[edit] Models of human evolution See also: Multiregional hypothesis See also: Recent single origin hypothesis
In a recent article, Leonard Lieberman and Fatimah Jackson have suggested that any new support for a biological concept of race will likely come from another source, namely, the study of human evolution.
They therefore ask what, if any, implications current models of human evolution may have for any biological conception of race.[20]
Today, all humans are classified as belonging to the species Homo sapiens and sub-species Homo sapiens sapiens. However, this is not the first species of hominids: the first species of genus Homo, Homo habilis, evolved in East Africa at least 2 million years ago, and members of this species populated different parts of Africa in a relatively short time. Homo erectus evolved more than 1.8 million years ago, and by 1.5 million years ago had spread throughout Europe and Asia. Virtually all physical anthropologists agree that Homo sapiens evolved out of Homo erectus. Anthropologists have been divided as to whether Homo sapiens evolved as one interconnected species from H. erectus (called the Multiregional Model, or the Regional Continuity Model), or evolved only in East Africa, and then migrated out of Africa and replaced H. erectus populations throughout Europe and Asia (called the Out of Africa Model or the Complete Replacement Model). Anthropologists continue to debate both possibilities, and the evidence is
technically ambiguous as to which model is correct, although most anthropologists currently favor the Out of Africa model.
Lieberman and Jackson have argued that while advocates of both the Multiregional Model and the Out of Africa Model use the word race and make racial assumptions, none define the term.[21] They conclude that
"Each model has implications that both magnify and minimize the differences between races. Yet each model seems to take race and races as a conceptual reality. The net result is that those anthropologists who prefer to view races as a reality are encouraged to do so" and conclude that students of human evolution would be better off avoiding the word race, and instead describe genetic differences in terms of populations and clinal gradations.[22]
[edit] Race as subspecies
Further information: Race (biology), Species, Subspecies, Systematics, Phylogenetics, Cladistics.
With the advent of the modern synthesis in the early 20th century, many biologists sought to use evolutionary models and populations genetics in an attempt to formalise taxonomy. The Biological Species Concept (BSC) is the most widely used system for describing species, this concept defines a species as a group of organisms that interbreed in their natural environment and produce viable offspring.
In practice species are not classified according to the BSC but according to typology by the use of a holotype, due to the difficulty of determining whether all members of a group of organisms do or can in practice potentially interbreed.[23] BSC species are routinely classified on a subspecific level, though this classification is conducted differently for different taxons, for mammals the normal taxonomic unit below the species level is usually the subspecies.[24] More recently the Phylogenetic Species Concept (PSC) has gained a substantial following. The PSC is based on the idea of a least-inclusive taxonomic unit (LITU), in phylogenetic classification no subspecies can exist because they would automatically constitute a LITU (any monophyletic group). Technically species cease to exist as do all hierarchical taxa, a LITU is
effectively defined as any monophyletic taxon, phylogenetics is strongly influenced by cladistics which classifies organisms based on evolution rather than similarities between groups of organisms.[23] In biology the term "race" is very rarely used because it is ambiguous, "'Race' is not being defined or used consistently; its referents are varied and shift depending on context. The term is often used colloquially to refer to a range of human groupings. Religious, cultural, social, national, ethnic, linguistic, genetic,
geographical and anatomical groups have been and sometimes still are called 'races'".[25] Generally when it is used it is synonymous with subspecies.[26][25][27] One of the main obstacles to identifying subspecies is that, while it is a recognised taxonomic term, it has no precise definition.[26]
Species of organisms that are monotypic (i.e. form a single subspecies) display at least one of these properties:
• All members of the species are very similar and cannot be sensibly divided into biologically significant subcategories.
• The individuals vary considerably but the variation is essentially random and largely meaningless so far as genetic transmission of these variations is concerned (many plant species fit into this category, which is why horticulturists interested in preserving, say, a particular flower color avoid propagation from seed, and instead use vegetative methods like propagation from cuttings).
