Cranio-facial variation is perhaps the most overrated phenotypic aspect of human body in societies, aside from skin tone. Not surprising, considering that it is the most regularly exposed part of the body that noticeably sports considerable variation, and goes without saying, forms a biological basis around which an individual's unique identity is established. Socio-ethnic identification is secondary to individual identity in society. Such is the obviousness of cranio-facial variation that images of certain archetypes or "idealized" types have been implanted or socially conditioned in the minds of folks, as socio-ethnic identifiers; this took a particularly notorious turn from ca. 19th century within European bio-anthropological circles, wherein human populations were grouped into narrow rigid "idealized" types, which were usually also identified with major geographical locations. These were presented as non-overlapping types, as though the so-grouped human populations didn't sport intra-group and intra-population variation, and by extension, variation manifestation that is shared with groups outside a reference point group. Howells types rely on such typological groups, and which Forensic science has embraced via Fordisc 2.0. Recent studies have shown that such approach is considerably limited in its ability to account for intra-group variation, and hence, fails the test of classifying individuals of a given population to the correct population of origin, because an 'unknown' or a 'known' specimen may well cluster with individuals in not one but multiple populations. A study on Spanish cranial series ended up having the said specimens cluster all over the map, with individuals in the series clustering with a multitude of groups spanning continents.
"Variation in racial classification represents the lack of a Spanish sample within the FORDISC 2.0 database as well as the human variation inherent within them. Individual crania were classified according to the best fit with the existing samples of the database, but the samples clearly were inadequate to elucidate the specific geographical origin of the overall Spanish sample…some crania were classified into groups with no clear geographic or ancestral relationship with the Spanish sample…
The authors also agree that additional and more complete samples from different geographical regions and groups are needed to augment the existing databases."
"race classification of all individuals in this sample using the Forensic Data Bank option. Of the 95 individuals, 42 (44 percent) were classified as white, 35 percent as black, 9 percent as Hispanic, 4 percent as Japanese, 4 percent as American Indian, and the remaining three individuals as Chinese and Vietnamese" - Ubelaker et al., Application of Forensic Discriminant Functions to a Spanish Cranial Sample, 2002.
Williams et al. 2005 provided another example of this, when their examination of Meroitic cranial series showed that the series couldn't be classified into a single homogeneous entity, but rather, produced clusters with multiple series from distinct geographical regions. They say:
The Howells series. Fordisc 2.0 could not effectively classify ten of the crania, and of the remainder, eight were identified as Late Period Dynastic Egyptian, six as Zalavar, four as Easter Islander, three as Lake Alexandrina Tribes, and three as Norse (Medieval Norway). Eight were not significantly different from eight separate populations: Teita, Andaman Islands, Zulu, Arikara, Santa Cruz Island, Ainu, Hokkaido, and Atayal.
"The Howells series. Fordisc 2.0 could not effectively classify ten of the crania, and of the remainder, eight were identified as Late Period Dynastic Egyptian, six as Zalavar, four as Easter Islander, three as Lake Alexandrina Tribes, and three as Norse (Medieval Norway). Eight were not significantly different from eight separate populations: Teita, Andaman Islands, Zulu, Arikara, Santa Cruz Island, Ainu, Hokkaido, and Atayal."
“Fordisc 2.0 classified the Nubian crania with populations over an enormous geopraphic range, including North and Central Europe, Easter Island, the Andaman Islands, Japan, Taiwan, South Africa, Australia, and North America. “
“If Fordisc 2.0 is revealing genetic admixture of Late Period Dynastic Egypt and Meroitic Nubia, then one must also consider these ancient Meroitic Nubians to be part of Hungarian, part Easter Islander, part Norse, and part Australian Aborigine, with smaller contributions from the Ainu, Teita, Zulu, Santa Cruz, Andaman Islands, Arikara, Ayatal, and Hokkaido populations. In fact, all human groups are essentially heterogeneous, including samples within Fordisc 2.0. Using Fst heritability tests, Relethford (1994) demonstrated that Howells’s cranial samples exhibit far more variation within than between skeletal series. There is no reason to assume that the heterogeneity of the Late Period Dynastic Egyptian population exceeds that characterizing our Nubian sample. This heterogeneity may also characterize the populations in the Forensic Data Bank; Fordisc 2.0 classified the Meroitic Nubians not as either all black or all white but as black, white, Hispanic, Chinese, Japanese, and Native American.”
