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Friedrich Leopold August Weismann

640px-August_Weismann

Educational Background/Training

Weismann was born on 17 January 1834 in Frankfurt am Main, in the German Confederation. His mother, Elise Eleanor Lübbren, was a musician and painter, and his father, Johann Konrad August Weismann, was a classics professor. Weismann studied music, particularly the works of Beethoven, and he studied nature, from which he collected butterflies. He noted diverse patterns and colors of butterflies, information that later informed his research on the development and evolution of butterflies and caterpillars.

In 1856 Weismann got his medical degree from the University of Göttingen in Göttingen, in the German Confederation. After graduation, Weismann worked as an assistant in a hospital for three years in Rostock, in the German Confederation, before becoming a physician in Frankfurt am Main in 1859. From 1861 to 1863, Weismann was the private physician for Archduke Stephen of Austria. In 1861, Weismann studied at the University of Giessen in Giessen in the German Confederation, with Rudolf Leuckart for two months, working on the ontogeny (development) and morphology (form) of animals, insects in particular. That year, Weismann read Charles Darwin’s On the Origin of Species two years after it was published in 1859, after which he adopted evolutionary theory. Weismann studied different factors he thought might cause morphological transformations in insects, including natural selection.

In 1863, Weismann became a docent in zoology and comparative anatomy, a mid-ranking academic position, in the University of Freiburg in Freiburg in Breisgau, also in the German Confederation. In 1864, Weismann’s eyesight declined, which left him partially blind and limited his ability to use microscopes. Nonetheless, he studied the metamorphosis and development of butterflies. Weismann became the founding director of the Zoological Institute at the University of Freiburg in 1867. That year, he married Marie Dorothea Gruber from Genoa, Italy. The couple had at least five children. Along with his students and assistants, Marie aided his experimental and observational studies after his eyesight failed. Marie died in 1886, but Weismann remarried at the age of sixty in the mid-1890s to Willemina Tesse from the Netherlands, a marriage that lasted six years.

Summary of Research

            August Friedrich Leopold Weismann studied how the traits of organisms developed and evolved in a variety of organisms, mostly insects and aquatic animals, in Germany in the late nineteenth and early twentieth centuries. Weismann proposed the theory of the continuity of germ-plasma, a theory of heredity. Weismann postulated that germ-plasma was the hereditary material in cells, and parents transmitted to their offspring only the germ-plasma present in germ-cells (sperm and egg cells) rather than somatic or body cells. Weismann also promoted Charles Darwin's 1859 theory of the evolution of species. Weismann argued that only changes to the germ cells, and not body cells, could be inherited, a theory that influenced theories of heredity throughout later centuries.

From 1881 onwards, Weismann published a series of essays about heredity. Those essays were collated in English in 1889's Essays upon Heredity and Kindred Biological Problems. The essays discussed topics including senescence, acquired characteristics, and the germ-plasma theory. For example, in the first chapter, "The Duration of Life," a translation of an essay originally published in German in 1881, Weismann detailed his evolutionary theory of senescence, the name given to the gradual deterioration of function of most life forms after they mature to adults. Weismann argued against theories that associated the length of an organism's life with the size or complexity of its body, or with how active it appears to be. Instead, he appealed to natural selection, arguing that it adapted organisms to reach reproductive maturity, and that it would not select for the capacity of the organism to live any longer once it was past reproductive age. He further argued that the death of male bees after they reproduced was selected for by nature to save nutrition for the colony, a phenomenon that precluded those organisms that had already reproduced from consuming resources.