• The variation among individuals is noticeable and follows a pattern, but there are no clear dividing lines among separate groups: they fade imperceptibly into one another. Such clinal variation displays a lack of allopatric partition between groups (i.e. a clearly defined boundary demarcating the subspecies), which is usually required before they are recognised as subspecies.[28]
A polytypic species has two or more subspecies. These are separate populations that are more genetically different from one another and that are more reproductively isolated, gene flow between these populations is much reduced leading to genetic differentiation.
[edit] Morphological subspecies
Traditionally subspecies are seen as geographically isolated and genetically differentiated populations.[26]
Or to put it another way "the designation 'subspecies' is used to indicate an objective degree of microevolutionary divergence"[25] One objection to this idea is that it does not identify any degree of differentiation, therefore any population that is somewhat biologically different could be considered a subspecies, even to the level of a local population. As a result it is necessary to impose a threshold on the level of difference that is required for a population to be designated a subspecies.[26] This effectively means that populations of organisms must have reached a certain measurable level of difference in order to be recognised as subspecies. Dean Amadon proposed in 1949 that subspecies would be defined
according to the seventy-five percent rule which means that 75% of a population must lie outside 99% of the range of other populations for a given defining morphological character or a set of characters. The 75 percent rule still has defenders but other scholars argue that it should be replaced with 90 or 95 percent rule.[29][30][31]
In 1978, Sewall Wright suggested that human populations that have long inhabited separated parts of the world should, in general, be considered to be of different subspecies by the usual criterion that most individuals of such populations can be allocated correctly by inspection. It does not require a trained anthropologist to classify an array of Englishmen, West Africans, and Chinese with 100% accuracy by features, skin color, and type of hair in spite of so much variability within each of these groups that every individual can easily be distinguished from every other. However, it is customary to use the term race rather than subspecies for the major subdivisions of the human species as well as for minor ones.[32]
On the other hand in practice subspecies are often defined by easily observable physical appearance, but there is not necessarily any evolutionary significance to these observed differences, so this form of classification has become less acceptable to evolutionary biologists.[26][25] Likewise this typological approach to "race" is generally regarded as discredited by biologists and anthropologists.
Because of the difficulty in classifying subspecies morphologically, many biologists reject the concept altogether, citing problems such as:[25]
• Visible physical differences do not correlate with one another, leading to the possibility of different classifications for the same individual organisms.[25]
• Parallel evolution can lead to the existence of the appearance of similarities between groups of organisms that are not part of the same species.[25]
• The existence of isolated populations within previously designated subspecies.[25]
• That the criteria for classification are arbitrary.[25]
[edit] Subspecies genetically differentiated populations
Another way to look at differences between populations is to measure genetic differences rather than physical differences, these should be less biased. Genetic differences between populations of organisms can be measured using the fixation index of Sewall Wright, which is often abbreviated to FST. This statistic is used to compare differences between any two given populations and can be used to measure genetic differences between populations for individual genes, or for many genes simultaneously.[33] For example it is often stated that the fixation index for humans is about 0.15. This means that about 85% of the variation measured in the human population is within any population, and about 15% of the variation occurs between populations, or that any two individuals from different populations are almost as likely to be more similar to each other than either is to a member of their own group.[26][25] It is often stated that human genetic variation is low compared to other mammalian species, and it has been claimed that this should be taken as evidence that there is no natural subdivision of the human population.[34][35][36][37][38]
Write himself believed that a value of 0.25 represented great genetic variation and that an FST of 0.15-0.25
represented moderate variation. It should however be noted that about 5% of human variation occurs between populations within continents, and therefor the FST between continental groups of humans (or races) is as low as 0.1 (or possibly lower).[33]
In their 2003 paper "Human Genetic Diversity and the Nonexistence of Biological Races"[39] Jeffrey Long and Rick Kittles give a long critique of the application of FST to human populations. They find that the figure of 85% is misleading because it implies that all human populations contain on average 85% of all genetic diversity. This does not correctly reflect human population history, they claim, because it treats all human groups as independent. A more realistic portrayal of the way human groups are related is to
understand that some human groups are parental to other groups and that these groups represent paraphyletic groups to their descent groups. For example under the recent African origin theory the human population in Africa is paraphyletic to all other human groups because it represents the ancestral group from which all non-African populations derive, but more than that, non-African groups only derive from a small non-representative sample of this African population. This means that all non-African groups are more closely related to each other and to some African groups (probably east Africans) than they are to others, and further that the migration out of Africa represented a genetic bottleneck, with a great deal of the diversity that existed in Africa not being carried out of Africa by the emigrating groups.
