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Classifying groups of humans

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Do races exist?

This can be a contentious subject because many people have emotional, religious, political or social definitions of words like “race”, or “human.” As such, many people respond based on emotions and social preconceptions. In our class we’re going to treat this topic scientifically.

One thing we notice in science and math is that one can’t answer a question, unless the question is first precisely asked. We need to know what the words mean. We need our questions formulated precisely.

So when someone asks “Do races exist?”, the answer depends on the definition of the word “race”. The only definitions we use in this class come from science. The only reasoning we use will be reasoning that can be applied to all forms of life on Earth (We’re not allowed to make special rules and terminology that apply only to humans.)

So let’s start with the givens:

ALL forms of life on Earth – including all mammals, including all humans – evolved over time from a common ancestor.

ALL forms of life on Earth are part of a branching tree of life, a phylogeny.

Phylogenetic Tree of Life by Ciccarelli in March 2006 Science

Image by Madeleine Price Ball. Simplified universal phylogenetic tree, made using information from the Interactive Tree of Life. Ciccarelli, et al., Mar 3 2006, Science Vol. 311

Now look just at the primate branch of the tree.

Again, groups and species are in a nested, tree-like structure.

Ape-Family-tree A family Portrait pasttime org episode-5-throwing-in-human-evolution

Now look just at the hominid (human-like) part of this family tree

Again, groups and species are in a nested, tree-like structure.

Dave Carlson Human family tree

Human family tree by Dave Carlson, carlsonstockart.com

What we learn from biology is that all forms of life exist like this

So when scientists ask “Do human races exist?” they are asking a very precise question: Does the human branch of the tree of life hav any further structure?

The answer is “yes” – humans do have populations with genetic differences that arose through evolution through natural selection.

DNA analysis offers us ways to understand human evolution. Populations based on DNA analysis can be shown on tree-like diagrams (phylogenetic trees.)

Jorde Wooding polymorphisms race genetics

from Genetic variation, classification and ‘race’, by Lynn B Jorde & Stephen P Wooding.

For more details see Genetic analysis of genes and “race”

Many genes that influence skin color.

Assuming something as simple as genes being only dominant or recessive:

* 2 genes create a total of 16 possibilities (see eye color chart above)

* 5 genes creates a 32×32 grid, for a total of 1024 possibilities.

* 10 genes creates a vast number of possibilities – a “spectrum” of skin colors.

* These possibilities cluster in groups that one can predict by evolutionary theory, and can be observed by looking at the genes (see gene “tree” image below)

“…Harrison and Owen predicted that a minimum of six to eight genes underlie the skin color differences between European and West African populations. More recently, researchers considering contemporary admixed populations, which had undergone admixture for many more generations, estimated that at least 10 genes contribute to population-level differences between European and West African populations”

“…Despite the many genes already implicated in human skin color variation, a substantial number of the differences among populations cannot be explained. There are more than 350 putative pigmentation loci identified in mouse models and cataloged in the IFPCS color genes database (http://www.espcr.org/micemut/); what, if any, role these genes play in the unexplained variation in human pigmentation remains unknown.”

The evolutionary genetic architecture of skin pigmentation in three populations. http://www.nature.com/milestones/skinbio4/full/skinbio20113a.html

The evolutionary genetic architecture of skin pigmentation in three populations.
http://www.nature.com/milestones/skinbio4/full/skinbio20113a.html

– Unpacking Human Evolution to Find the Genetic Determinants of Human Skin Pigmentation, Ellen E Quillen1 & Mark D Shriver, Milestones cutaneous Biology, 17 Nov 2011

Why do scientists care about genetics?

What Controls Variation in Human Skin Color?, Gregory S Barsh
PLoS Biol. 2003 Oct; 1(1): e27.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC212702/

From a clinical perspective, inadequate protection from sunlight has a major impact on human health (Armstrong et al. 1997; Diepgen and Mahler 2002).