Linking his work to broader context

When Weismann’s germ theory is paired with that of Gregor Mendel’s on inheritance we are provided the basic understandings of how humans and other animals inherit their traits from their parents. Weismann used his germ theory to explain that, “natural selection favors organisms that pass on their germlines before conspecific and before extrinsic factor cause their death (Crews and Ice, 2012: 639).” This statement has been used to create the concept of life history theory for many different species, including humans. For life history theory, we see that the goal to reproduce is in conflict with the maintenance of the body. This result in trade-offs that the body goes through in order to chose one of these actions over the other. The allocation of resources between reproducing and somatic maintenance created a way for researchers to compare and to structure their research as to why these chronic diseases and changes in old age occur. Through these trade offs in life history theory is how Weismann can be connected to this chapter. This chapter discusses the aging and senescence of humans. Senescence occurs when the body begins to function less efficiently and cell begins to functioning deteriorate as life progresses. The chapter discusses the different ways that the body ages in multiple areas, including hormones, immune system, cardiovascular, body composition, bone, dementias, and reproductive aging. All of these areas are altered through the aging of an individual based on the life history course that has been taken. One problem with Weismann’s concept of life history is that it does not allow for the environment and culture to alter these stages. Even in the chapter there is little discussion on the environment and its effects on life history. Yet, with the time that Weismann came up with this theory there must be credit given for his ability to come up with these conclusions that further lead to our understanding of how the body works.

Crews, D.E., Ice, G.H. (2012). Aging, senescence, and human variation in Human Biology: An Evolutionary and Biocultural Perspectives, Second Edition. Edited by Stinson, Bogin, O’Rourke. John Wiley and Sons, Inc.: Hoboken, New Jersey.

(2008). August Weismann found at http://www.encyclopedia.com/topic/August_Weismann.aspx.

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William R. Leonard is a leading anthropologist in the field of human nutrition. He was born in Jamestown, NY and received his PhD in biological anthropology from the University of Michigan at Ann Arbor in 1987. He is now an Abraham Harris Professor in the Department of Anthropology and the Chair of Anthropology at Northwestern University. He is also the Director of the Global Health Studies Program.

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Dr. William R. Leonard (left) with former student Josh Snodgrass, Univeristy of Oregon, conducting fieldwork in Siberia. (Photo provided by William Leonard)

Much of his research focuses on nutrition, energetics, and child growth in both modern and prehistoric human populations. He has traveled and studied in regions of South America, including Bolivia, Ecuador, and Peru, and also Siberia. In these regions, Leonard conducts research on population adaptation to their specific nutritional environment and how these adaptations affect their health, as well as contribute to chronic disease risks. Additionally, Leonard compiles information about human and primate ecology in order to examine the evolution of nutritional requirements in our hominid ancestors. This research leads to insight regarding the origins of obesity and metabolic diseases in contemporary human populations.

One recently published paper by Leonard, titled “The global diversity of eating patterns: Human nutritional health in comparative perspective” highlights Leonard’s work surrounding human nutrition, dietary trends, and the raising rates of obesity in the US. In the paper, he focuses on the different types of subsistence in the US versus less modern, more traditional societies. He notes that the energy intake between industrialized and non-industrialized societies is not different, but that the composition of nutrition includes higher levels of fats and carbohydrates in industrialized cultures. He also compares humans’ nutritional needs to primates, noting that the increase in brain size in higher-level primates such as humans has led to humans requiring higher quality foods than some of our close evolutionary relatives. As rates of obesity and chronic metabolic diseases continue to rise in the US and other industrialized societies, research such as Leonard’s studying the causes and origins of such nutritional deficiencies is of growing importance.

References:

Leonard, William R.

2014 The global diversity of eating patterns: Human nutritional health in comparative perspective. Physiology & Behavior 134:5-14.

Background information based on biosketch provided by Dr. William R. Leonard.

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bogin_528

Barry Bogin is an American physical anthropologist trained at Temple University that researches physical growth in Guatemalan Maya children, and is a theorist upon the evolutionary origins of human childhood. He is currently at Loughborough University in the UK. He is noted for the idea that evolution added two new stages into human development; childhood and adolescence.

 

Smith,B Holly

B. Holly Smith is a Associate Research Scientist in the Department of Anthropology at the University of Michigan at Ann Arbor where she got both her Master and Ph.D. She is interested in  how humans differ from other mammals in life cycle and life span, why we differ and whether we can reconstruct the evolutionary history of our life cycle from the fossil record.

This chapter was about the evolution and alterations of the human life cycle. The main questions that guided this research were:

  • How can human biologists identify the shared and novel features of the human life cycle?
  • Can the time of origin of the novel features be determined?
  • Can the reasons for the evolution of new growth development and maturation patters be determined?