This view produces a version of human population movements that do not result in all human populations being independent, but rather produces a series of dilutions of diversity the further from Africa any population lives, each founding event representing a genetic subset of it's parental population. Long and Kittles find that rather than 85% of human genetic diversity existing in all human populations, about 100% of human diversity exists in a single African population, whereas only about 70% of human genetic diversity exists in a population derived from New Guinea. Long and Kittles make the observation that this still produces a global human population that is genetically homogeneous compared to other mammalian populations.
Wright's F statistics are not used to determine whether a group can be described as a subspecies or not, though the statistic is used to measure the degree of differentiation between populations, the degree of genetic differentiation is not a marker of subspecies status.[33] Generally taxonomists prefer to use phylogenetic analysis to determine whether a population can be considered a subspecies. Phylogenetic analysis relies on the concept of derived characteristics that are not shared between groups, this means that these populations are usually allopatric and therefore discretely bounded, this makes subspecies, evolutionarily speaking, monophyletic groups.[26] The clinality of human genetic variation in general rules out any idea that human population groups can be considered monophyletic as there appears to always have been a great deal of gene flow between human populations.[26]
[edit] Population genetics: population and cline
At the beginning of the 20th century, anthropologists questioned, and eventually abandoned, the claim that biologically distinct races are isomorphic with distinct linguistic, cultural, and social groups. Shortly thereafter, the rise of population genetics provided scientists with a new understanding of the sources of phenotypic variation. This new science has led many mainstream evolutionary scientists in anthropology and biology to question the very validity of race as a scientific concept describing an objectively real phenomenon. Those who came to reject the validity of the concept of race did so for four reasons:
empirical, definitional, the availability of alternative concepts, and ethical (Lieberman and Byrne 1993).
The first to challenge the concept of race on empirical grounds were anthropologists Franz Boas, who demonstrated phenotypic plasticity due to environmental factors (Boas 1912), and Ashley Montagu (1941, 1942), who relied on evidence from genetics. Zoologists Edward O. Wilson and W. Brown then challenged the concept from the perspective of general animal systematics, and further rejected the claim that "races" were equivalent to "subspecies" (Wilson and Brown 1953).
[edit] Clines
One of the crucial innovations in reconceptualizing genotypic and phenotypic variation was
anthropologist C. Loring Brace's observation that such variations, insofar as it is affected by natural selection, migration, or genetic drift, are distributed along geographic gradations or clines (Brace 1964).
This point called attention to a problem common to phenotype-based descriptions of races (for example, those based on hair texture and skin color): they ignore a host of other similarities and differences (for example, blood type) that do not correlate highly with the markers for race. Thus, anthropologist Frank Livingstone's conclusion that, since clines cross racial boundaries, "there are no races, only clines"
(Livingstone 1962: 279).
In a response to Livingston, Theodore Dobzhansky argued that when talking about "race" one must be attentive to how the term is being used: "I agree with Dr. Livingston that if races have to be 'discrete units,' then there are no races, and if 'race' is used as an 'explanation' of the human variability, rather than vice versa, then the explanation is invalid." He further argued that one could use the term race if one distinguished between "race differences" and "the race concept." The former refers to any distinction in gene frequencies between populations; the latter is "a matter of judgment." He further observed that even when there is clinal variation, "Race differences are objectively ascertainable biological phenomena ....
but it does not follow that racially distinct populations must be given racial (or subspecific) labels."[40] In short, Livingston and Dobzhansky agree that there are genetic differences among human beings; they also agree that the use of the race concept to classify people, and how the race concept is used, is a matter of social convention. They differ on whether the race concept remains a meaningful and useful social convention.
