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Chapter 4

Textbook Chapter 4 Notes

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Heather Miller

Notes From Reading CHAPTER 4:M ODERN H UMAN VARIATION AND A DAPTATION (PGS. 69-94) Learning Objectives  Describe how the history of race studies, particularly in Europe and the United States, has influenced current popular views of human variation  Explain why anthropologists no longer consider these historical approaches as biologically useful  Compare and contrast the historical views with a modern biological perspective based on molecular biology and population genetics  Explain why recent population history of the sickle-cell trait as well as lactase persistence are good examples of biocultural evolution Introduction  Human diversity have played an enormous role in human relations for at least a few thousand years o Still influencing political and social perceptions  The record of genocidal and ethnic cleansing atrocities tell us that, worldwide, we have a long way to go before tolerance becomes the norm Historical Views of Human Variation  1350BC. The ancient Egyptians had classified humans based on their skin color o Red: Egyptians, yellow: people to the east, white: people in the north, and black: sub-Saharan Africans  After the discovery of the New World (16 century), several European countries embarked on a period of intense exploration and colonization o There was an increased awareness of human diversity  In the 18 and 19 centuries, scientists concentrated primarily on describing and classifying biological variation in humans as well as in nonhuman species  The first scientific attempt to describe variation among human populations was Linnaeus’ taxonomic classification (playing humans in 4 separate categories)  German anatomist, Johann Friedrich Blumenbach, classified humans into five races (white, yellow, red, black and brown)  Biological Determinism – the concept that phenomena, including various aspects of behavior (ie. intelligence, values, morals) are governed by biological (genetic) factors; the inaccurate association of various behavioral attributes with certain biological traits, such as skin color o Association between physical characteristics and such attributes as intelligence, morals, values, abilities, and even social and economic status th  Francis Galton shared an increasingly common fear among 19 century Europeans that “civilized society” was being weakened by the failure of natural selection to eliminate unfit and inferior members  Eugenics – the philosophy of “race improvement” through the forced sterilization of members of some groups and increased reproduction among others; an overly simplified, often racist view that’s now discredited The Concept of Race  Polytpic – referring to species composed of populations that differ in the expression of one or more traits o Is composed of local populations that differ in the expression of one or more traits  Within local populations, there’s a great deal of genotypic and phenotypic variation between individuals  The term race has been used synonymously with species as in “the human race”  Although race is usually a term with biological connotations, it also has enormous social significance  Within anthropology, the term ethnicity was proposed in the early 1950s to avoid the more emotionally charged term race Notes From Reading CHAPTER 4:M ODERN H UMAN VARIATION AND A DAPTATION (PGS. 69-94)  Ethnicity refers to cultural factors, but the fact that the words ethnicity and race are used interchangeably reflects the social importance of phenotypic expression and demonstrates how phenotype is mistakenly associated with culturally defined variables  In biological usage, race refers to geographically patterned phenotypic variation within a species o Biologists now almost never refer to “races” of other species, but more typically talk about populations or, for major subdivisions, subspecies th  In the 20 century, the application of evolutionary principles to the study of modern human variation replaced the superficial 19 century view of race based on observable phenotypes  The amount of genetic variation accounted for by differences between groups is vastly exceeded by the variation that exists within groups  Anthropologists argue that race is an outdated creation of the human mind that attempts to simplify biological complexity by organizing it into categories  A limitation of traditional classification schemes derives from their inherently typological nature o Categories are distinct and based on stereotypes or ideals that comprise a specific set of traits  Polygenic – influenced by more than one gene and therefore exhibit a continuous range of expression  The scientific controversy over race will fade as we enhance our understanding of the genetic diversity of our species Contemporary Interpretations of Human Variations  It is now possible to directly sequence DNA, and we can actually identify entire genes and even large DNA segments and make comparisons between individuals and populations  We now have the capacity to obtain DNA profiles for virtually every human population on earth Human Polymorphisms  Polymorphisms – loci with more than one allele. Polymorphisms can be expressed in the phenotype as the result of gene action (as in ABO), or they can exist solely at the DNA level within noncoding regions  Polymorphisms have been essential to the study of evolutionary processes in modern populations  Examining single traits can be informative regarding potential influences of natural selection or gene flow Polymorphisms at the DNA Level  Due to the Human Genome Project, we’ve recently discovered many variations in the human genome  Researchers are expanding their approach to map patterns of variation for individual nucleotides  Single nucleotide changes also frequently occur in non-protein-coding portions of DNA  These point mutations, together with those in coding regions of DNA, are all referred to as single-nucleotide polymorphisms (SNPs)  The most recent and most comprehensive population data regarding worldwide patterns of variation come from