 

Stages in the Life cycle

There are four main stages in the human life cycle Birth, Postnatal Development, Adulthood, and Death. Of these, both postnatal development and adulthood are divided up into sections. Pregnancy (the period before birth) is divided into trimesters and during this gestational period, the fetus grows and changes and experiences critical periods. These are times when a fetus is particularly susceptible to outside factors such as diseases or lack of nutrients. During this time the fetus can undergo epigenetic modification.

What other outside factors can affect a fetus in vivo?

 

After the pregnancy comes birth, a rapid transition from a fairly stable liquid environment to a volatile gaseous one. And after this period come the postnatal development. This is the most complex of the stages and is divided up into these sections

  • Neonatal period
  • Infancy
  • Childhood
  • Juvenile
  • Puberty
  • Adolescence

Which of these sections is the longest and why do you think that is?

In which of these stages is proper nutrition the most critical for brain development, and why is this so?

 

Why did new life stages evolve?

If we look at the life cycles of other large primates we see that although humans experience delays in Molar 1 eruptions, menarche and 1st births, humans have less spacing between births (3 years for humans, 6 for chimpanzees). This gives humans the advantage to give birth to more offspring. So we find that our evolution of childhood gives us the reproductive advantage although it does come with some drawbacks. Children need specialized diets and extended periods of care, as they do not become self sufficient until post-adolescence.

Although we cannot study the life cycles of an extinct organism, we can postulate it by looking at currently living species. In looking at archaeological evidence, we can see that there is an increase in brain size in cubic centimeters and that because of this, there had to have been an increase in postnatal stages. When we get to homo sapiens we see the appearance of adolescence.

Which organism(s) would be useful in looking at early human life cycles?

What physiological changes needed to occur in early human ancestors to accommodate larger brains?

 

Food for thought

  • How would we be different if we had a shorter postnatal period?
  • Would anything be different if humans waited (on average) twice as long between children?

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will-leonard3

The Author of this chapter, William R. Leonard, is currently a professor of anthropology at Northwestern University. He holds the title at this university as the Abraham Harris Professor of Anthropology. He He received his PhD from the University of Michigan in 1987. His research interests include biological anthropology, adaptability, growth and development, and nutrition focusing on populations in South America, Asia, and the United States. His most recent publication was on the topic of precursors to over-nutrition and the effects of household market food expenditures on body composition among the Tsimane in Bolivia.

The ecological variation of available food has been an important factor throughout the history of human evolution and continues to shape the biology of traditional human populations today. The relationship that humans share with their environments (i.e., acquisition and expenditure of energy) has adaptive consequences for both survival and reproduction. Humans are similar to other primates in that we are omnivorous (i.e., we eat both plants and animals) and we have nutritional requirements (e.g., the inability to synthesize vitamin C) that has caused us to adapt diets that include large quantities of fruit and vegetable material. However, what is unique to humans is our highly diverse diet (i.e., dietary plasticity) that evolved because of cultural and technological innovations that developed for processing various resources. This has allowed humans to expand into the many different ecosystems that we inhabit today.

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In order tomaintain our health, humans require six classes of nutrients:

(1)   Carbohydrates are the largest source of dietary energy for most human groups. For example, carbs account for about 40-50% of the daily calories of U.S. adults. There are three type of carbohydrates including monosaccharides (i.e., simple sugars), disaccharides (i.e., sugars formed by two monosaccharides), and polysaccharides (i.e., complex sugars made up of three or more monosaccharides).

(2)   Fats are the most calorically dense source of dietary energy and provide the largest store of potential energy for the body to do biological work. Fats are divided into three groups. The first, simple fats, is mostly made up of triglycerides (i.e., glycerol and fatty acid). Fatty acids can be further divided into saturated (i.e., found in animal products) and unsaturated fats (i.e., monounsaturated and polyunsaturated mostly found in vegetable oil). Compound fats are the second type of fat that consist of a simple fat in combination with another type of chemical compound, such as a sugar or a protein. Compound fats are important for blood clotting and insulating nerve fibers. The third category of fats is known as derived fats, which are a combination of simple and compound fats (e.g., cholesterol). Cholesterol is important for normal development and function. It is also a precursor in the synthesis of vitamin D and hormones like estradiol, progesterone, and testosterone.