In 1964, biologists Paul Ehrlich and Holm pointed out cases where two or more clines are distributed discordantly—for example, melanin is distributed in a decreasing pattern from the equator north and south; frequencies for the haplotype for beta-S hemoglobin, on the other hand, radiate out of specific geographical points in Africa (Ehrlich and Holm 1964). As anthropologists Leonard Lieberman and Fatimah Linda Jackson observe, "Discordant patterns of heterogeneity falsify any description of a population as if it were genotypically or even phenotypically homogeneous" (Lieverman and Jackson 1995).
Patterns such as those seen in human physical and genetic variation as described above, have led to the consequence that the number and geographic location of any described races is highly dependent on the importance attributed to, and quantity of, the traits considered. For example if only skin colour and a "two race" system of classification were used, then one might classify Indigenous Australians in the same
"race" as Black people, and Caucasians in the same "race" as East Asian people, but biologists and
anthropologists would dispute that these classifications have any scientific validity. On the other hand the greater the number of traits (or alleles) considered, the more subdivisions of humanity are detected, due to the fact that traits and gene frequencies do not always correspond to the same geographical location, or as Ossario and Duster (2005) put it:
Anthropologists long ago discovered that humans' physical traits vary gradually, with groups that are close geographic neighbors being more similar than groups that are geographically separated. This pattern of variation, known as clinal variation, is also observed for many alleles that vary from one human group to another. Another observation is that traits or alleles that vary from one group to another do not vary at the same rate. This pattern is referred to as nonconcordant variation. Because the variation of physical traits is clinal and nonconcordant,
anthropologists of the late 19th and early 20th centuries discovered that the more traits and the more human groups they measured, the fewer discrete differences they observed among races and the more categories they had to create to classify human beings. The number of races observed expanded to the 30s and 50s, and eventually
anthropologists concluded that there were no discrete races (Marks, 2002). Twentieth and 21st century biomedical researchers have discovered this same feature when evaluating human variation at the level of alleles and allele frequencies. Nature has not created four or five distinct, nonoverlapping genetic groups of people.[41]
[edit] Populations
Population geneticists have debated as to whether the concept of population can provide a basis for a new conception of race. In order to do this a working definition of population must be found. Surprisingly there is no generally accepted concept of population that biologists use. It has been pointed out that the concept of population is central to ecology, evolutionary biology and conservation biology, but also that most definitions of population rely on qualitative descriptions such as "a group of organisms of the same species occupying a particular space at a particular time"[42] Waples and Gaggiotti identify two broad types of definitions for populations, those that fall into an ecological paradigm and those that fall into an evolutionary paradigm. Examples such definitions are:
• Ecological paradigm: A group of individuals of the same species that co-occur in space and time and have an opportunity to interact with each other.
• Evolutionary paradigm: A group of individuals of the same species living in close enough proximity that any member of the group can potentially mate with any other member.[42]
Richard Lewontin, claiming that 85 percent of human variation occurs within populations, and not among populations, argued that neither "race" nor "subspecies" were appropriate or useful ways to describe populations (Lewontin 1973). Nevertheless, barriers—which may be cultural or physical— between populations can limit gene flow and increase genetic differences. Recent work by population geneticists conducting research in Europe suggests that ethnic identity can be a barrier to gene flow.[43][44][45][46] Others, such as Ernst Mayr, have argued for a notion of "geographic race" [4]. Some researchers report the
variation between racial groups (measured by Sewall Wright's population structure statistic FST) accounts for as little as 5% of human genetic variation². Sewall Wright himself commented that if differences this
variation between racial groups (measured by Sewall Wright's population structure statistic FST) accounts for as little as 5% of human genetic variation². Sewall Wright himself commented that if differences this