the analysis of extremely large portions of DNA, called “whole-genome” analysis  Results of these new studies are significant because they confirm earlier findings from more restricted molecular data as well as providing new insights  All human populations outside Africa have much less genetic variation than is seen in Africa  New data shed light on the genetic relationships between populations worldwide and the nature of human migrations out of Africa  Preliminary results suggest that the patterning of human variation at the global level may help scientists identify genetic risk factors that influence how susceptible different populations are to various diseases Notes From Reading CHAPTER 4:M ODERN H UMAN VARIATION AND ADAPTATION (PGS. 69-94)  5,000 years ago, populations of north-central Europe were selectively breeding cattle for higher milk yields o As the population were increasing their dependence on fresh milk, they were inadvertently selecting for the gene that produce lactase persistence in themselves  In East Africa, the domestication of cattle, partly to provide milk, was a cultural and dietary shift of sufficient important to cause allele frequencies to change in two distinct areas Human Biocultural Evolution  Culture = human strategy of adaptation  Humans live in cultural environments that are continually modified by their own activities  Evolutionary processes are understandable only within a cultural context  Slash-and-Burn Agriculture – a traditional land clearing practice involving the cutting and burning of trees and vegetation. In many areas, fields are abandoned after a few years and clearing occur elsewhere  The increase in the frequency of the sickle-cell allele is a biological adaptation to an environmental change  Heterozygous (people with sickle-cell trait) have increased resistance to malaria and presumably higher reproductive success  There is a counterbalance between selective forces with an advantage for carriers only in malarial environments  Lactose intolerance, which involves a person’s ability to digest milk, is another example of human biocultural evolution  Lactase Persistence – in adults, the continued production of lactase, the enzyme that breaks down lactose (milk sugar). This allows adults in some human populations to digest fresh milk products. The discontinued production of lactase in adults leads to lactose intolerance and the inability to digest fresh milk Population Genetics  Population Genetics – the study of the frequency of alleles, genotypes, and phenotypes in populations from a microevolutionary perspective  Population – is a group of interbreeding individuals that share a common gene pool  Gene Pool – the total complement of genes shared by the reproductive members of a population  The genes (alleles) in a gene pool are mixed by recombination and then reunited with their counterparts through mating  Factors that determine mate choice are geographical, ecological and social  Breeding Isolates – Populations that are clearly isolated geographically and/or socially from other breeding groups  Geography plays a dominant role in producing these isolates by strictly determining the range of available mates  Within these limits, cultural rules can play a deciding role by prescribing who is most appropriate among those who are potentially available  To determine whether evolution is taking place at a given locus, population geneticists measure allele frequencies for specific traits and compare these observed frequencies with a set predicted by a mathematical model  Hardy-Weinberg Equilibrium – the mathematical relationship expressing – under conditions in which no evolution is occurring – the predicted distribution of alleles in populations; the central theorem of population genetics Notes From Reading CHAPTER 4:M ODERN HUMAN VARIATION AND ADAPTATION (PGS. 69-94)  Factors that act to change allele frequencies include: o New variation (that is, new alleles produced by mutation) o Redistributed variation (that is, gene flow or genetic drift) o Selection of “advantageous” allele combinations that promote reproductive success (that is, natural selection) The Adaptive Significance of Human Variation  Biological anthropologists view human variation as the result of the evolutionary factors: mutation, genetic drift, gene flow and natural selection  To survive, all organisms must maintain the normal functions of internal organs, tissues, and cells within the context of an ever-changing environment  Stress – in a physiological context, any factor that acts to disrupt homeostasis; more precisely, the body’s response to any factor that threatens its ability to maintain homeostasis  Homeostasis – a condition of balance, or stability, within a biological system, maintained by the interaction of physiological mechanisms that compensate for changes (both external and internal)  Adaptation refers to long-term evolutionary changes that characterize all individuals within a population or species  Acclimatization – physiological responses to changes in the environment that occur during an individual’s lifetime. Such responses may be temporary or permanent, depending on the duration of the environmental change and when in the individual’s life it occurs. The capacity for acclimatization may typify an entire species or population, and because it’s under genetic influence, it’s subject to evolutionary factors such as natural selection and genetic drift  Simplest type of acclimatization is a temporary and rapid adjustment to an environmental change o Ie. Tanning o Very rapid increase in hemoglobin production that occurs when people who live at low elevations travel to higher ones  Developmental Acclimatization – is irreversible and results from exposure to an environmental challenge during growth and development Solar Radiation, Vitamin D, and Skin Color  Skin color is often cited as an example of adaptation through natural selection in humans  Skin color is mostly influenced by the pigment melanin, a granular substance produced by specialized cells (melanocytes) in the epidermis  Melanin is impo
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