(3)   Proteins are an important energy source, but they are also crucial for the growth and replacement of living tissues. In order to get theadequate nutrition per day a person needs a sufficient quantity and quality of protein. The digestibility and amino acid composition determine the quality of a protein. Complete proteins have the necessary amino acids in the quantity and proportions that are needed to maintain healthy tissue repair and growth. Good sources of complete proteins come from animal foods including eggs, milk, meat, fish, and poultry. Incomplete proteins are those that lack one or more essential amino acids. Incomplete proteins are found inplant foods, such as grains, legumes, seeds, and nuts. So if you want to be a vegetarian it will require combining different sources of plant foods in order to get all of the essential amino acids you need.

(4)   Vitamins are not a source of energy, because they just help the body use energy and carry out other metabolic activities. There are two categories of vitamins: water-soluble vitamins (i.e., B vitamins and vitamins C are needed on a daily basis because they are not stored in the body) and fat-soluble vitamins (i.e., vitamins A, D, E, and K are stored in the body so they don’t have to be taken every day). Be careful because if you take too many fat-soluble vitamins over a long period of time it can be toxic.

(5)   Minerals, such as iron, are inorganic elements that are needed in many biological molecules (e.g., hemoglobin) and are vital formaintaining various physiological functions.  

(6)   Water makes up a large portion of our body weight at 40-60% for adults. Humans get water from liquid intake, food, and “metabolic water” that is produced as the result of energy-yielding reactions.

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Recent research has focused on developing and refining energy and nutrient requirements for the various human populations around the world. Many factors must be considered in order to efficiently estimate a person’s daily energy needs including diet, daily activities and exercise, energy costs for reproduction, sex and age. According to the World Health Organization (WHO), women who are pregnant need an extra 85 kcal/day during the first trimester, an extra 285 kcal/day during the second trimester, and an extra 475 kcal/day during the final trimester. Children’s and adolescents’ energy requirements are measured differently from adults, because they have extra energy costs that are associated with growth. Pregnant women, children and adolescents also require more protein than the average adult.

The dietary patterns and metabolism of humans has been shaped by the energy demands of our relatively large brain. The energy demands of humans are usually divided into maintenance energy (i.e., needed for day-to-day survival) and productive energy (i.e., needed for growth and reproduction). Humans spend a larger portion of their daily energy budget on brain metabolism when compared to other organs in the body. We use 20-25% of our BMR (basal metabolic rate) on brain metabolism compared to the 8-10% used by primates and only 3-5% used by other mammals. It has been hypothesized that because of the high metabolic costs of our brains we require high-quality diets. Animal foods contribute to about 45-65% of the diet amonghunter-gatherers, which is much higher quality than expected for primates of our body size. Humans also have small gut volumes for our size, because most large-bodiedprimates have large intestines for digesting fibrous, low-quality diets. So, we probably evolved to have smaller intestines and a reduced colon because of our high-quality diets.  

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Throughout the evolution of the different hominin species there has been changes in brain and body size. The australopithecines had smaller brains relative to their body size, but with the emergence of the genus Homo there was a dramatic increase in brain size. The body size of Homo erectusalso increased, but the changes of the brain size were much larger than those that occurred with body mass. Homo erectus had a larger brain and body but smaller teeth, which suggests that this species relied on a different subsistence source than the australopithecines that was probably easier to digest (i.e., less fibrous plant foods) and richer in calories. The greater nutritional stability of the genus Homo provided the fuel for the energy demands of their larger brains.   

While Humans do have a diverse range of diets across the world, environmental pressures have contributed to adaptations such as lactose tolerance and the ability to digest starch. Some adaptations have become maladaptive in modern society, such as increased fat storage, which has lead to increasing rates of obesity. The amount of animal foods (meat, eggs, milk, etc.) varies across cultures and geographic location. Contemporary foraging groups consume animal foods for approximately 45-65% of their diets. However in the US our animal foods consumption is approximately 26% of our diet. Macronutrient consumption also varies across populations. Americans derive 15% from protein, 34% from fat, and a very high 51% of their energy from carbohydrates. This carbohydrate % is higher than every other population except for small-scale farmers. Another interesting statistic is the estimated consumption percentages estimated for modern foragers: 20-31% protein, 38-49% fat, and 31% carbohydrates. What do you think about these forager percentage estimates in comparison to American percentages?  

