Unit 3: Mechanics of Natural Selection and Human Variation

Genetics and Evolution

(This information is also found in Embers Chapter 3
p.32)

Universe = 15 billion years old (Astronomers)

Primates appeared 55 to 65 million yrs ago.

They were distinctive because of flexible digits. May or may not have lived in trees.

35 million – first monkeys and apes appear
15 million—apelike ancestors of humans appear
4 million – first humanlike beings appear
100,000 – modern looking humans evolve

This chapter focuses on the evolution of evolutionary theory and how it accounts for change over time.

Plato and Aristotle believed that plants and animals were in a continuum going from more perfection to less perfection – humans were at the top of the scale.

 

Later Greek philosophers believed that the creator gave life or radiance to humans first but that it faded with each subsequent creation

Plotinus believed that all beings were linked in a “chain of being” from God down through living beings. All beings were linked together and were dependent on each other. This was a popular belief and encourages study of previously unknown creatures, and comparative anatomical studies which clears the way for people to think of evolution – kind of a chain of being. The idea that humans were closely related to apes fit with this, humans were perfected apes.

Linnaeus’ classification of plants and animals into a systema naturae places humans in the same order (Primates) as apes and monkeys. the hierarchical classification scheme in descending order going from kingdom to class, order, genus, and species, provided a framework for the idea that humans, apes and monkeys had a common ancestor.

All of these people believed that species were created in a fixed form, and did not change

Lamarck believed that acquired characteristics could be inherited and therefore species could evolve; individuals who in their lifetime developed characteristics helpful to survival would pass those characteristics on to future generations, and change the physical makeup of the species. Giraffes necks grew because successive generations of giraffes stretched out to eat from high trees, and successive generations would come to have long necks.

Cuvier believed that catastrophies accounted for changes in the earth and fossil record, catastrophies wiped out species that were then replaced by subsequent species.

Geologists studying formation of the earth came up with the idea of uniformitarianism that natural forces shaped and reshaped the earth over time. The fossils of flora and fauna in layers of rock strata were used to define different geological epochs.

Darwin after studying changes in plants, fossils and living pigeons decided he disagreed with the idea of the fixed form – and believed that species evolved through the mechanism of natural selection. Wallace came to the conclusion about the same time, they presented their ideas in 1858 to the Linnean Society of London.

1859 “Origins of Species” – the idea that species belonging to the same genera are lineal descendants of some other and generally extinct species……outrages the creationist….
and then 1871 “Descent of Man” it became clear that Darwin believed that man descended from nonhuman forms, and the controversy began.

PRINCIPLES OF NATURAL SELECTION
Darwin is not the first to believe that species evolved but he was the first to credit natural selection for the way evolution had occurred.

Natural selection is the main process that increases the frequency of adaptive traits through time. The operation of natural selection involves three conditions or principles…

variation– each species is composed of a great variety of individuals, some of which are better adapted to their environment than others. This variety allows natural selection to favor certain characteristics over others.

heritability – offspring inherit traits from their parents, to some degree and in some way

differential reproductive opportunity/success – better adapted individuals generally produce more offspring over the generations than the less well adapted, so the adaptive traits’ frequency increases. a new species emerges when changes in traits, or geographic barrier result in the reproductive isolation of that population.

Adaptive or advantageous traits may result in greater reproductive success in a particular environment.

The theory of natural selection suggests that disadvantageous or maladaptive traits will generally decline in frequency or disappear – not necessarily the case. Traits may be linked to other traits and preserved from earlier forms…eg. choking – vertebrates often have a place where the airway and the digestive systems cross….and have not corrected through natural selection.

Environmental changes or new members of the species may cause natural selection. When environment changes the individuals that posses the traits that allow for survival will be most likely to reproduce and their traits will become dominant. Variation will still exist, but less adaptive traits may decrease in frequency.