“We suggest that skeletal specimens or samples cannot be accurately classified by geography or by racial affinity because of (1) the wide variation in crania of the known series that crosscuts geographic populations (polymorphism), (2) the clinal pattern of human variation, and (3) cultural and environmental factors. Even a presumably homogeneous population such as the Meroitic Nubians shows extensive variation that preclude its classification as a geographic group.”
Apparently multiple variables go into shaping variations in human crano-facial development. Williams et al. put this simply, when they say:
“Finally, the assumption that cranial form is an immutable “racial” character is very likely to be false, given the diversity of studies of immigrants and the known effects of food preparation and masticatory stress upon cranial form. Cranial form, like other aspects of the body, is a phenotype partly determined by heredity but also strongly influenced by the conditions of life.”
As far as the "conditions of life" is concerned, "acclimatization" is one notable factor. Michael A. Little and Jere D. Haas summarize it, when they say:
“there is a recognition of the importance of the process of acclimatization. Acclimatization is defined as a biological response to repeated exposure to a climatic stressor (Prosser 1964). It is an expression of the genetic plasticity of the population, and with acclimatization, it is assumed that an adaptive response can occur without genetic change. Three forms of physiological acclimatization have been recognized: (1) simple, reversible acclimatization-a physiological response to a stressful environment that gradually disappears at the cessation of the stress; (2)irreversible acclimatization—acquired as a result of climatic stress but remaining after the stress is removed; and (3) developmental acclimatization— much like irreversible acclimatization except that the exposure must occur at a particular time during the growth process. Until the process of acclimatization was appreciated and its varieties identified much inter-operation variation was attributed to genetic difference. Attempting to define the limits of acclimatization has become as important as defining the underlying genetic basis of adaptation. Indeed it is only after acclimatization is accounted for that genetic adaptations can be identified.
A third manner by which contemporary studies of climatic adaptation differ from earlier efforts is through an appreciation of the role of culture (Baker 1960). Anthropologists since the time of Darwin have played lip service to cultural factors using such generalities as “sexual selection” and “cultural selection.” But it is only with recent empirical studies that cultural mechanisms are being identified and quantified. These have shown that culture is a buffer that modifies rather than eliminates climatic exposure (Wulson 1949, Planalp 1971, Little and Hanna 1978).” - Courtesy of Michael A. Little and Jere D. Haas—Human Population Biology: A Transdisciplinary Science, 1989.
From the genetic standpoint, it is generally known that when only a small segment of a larger population diverges and then locates elsewhere to assume the role as a founder group, the likelihood of loss of diversity is a strong possibility that goes along with it. Hence, pronounced reduction of diversity characterizing the newly-divergent offshoot group allows the distribution of certain traits to figure more prominently than the case would be in the parent population, and alternatively, other traits die out more dramatically. The result of such development, has been invoked in marked departure of offshoot founder groups from their ancestral population. As Little & Haas note, cultural behavior patterns chime in to "modify" these variations. Hence, putting acclimatization effects aside [not to leave out natural selection in the complex mix of factors], on one hand, certain variations brought upon by genetic mutation can be quite dramatic secondary to random genetic drift, wherein certain variations are magnified while others not so much; on another hand, cultural behavior patterns, which can be exemplified in "sexual selection" tendencies, factor in and contribute further to the sustenance and prevalence of certain elements of the overall variation over other elements, and hence, lending hand in certain intra-population morphological tendencies.
*Subject to modification upon new information without notice.
—Ubelaker et al., Application of Forensic Discriminant Functions to a Spanish Cranial Sample, 2002
—Williams et al. 2005, Forensic Misclassification of Ancient Nubian Crania: Implications for Assumptions about Human Variation.
—Michael A. Little and Jere D. Haas—Human Population Biology: A Transdisciplinary Science, 1989.
—Discussion link: Nile Valley discussion board.