In Australia, the lifetime cumulative incidence of skin cancer approaches 50%, yet the oxymoronic “smart tanning” industry continues to grow, and there is controversy over the extent to which different types of melanin can influence susceptibility to ultraviolet (UV) radiation (Schmitz et al. 1995; Wenczl et al. 1998).

At the other end of the spectrum, inadequate exposure to sunlight, leading to vitamin D deficiency and rickets, has been mostly cured by nutritional advances made in the early 1900s.

In both cases, understanding the genetic architecture of human skin color is likely to provide a greater appreciation of underlying biological mechanisms, much in the same way that mutational hotspots in the gene TP53 have helped to educate society about the risks of tobacco (Takahashi et al. 1989; Toyooka et al. 2003).

From a basic science perspective, variation in human skin color represents an unparalleled opportunity for cell biologists, geneticists, and anthropologists to learn more about:

* the biogenesis and movement of subcellular organelles,
* to better characterize the relationship between genotypic and phenotypic diversity
* to further investigate human origins
* and to understand how recent human evolution may have been shaped by natural selection.

Further reading

http://learn.genetics.utah.edu/

Learning Standards

HS-LS1-1. Construct a model of transcription and translation to explain the roles of DNA and RNA that code for proteins that regulate and carry out essential functions of life.

Clarification Statements: Proteins that regulate and carry out essential functions of life include enzymes (which speed up chemical reactions), structural proteins (which provide structure and enable movement), and hormones and receptors (which send and receive signals). The model should show the double-stranded structure of DNA, including genes as part of DNA’s transcribed strand, with complementary bases on the nontranscribed strand.

State Assessment Boundaries: Specific names of proteins or specific steps of transcription and translation are not expected in state assessment. Cell structures included in transcription and translation will be limited to nucleus, nuclear membrane, and ribosomes for state assessment.

HS-LS3-1. Develop and use a model to show how DNA in the form of chromosomes is passed from parents to offspring through the processes of meiosis and fertilization in sexual reproduction.

Clarification Statement: The model should demonstrate that an individual’s characteristics (phenotype) result, in part, from interactions among the various proteins expressed by one’s genes (genotype)

State Assessment Boundary: Identification of specific phases of meiosis or the biochemical mechanisms involved are not expected in state assessment.

HS-LS3-2. Make and defend a claim based on evidence that genetic variations (alleles) may result from (a) new genetic combinations via the processes of crossing over and random segregation of chromosomes during meiosis, (b) mutations that occur during replication, and/or (c) mutations caused by environmental factors. Recognize that mutations that occur in gametes can be passed to offspring.

Clarification Statement: Examples of evidence of genetic variation can include the work of McClintock in crossing over of maize chromosomes and the development of cancer due to DNA replication errors and UV ray exposure.

State Assessment Boundary: Specific phases of meiosis or identification of specific types of mutations are not expected in state assessment.

HS-LS3-3. Apply concepts of probability to represent possible genotype and phenotype combinations in offspring caused by different types of Mendelian inheritance patterns.

Clarification Statements: Representations can include Punnett squares, diagrams, pedigree charts, and simulations. Inheritance patterns include dominant-recessive, codominance, incomplete dominance, and sex-linked.

HS-LS3-4(MA). Use scientific information to illustrate that many traits of individuals, and the presence of specific alleles in a population, are due to interactions of genetic factors
and environmental factors.

Clarification Statements: Examples of genetic factors include the presence of multiple alleles for one gene and multiple genes influencing a trait.
An example of the role of the environment in expressed traits in an individual can include the likelihood of developing inherited diseases (e.g., heart disease, cancer) in relation to exposure to environmental toxins and lifestyle; an example in populations can include the maintenance of the allele for sickle-cell anemia in high frequency in malaria-affected regions because it confers partial resistance to malaria.

State Assessment Boundary: Hardy-Weinberg calculations are not expected in state assessment.

 

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