Natural selection does not account for all variation in the frequencies of traits – ex. the frequencies of neutral traits (no advantages or disadvantages)

Observed examples of evolution –

Moths in England – in polluted areas the dark variety of the moth is dominant, in less polluted areas the lighter variety is more prevalent….

direct selection – population shifts over time to the more adaptive trait.

normalizing selection – average value doesn’t change but the extremes fade through natural selection – birthweight of babies

balancing selection – heterozygous gene combinations favored over homozygous combinations… sickle cell anemia, DDT resistance housefly, antibiotic resistant bacteria

what happens to traits when 1) new individuals join the population
2) mate with those who don’t have the trait?

Mendel – dominant and recessive traits — green and yellow peas, smooth and wrinkled peas – how does phenotype (how it looks) vary from genotype (genetic make up) –

xx — recessive gene pair = recessive trait

XX — dominant trait gene pair = dominant trait

Xx — offspring with one of each will look like dominant parent

xX + Xx == offspring can be XX or xx or Xx

genes — Mendel’s units of heredity
allele – pairs of genes
homozygous – alleles are the same
heterozygous – different

genes of higher organisms are on ropelike structures called chromosomes in the nucleus of the cell.

Two concepts you should also remember as we go forward:

Genetic drift – random processes that affect gene frequencies in isolated populations, and cause normally rare traits to become common in the population

Gene flow – mating between populations, how gene crossover at the margins will cause a trait to flow through an area A->B B->C C->D Red hair in area A may eventually travel to area D.

Some discussion about how evolution is thought to work from the BBC — Ape Man to Neanderthal

Also consider this:  Can adaptive traits can be behavioral as well as genetically based?

 

Chapter 12 Human Variation and Adaptation (Ember p.198)

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All Humans are Homo sapien sapiens

Anthropologists are concerned with variation in recent human populations

Why do physical differences exist?

genetic

environmental

Adaptation –

Mutation – changes in the structure of a gene – the ultimate source of all genetic variation

Natural selection results in more favorable genes becoming more frequent in a population over time – this is called adaptation

Adaptations are genetic changes that give their carriers a better chance to survive and reproduce than non-carriers who live in the same environment – the environment favors reproductive success of some traits rather than others

Effectiveness of an adaptive gene depends on the environment – not necessarily equally successful in all environments

Neutral traits may flow also, from gene flow and genetic drift

Gene flow, and genetic drift are not adaptive processes — drift may increase differences, flow decreases differences between populations

Acclimatization –

Physical environment can produce variation even in the absence of genetic change.

Climate may influence the way the human body grows and develops, and therefore some kinds of human variation may be explainable as a function of environmental variation. – this is called acclimatization

Acclimatization involves physiological adjustments in individuals to environmental conditions – they may have underlying genetic factors but they are traits individuals develop over a lifetime instead of being born with them.

Some long term acclimatizations are hard to distinguish from adaptations because they become established as normal operating processes and may persist even after the individual moves to a different environment than the one that triggered the variation.

Common acclimatization – tanning

INFLUENCE OF CULTURAL ENVIRONMENT – p. 199

Humans can affect their environments

culture allows humans to modify their environments and these modifications may lessen the likelihood of genetic adaptations and physiological acclimatizations

cold weather – heat our buildings, put on warmer clothes

iron deficiencies – cook in iron pots

Culture can influence the direction of natural selection – dairying increased frequency of genes that allow adults to digest milk

Cultures can practice behaviors hat lead to physical variations between their culture and others, or to indicate elite status in their own cultures

foot binding   — Chinese women

headbinding  —  highland Andean societies, NW Native Americans

circumcision – Jews

plastic surgery — Brazil and Argentina

PHYSICAL VARIATION IN HUMAN POPULATIONS

Genes, environment and culture interact to create human variation.

Physical –

Body build and facial construction

Scientists have suggested that the body build of many birds and mammals may vary according to the temperature of the environment in which they live.

