Download Human Variation & Adaptation

12. Human Variation &
Adaptation
Oct. 30, 2008
Introduction
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Human Diversity?
Race? Biological or Cultural?
Ethnic Identity or Group?
Macroevolution and Microevolution
mtDNA and DNA comparisons reveal
remarkable uniformity (more differences
within populations than between them)
• But considerable phenotypic differences
Historical Views of Human Variation
• 1350 BC Egyptians classified humans based on skin color; red
for Egyptians, yellow for the east, white for the north and
black for sub-Saharan.
• Subsequent to the “discovery” of the New World, several
European countries embarked on a period of intense
exploration and colonization in both Old and New Worlds.
• Through the 18th to 19th centuries, European and American
scientists concentrated on primarily describing and classifying
the biological variation in humans.
• Linnaeus 1758 classified humans in four separate categories
(European, African, East Asian, American Indian) based on
skin color, form of dress and supposed behaviors.
Historical Views of Human Variation
• Blumenbach classified humans in five separate
categories (white, yellow, red, black and brown) but
emphasized that racial categories based on skin color
were arbitrary and many traits weren’t discrete
phenomena.
• Nevertheless, by the mid-19th century, populations
were ranked essentially on a scale based on skin
color; with sub-Saharan Africans at the bottom.
• Anders Retzius 1842 and the cephalic index
describing shape (max. head breadth divided by max.
head length) – dolichocephalic <75%, brachycephalic
>80%, mesocephalic 75-80%.
Historical Views of Human Variation
• Biological Determinism – during the 1800s, the concept that
phenomena, including various aspects of behavior are
governed by biological factors; the inaccurate association of
various behavioral attributes with certain biological traits, such
as skin color.
• Ethnocentric – viewing cultures from the inherently biased
perspective of one’s own culture, which usually results in
seeing one culture superior or inferior to another
• Cultural relativism – is the view that cultures have merits
within their own historical and environmental contexts and
that they shouldn’t be judged through comparison with one’s
own culture
Historical Views of Human Variation
• Eugenics – developed in the late 1800s and early
1900s (into the 1930s), 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 has been discredited.
• Francis Galton, a cousin of Charles Darwin!
• Provided “scientific” reasoning for the Nazi
campaign
Concept of RACE
• Polytypic – referring to species composed of
populations that differ with regard to the expression
of one or more traits. There is a great deal of
genotypic and phenotypic variation.
• Most obvious - skin and eye color; shape of face and
nose; hair form, density and distribution
• The word ‘race’ began in 1500s
• Synonymously used with species, e.g. ‘the human
race’
• 1600s race referred to various culturally or nationally
defined groups, e.g. ‘Jewish race’ or ‘Japanese race’
Concept of RACE
• Unfounded assumption that there is an association between physical
traits and cultural attributes.
• In a biological context – race (or subspecies) refers to
geographically patterned phenotypic variation within a species. But
there are no criteria by which races of plants and animals are
assessed.
• Prior to WWII studies focused on physical attributes and variation,
which are largely superficial.
• Races are not fixed or immutable biological entities but rather the
product of societal and typological thinking.
• Furthermore, characteristics that have been used to define races are
polygenic (i.e. influenced by more than one gene and interaction
with the environment).
Intelligence
• Intelligence - Mental Capacity; ability to learn,
reason, or comprehend and interpret information,
facts, relationships, and meanings; the capacity to
solve problems, whether through the application of
previously acquired knowledge or through insight.
• IQ scores and intelligence aren’t the same; IQ scores
overlap between populations; complex cognitive
abilities, however measured, are influenced by
multiple loci and are thus polygenic.
• There is no evidence whatsoever that populations
vary with regard to cognitive abilities.
Contemporary Interpretations of
Human Variation
• Polymorphisms – Loci with more than one allele.
Polymorphisms can be expressed in the phenotype as
the result of gene action, or they can exist solely at
the DNA level within noncoding regions.