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Carbohydrates consumed in subsistence-level societies are typically more complex with a small percentage of their carb consumption coming from simple carbs. American carbohydrates however come mostly from simple carbs and processed grains. These simple and processed carbs are absorbed faster into the blood stream than more complex varieties. A high glycemic level in the blood stream may lead to insulin resistance, which may lead to obesity, type II diabetes, hypertension, hyperlipidemia, and coronary heart disease. In comparison to subsistence-level populations, industrialized men weigh approximately 26.5 lbs more and require 150-200 kcal less. Industrialized women weigh 17.7 lbs more and demand approximately 90kcals. The US Department of Health and Human Services has also released guidelines that adults do approximately 150min/week of moderate physical activity. Another recommendation by IOM set the bar higher at 1 hour/day.

Another interesting fact from later on in the chapter is associated with the enzyme amylase. Carbohydrate digestionbeings in the mouth with amylase (enzyme found in saliva). Populations with high-carb diets have more copies of the AMY1 gene and therefore more amylase. So differences in dietin recent human evolution have exerted strong selection at the AMY1 locus. Also humans have three times as many AMY1 genes as chimps and bonobos. This implies that there was strong evolutionary selection on this gene during the early divergence of hominins from apes.

Food processing techniques are developed to fit the needs of the subsistence-level society that grows that particular crop. Corn, a major crop in the Americas, is high in protein but low in the amino acids lysine and tryptophan as well as the B vitamin niacin. To solve this problem, corn is processed in the presence of alkaliproducts (e.g., ash, lime, and lye) adding back these key nutrients. Andean populations processed potatoes in a way that removes the hazardous glycoalkaloids. Also, Asian populations processed the antitrypsin factor out of soybeans.

Climate may also have an effect on metabolic rates. Studies show that populations living in warmer climates have a lower metabolic rate than those living in colder environments. This attributes to a variation in dietary needs in different climates. It is being questioned whether these population differences are genetic or part of acclimatization.

lactose-intolerance

The ability to digest lactase disappears after weaning for most mammals, however some human populations have developed the ability to digest lactose and are thus lactose-tolerant. This change is a relatively recent evolutionary event occurring within the last 10,000 years. Genetic analysis shows that selection for the lactase persistence appeared about 7500 years ago. The allele spread across Europe in association with dairy/farming subsistence. It also appears to have evolved independently in some African populations approximately 6000-7000 years ago. However, some malabsorbers (genetically intolerant) people are able to digest lactose, and some genetically tolerant people are unable to digest milk. This suggests that dietary habits during development may contribute to lactose tolerance. In the malabsorbers this is due to an increased tolerance in the colon instead of an increase in lactase (enzyme that digests lactose. Life tip:If it ends in –ase it is an enzyme).

African-Americans have an increased risk of cardiovascular diseases. One model says that the problem is a consequence of genetic adaptation for efficient sodium (Na+) storage. Na+ is readily lost in sweat and was rare in many tropical societies. These groups have lower sweat rates and lower sodium concentrations in their sweat than European control groups. Now with salt being readily available to people who have genetically evolved to retain it, these people have higher bloodpressure. In relation to this model, the same scientist says that slaves brought over on slave ships would have been exposed to severe dehydration, and those with salt-retention would have been more likely to survive. So dependents of slaves have a high probability of having this recently selected for trait. (This study focuses on the West Indies and thereforemay not be representative of the US). Some argue that the slavery hypothesis is overly simplistic and a modern representation of racism in science. Still others argue that this increased risk is related to socioeconomic stress. Increased stress leads to increasedsympathic nervous system activity. The release of norepinephrine and adrenocorticotropic hormone elevate blood pressure by increasing sodium retention.  Do you think the slavery hypothesis is racist? Which of these models makes more sense to you?

