should satisfy several constraints. For example, under the evolutionary tree model, all non-African human populations “split” from the Africans at the same time, and therefore all genetic distances between African and non-African populations have the same expected value (  Fig. 2.2  a). When genetic distances instead reflect the amount of gene flow, “treeness” constraints are no longer applicable. Because gene flow is commonly restricted by geographical distance (Wright, 1943), gene flow models are expected to yield a strong positive relation between geographical and genetic distance. Statistical procedures exist to quantify the degree of fit of the genetic distance data to treeness (Templeton, 1998a). All human genetic distance data sets that have been tested fail to fit treeness (Bowcock et al., 1991; Cavalli-Sforza et al., 1996; Nei & Roychoudhury, 1974, 1982; Templeton, 1998a). In marked contrast, the genetic distance data fit well to a restricted gene flow model. For example, Cavalli-Sforza et al. (1996) assembled a comprehensive human data set and concluded that 'the isolation-by-distance models hold for long distances as well as for short distances, and for large regions as well as for small and relatively isolated populations' (p. 124)"

(and in closing) "[H]uman populations do not define races under any of the definitions currently applied to nonhuman organisms. The genetic data are consistently and strongly informative about human races. Humans show only modest levels of differentiation among populations when compared to other large-bodied mammals, and this level of differentiation is well below the usual threshold used to identify subspecies (races) in nonhuman species. Hence, human races do not exist under the traditional concept of a subspecies as being a geographically circumscribed population showing sharp genetic differentiation. A more modern definition of race is that of a distinct evolutionary lineage within a species. The genetic evidence strongly rejects the existence of distinct evolutionary lineages within humans. The widespread representation of human “races” as branches on an intraspecific population tree is genetically indefensible and biologically misleading, even when the ancestral node is presented as being at 100,000 years ago. Attempts to salvage the idea of human “races” as evolutionary lineages by invoking greater racial purity in the past followed by admixture events are unsuccessful and are falsified by multilocus comparisons of geographical concordance and by haplotype analyses. Instead, all of the genetic evidence shows that there never was a split or separation of the “races” or between Africans and Eurasians. Recent human evolution has been characterized by both population range expansions and recurrent genetic interchange among populations. There has been no split between any of the major geographical populations of humanity.

Because of the extensive evidence for genetic interchange through population movements and recurrent gene flow going back at least hundreds of thousands of years, there is only one evolutionary lineage of humanity and there are no subspecies or races under either the traditional or phylogenetic definitions. Human evolution and population structure have been and are characterized by many locally differentiated populations coexisting at any given time, but with sufficient genetic contact to make all of humanity a single lineage sharing a common, long-term evolutionary fate. The genetic differences that do exist among human populations are explained primarily by geography under an isolation by distance model, with some extreme differentiation being due to recent founder events and local adaptations."

"The Genetic and Evolutionary Significance of Human Races", Alan Templeton, from Race and Intelligence: Separating Science from Myth, Jefferson Fish editor, 2002.