• Human Polymorphisms – human ABO blood types.
• Polymorphisms at the DNA level – single nucleotide
polymorphisms.
• Cline – A gradual change in frequency of genotypes
and phenotypes from one geographical region to
another. E.g. skin color, lactose (in)tolerance, sicklecell trait.
Human Biocultural Evolution
• Slash and burn agriculture – a traditional landclearing practice whereby trees and vegetation are cut
and burned. In many areas, fields are abandoned after
a few years and clearing occurs elsewhere.
• Perhaps in just the last 2000 years.
• Lactase persistence – the ability to continue to
produce the enzyme lactase in adults. Most mammals,
including humans, lose this ability after they are
weaned.
Population Genetics
• Population genetics – the study of the
frequency of alleles, genotypes, and
phenotypes in populations from a
microevolutionary perspective.
• Gene pool – the total complement of genes
shared by the reproductive members of a
population.
• Breeding isolates – Populations that are clearly
isolated geographically and/or socially from
other breeding groups.
Population Genetics
• Endogamy – mating with individuals from the same
group. Results in populations that are genetically
homogenous.
• Exogamy – mating pattern whereby individuals
obtain mates from groups other than their own.
Results in populations that are genetically
heterogeneous.
• Hardy-Weinberg – the mathematical relationship
expressing – under ideal conditions – the predicted
distribution of alleles in populations; the central
theorem of population genetics (see Appendix C).
Adaptive Significance of
Human Variation
• Stress – any factor that acts to disrupt homeostasis
• Homeostasis – a condition of balance or stability maintained by the
interaction of physiological mechanisms that compensate for
changes, both external and internal.
• Acclimatization – physiological responses to changes in the
environment that occur during an individuals lifetime; responses can
be temporary or permanent. E.g. visiting high-altitude regions.
• Solar Radiation, Vitamin D, & Skin Color
Skin color influenced by the pigment melanin, granular substance
produced by melanocytes.
Melanin absorbs UV rays
Vitamin D is produced in the body as a result of the interaction
between UV and a substance similar to cholesterol. Vitamin D is
essential for normal bone growth.
Adaptive Significance of
Human Variation
• Thermal Environment: responses to heat and
cold.
- Vasodiliation – expansion of blood vessels,
permitting increased blood flow to the skin. This
permits warming of the skin and also facilitates
radiation of warmth as a means of cooling. This is an
involuntary response to warm temperatures,
various drugs, and even emotional states.
- Vasoconstriction – narrowing of blood vessels to
reduce blood flow to the skin. This is an involuntary
response to cold temperatures and reduces heat loss
at the skin’s surface.
Bergmann’s & Allen’s Rules
Bergmann’s rule (concerns
the relationship of body
mass or volume to surface
area)
Allen’s Rule (concerns
shape of the body,
especially appendages, i.e.
arms and legs)
Adaptive Significance of
Human Variation
• High Altitude
- Hypoxia – lack of oxygen. This can refer to
reduced amounts of available oxygen in the
atmosphere (due to lowered barometric
pressure) or to insufficient amounts of oxygen
in the body; e.g. consider people living in the
Himalayan or Andean regions.
Adaptive Significance of
Human Variation
• Infectious Disease
- Vectors – agents that serve to transmit disease from one
carrier to another; e.g. mosquitoes.
- Endemic – continuously present in a population.
- Zoonotic – pertaining to zoonosis, a disease that is transmitted
to humans through contact with nonhuman animals.
- Pathogens – any agents, especially microorganisms such as
viruses, bacteria, or fungi, that infect a host and cause disease.
- Pandemic – an extensive outbreak of disease affecting large
numbers of individuals over a wide area; potentially a
worldwide phenomenon; e.g. HIV-AIDS.
Continuing Impact of Disease
• HIV (see also SIV & FIV), drug-resistant TB,
malaria, bird flu, SARS & Ebola – all are
powerful selective agents which influence
gene frequencies.