Bergmann and Allen in the 1950’s – applied the rules about animals to humans and found that there is a general relationship between body size and temperature –

Bergmann’s rule — slenderer populations of a species inhabit the warmer parts of its geographic range and more robust populations inhabit the cooler areas.

Allen’s rule – refers to another kind of variation in body build – protruding parts (arms, legs) are relatively shorter in cooler areas of a species range than in warmer areas.

so – slender long limbed people have more surface area in relation to body mass to facilitate dissipation of body heat – shorter limbed people have less surface area and can retain heat.  (research with rats mimics this)

facial features may be acted on the same way – cold climate – -wider faces, narrower nasal openings are smaller.  people in the tropics have broad, short, flat noses – climates with low humidity tend to have long, thin noses – may be a more efficient humidifier in hot or cold temperatures.

SKIN COLOR –

Human populations vary greatly in skin color –

people who live in northern climates tend to have less melanin or pigment – allows them to absorb vitamin D from sunlight (Gloger’s rule) – those in warmer climates have more melanin and darker skin (feathers, whatever) to protect sensitive inner layers of skin from damaging ultraviolet rays.  – less sunburn, and skin cancer than light skinned people – and maybe more resistance to tropical diseases.

ADAPTATION TO HIGH ALTITUDES

Humans seem to adapt to hypoxia as they mature – large chest/lung structure seems to be from use at altitude during childhood and  not biological

HEIGHT

Nutrition affects height.

Good health affects height.

Reduction in stature during times of war and famine support this especially before age 3

New research says that stress in infancy seems to affect height and weight. – including mother/infant separation if practiced before 2 years of age

Vaccination of children under 2 – taller on average.

Stress in infancy also seems to affect height.

SUSCEPTIBILITY TO INFECTIOUS DISEASES

genetic basis for susceptibility to disease

genetic homogeneity in a population may increase susceptibility

and are more likely to face epidemic disease  — Native Americans – small founding population and little opportunity for genetic drift

cultural practices may help – exposure to other communities may spread disease

Yanamamo “visiting”  Native American trade practices

Large populations in dense areas give microorganisms more opportunity to spread and mutate  — poor sanitation and contaminated water also impact health and resistance

SICKLE CELL ANEMIA

a mutation of the genetic instruction for hemoglobin  causes mutated sickle shaped red blood cells that do not provide enough oxygen to the body and can damage heart, lung and brain

may be a mutation to counterbalance malarial infection as an example of “balancing selection” – found in tropical areas, where the incidence of malaria is high, children with sicklemia have fewer malarial parasites in their bodies and are more likely to survive – possibly because it is harder for the parasites to survive in an abnormal blood stream since the mutated cells do not survive long, and cannot support the parasite’s needs

LACTASE DEFICIENCY

Lactase I enzyme necessary for breaking down sugar in milk, lactose, into simpler sugars that can be absorbed into the bloodstream

Without this people can’t digest milk properly and it may cause bloating, cramps diarrhea.

Lactose intolerance is common among adults in all parts of the world, and exists in children in non-dairying regions after infancy.

Some adults retain the ability to produce lactase I in adulthood – lactose absorption in adulthood seems directly connected to dairying culture – and may be a product of natural selection – greater reproductive success if genetically able to process lactose as an adult

Some dairying societies do not retain, they reduce lactose by transforming milk into cheese, yogurt, sour cream and other low lactose foods.

Studies show that this chemical solution happens closer to the equator while the biological one happens further away – perhaps because lactose behaves like vitamin D biochemically and allows the absorption of calcium but only in people with the lactase producing gene – natural selection may favor lactase production in adulthood, in conjunction with lighter skin, at higher latitudes (less sunlight)

Genes, environment and culture interact to create human variation.

 

 

 

 

 

 

Cloning and genetic engineering, laboratory fertilization may give us an increased ability to affect the genes of humans, what are the implications for the interference with natural selection?