sugar-consumption-graph

Type 2 diabetes is when your cells reduce the number of insulin receptors and then become insensitive to insulin (your insulin levels are not necessarily affected). “Thrifty Genotype” is the current hypothesis for why we evolved to be sensitive to insulin. Hunter gather societies were faced with seasonal and year-to-year fluctuations in availability of nutrients and therefore would have developed a “thrifty genotype” that would have allowed for a quick release of insulin and an increase in glucose storage during times of plenty. Nowadays we live in a constant state of plenty, and this “thrifty genotype” is now maladaptive and a contributor to diabetes and obesity. Native Americans have a very high rate of diabetes which could beassociated with the fact that they were part of a population with many “thrifty genotype” traits due to their old lifestyle, and due to the recent change in diet they are especially at risk. In addition to the ancestry view of “thrifty genotype”, recent studies also show that babies with poor nutritional conditions in early life select for “thrifty phenotype” which can also lead to increased rates of diabetes and obesity in adulthood. Could thrifty phenotype be epigenetic and passed on to offspring?

obesity_trends_20092

The obesity epidemic is a combination of all the above traits, and is associated with the transition from subsistence-level nutrition to modern-day industrial nutrition styles (processed foods, growth hormones, etc.). Thrifty genotype and phenotype are playing a huge role in populationsthat are just now gaining access to stable food supplies. Urbanization and rising incomes throughout the developing world have increased rates of overweight and obesity. Trends in US food use patterns the global trends. Energy consumed from soft drinks has increased 70% since the mid-1970’s. Available energy from vegetable oils has increased by 30% over that same time period. Other factors include the increase in eating away from home and snacking. Sugars, processed grains, and added fats are some of the cheapest food options, and with today’s bad economy poorer people are consuming more of these bad nutrients. Our modern environment has been characterized as “obseogenic”—that is, providing abundant food energy, while requiring little work or activity to produce that energy. What do you think about the obesity epidemic? Is genetics an excuse?

obesity-and-fastfood-nations

 

 

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Epigenetic Mechanisms, Quick &  Dirty

Jablonka & Raz (2009) show us this elegant illustration of broad and narrow epigenetic transmission.

Epigenetic inheritance in the broad sense is the inheritance of developmental variations that do not stem from differences in the sequence of DNA...information transference that can take place through developmental interactions between mother and offspring..., through social learning..., and through symbolic communication.

We...define cellular epigenetic inheritance as the transmission from mother cell to daughter cell of variations that are not the result of differences in DNA base sequence and/or the present environment.  Transmission can be through chromatin marks, through RNAs, through self-reconstructing three-dimensional structures, and through self-sustaining metabolic loops.

In the single-cell "bottleneck" variety of epigenetic inheritance (pathway a in the above diagram) Jablonka &  Raz focus on...

The environment may induce epigenetic variation by directly affecting the germline or by affecting germ cells through the mediation of the soma, but, in either case, subsequent transmission is through the germline.

Evolutionary Implications

According to Jablonka & Raz (2009), there are 5 effects of epigenetic mechanisms & inheritance vis-a-vis evolution:

(i) evolutionary change occurring through selection of epigenetic variants, without involvement of genetic variation; (ii) evolutionary change in which an initial epigenetic modification guides the selection of correlated genetic variations; (iii) evolutionary change stemming from the direct effects of epigenetic variations and epigenetic control mechanisms on the generation of local and systemic epigenomic variations; (iv) evolutionary change resulting from the constraints and affordances that epigenetic inheritance imposes on development; and (v) evolutionary change that leads to new modes of epigenetic inheritance.

Siberian Silver Fox Experiments

The Siberian silver fox experiments are so cool, & often cite them as an example of gene linkage.  Honestly, I was just BSing in suggesting that the curly tails, rounded nose, etc. were possibly linked on the same chromosome to tameness & recognized that there might be other factors involved.  Lo & behold, a citation in Jablonka & Raz (2009) pointed us toward epigenetic studies to come out of that body of research.

Cute & cuddly silver foxes
Cute & cuddly silver foxes

It turns out that the coat spotting & non-spotting variation that we associate with domestication occurs too quickly to be pure mutation, though it behaves like a dominant & semi-dominant trait, & couldn't be explained by inbreeding because the inbreeding coefficient was too low (0.03).  Instead, they believe

the stress of domestication and selection for tameness targeted genes with large effects in the neuro-hormonal system...and may have heritably reactivated some of them...This epigenetic interpretation, in terms of new epimutations rather than new mutations, explains the high rate of appearance and disappearance of some phenotypes, and support for this comes from the fact that at least two of the genes (Agouti and C-kit) that seem to be involved in the changes are known to have heritable epigenetic variants in mice...

One aspect of epigenetics that seems important here is the concept of canalization, introduced by Waddington several decades back (he also introduced the concept of epigenetics in general, which everyone rightly thought was Lamarkian & wrongly ignored--turns out he was on the money).  Roughly, canalization means that some environmental perturbation pushes a phenotype into a canal or valley, whereafter selection pressures prevent the phenotype from returning to its previous state because the "climb" up the sides of the canal or out of the valley are too steep.  Think of a marble on a tabletop that is essentially flat but has a valley to one side of it.  Stochastic chance dictates that the marble can roll any which way, but if it happens to roll toward the valley, it gets stuck there & can only roll further in the valley.  Or as this image illustrates, there are several possible environmental variations possible, but once a phenotype goes one way (plastically), it cannot go back.

So it seems to be with the silver foxes.  Once an environmental condition pushes silver foxes (or wolves before them) one way (luring tame ones to their yummy debris & handouts) or another (spooking the nervous ones to run away), a cascade of epigenetic mechanisms pushes them further along.  At that point, according to this model, tame ones cannot become anxious/aggressive & vice versa.

While cute silver foxes that you can cuddle with get all the press, the less publicized but equally fascinating is the aggressive foxes that want to rip your face off.

Aggressive domesticated silver fox Courtesy of Lyudmila Trut / Institute of Cytology andGenetics / The Siberian Division of the Russian Academy of Sciences (Source: Dugatkin 2003, http://www.dana.org/news/cerebrum/detail.aspx?id=2922)
Aggressive domesticated silver fox Courtesy of Lyudmila Trut / Institute of Cytology andGenetics / The Siberian Division of the Russian Academy of Sciences (Source: Dugatkin 2003, http://www.dana.org/news/cerebrum/detail.aspx?id=2922)

So what's going on with these aggressive foxes?  According to Popova (2006), there are at least 16 genes that influence aggression, but most aggression behavior is influenced by just a few of those.  A major player seems to be serotonin (5-HT).  The 5-HT pathway in the brain suppresses aggression.  5-HT is not a gene though, it is a hormone; & genes code for proteins.  So if there's a gene change, what is/are the gene(s)?  It could be any gene that produces an enzyme involved in the essential mechanisms of the 5-HT system, which include synthesis/degradation, reuptake in the synaptic cleft, & density/sensitivity of receptors (for more background on 5-HT, I've written on this before here).  As the figure below illustrates, there are enzymes that catalyze serotonin synthesis (TPH & decarboxylase of aromatic l-amino acids), two enzymes that help break serotonin down (MAO A & B), & an enzyme (SERT) that transports serotonin.

There are two TPH genes, & it is the 2nd one (TPH2), expressed in the brain & responsible for the central nervous system, that effects 5-HT & seems to be responsible for aggressive behavior.

Silver foxes displaying friendly responses to human contact were shown to have higher 5-HT and 5-HIAA levels, and higher TPH activity in the midbrain and hypothalamus in comparison to nonselected wild-type silver foxes bred in captivity. Importantly, the changes were found in the midbrain representing the area of main location of TPH2-synthesizing cell bodies.  These findings were interpreted as an indication of an increased activity of the brain 5-HT system in the tame animals and, subsequently, a decreased activity of this system in highly aggressive animals.

MAO A has a higher affinity for 5-HT & is considered the principle enzyme in breaking down serotonin.  When MAO A is disrupted in mice, they get more aggressive.  Deletion of SERT (the transporter that allows 5-HT molecules not taken up by post-synaptic receptors to be recycled & reused) in knockout mice also produces aggressive behavior.  Finally, there are 14 different subtypes of 5-HT post-synaptic receptors.  Genetically low aggression has been associated with increased expression of specific subtypes of these receptors in the midbrain & specific densities & function in specific regions of the brain.  These likely function to suppress aggressive behavior.

The figure below depicts this as essentially two pathways, which we can compare analogously to the Jablonka & Raz depiction of the narrow "bottleneck" pathway, albeit via two cells (or genes).  I think.

If any one of these mechanisms or either of these pathways influences aggression, they will interact with the environment to mutually reinforce themselves & push the marble down toward the other pathway too.  In other words, if the stress of domestication bumped the marble off the plane, having even only a slightly higher tendency of aggression relative to tameness will result in amplification of the entire aggression pathway, even if the environmental conditions of captivity are thereafter removed (i.e., the animal is released).  What still remains to be clarified is how the initial brain changes occur & the roles of other mechanisms in the system.

Zane Thayer & Chris Kuzawa review data that offer a clue.  They point out that "psychosocial stress contributes to the social gradient in health" (2011:799).  This is well-established by now, but the mechanisms are interesting.  In two studies particularly relevant to our question of how the stress of domestication may influence aggression in silver foxes, childhood abuse was associated w/ methylation differences at the GR (glucocorticoid) locus in the hippocampus & the serotonin transporter protein (SLC6A4) locus.  Another study found that maternal depression during pregnancy predicted stress reactivity & methylation of the GR locus in buccal cells of their infants 3 months after birth.  Methylation is the addition of a methyl group to a substrate or substitution of an atom by a methyl group.  This can take place in DNA or proteins.  In DNA, it can result in the change of an amino acid base, thus a change in the genetic code resulting in production of different or altered proteins.  In proteins, it results in regulatory changes in the protein functions, so methylation can have wide-ranging effects.  The methylation of the GR receptor locus may affect things like the binding of glucocorticoids to the receptor.  Glucocorticoids are best known for their role in stress response, but, relevant to this discussion, they are also operative in memory consolidation and learning, as contextual fear conditioning, among many other functions.  The SLC6A4 serotonin transporter protein terminates action of serotonin in the synapse & recycles it, which is a key function in mood stabilization.  Low serotonin is associated with high fear response.

So the psychosocial stress of domestication in some silver foxes could result in methylation of glucocorticoid & serotonin receptors, directly influences fear/aggression response in pups, that persists throughout their lives.  It can also influence depression in mothers that is passed on in the receptor activity of their pups.  We can link this with Larry Schell's model of risk-focusing.  Replace "SES" with "personality."  Fear/aggression in mothers increases the risk for fear/aggression in descendants, as tameness in mothers increases the risk of tameness in descendants.

Larry Schell's risk-focusing modelAlthough the model above suggests gene line changes, the broad epigenetic view suggests that some of these influences may not influence the germ yet still persist over multiple generations because of the influence that maternal disposition has on offspring gene expression.  Or, as Thayer & Kuzawa note

...Genes are regulated by biological "memories" of experiences acquired earlier in our own lives, and even by recent predecessors... (2011:801)

I'm really excited about having the opportunity to have my genes tested with my Biology, Culture, & Evolution class. I am looking forward to learning more about my ancestry. All I know is that at some point my ancestors lived in Ireland and Scotland, and on my mother's side my great-great grandmother was a Choctaw Indian. I think it will be really cool to find out more about my ancestry even further back than that. I also think it will be really beneficial for me to know which diseases I'm at risk for. High blood pressure and cardiovascular disease run in my family on both my maternal and paternal side. If I am at greater risk for that, I can make sure to maintain a healthy diet and exercise in order to prevent those diseases. However, I am a little worried about finding out that I may be at a greater risk for developing Alzheimer's because my granddad on my mother's side had it. If I am at a higher risk for it, there isn't anything I can do to prevent it. I'm haven't decided if it will be a good thing to know ahead of time, or if it would just cause unnecessary worrying.
I think it will also be advantageous to know which types of medication would work best for me and which ones I should avoid. I do think it would be beneficial to know what diseases I am a carrier for so that I can be aware of any problems that might occur when I do have children.

8

About the Authors

Dr. Dennis O'Rourke is the Interim Chair of the Anthropology Department at the University of Utah in Salt Lake City, Utah. He is also the Co-Chair of the International Review Board and Vice President of Research. Dr. O'Rourke received his BA (with Honors), MA and PhD in Anthropology  from the University of Kansas in 1973, 1976 and 1980. He then went on to his Post-Doctoral fellowship in the St. Louis School of Medicine at Washington University. There he focused on Psychiatry/Genetic Epidemiology.

As mentioned before in @rebeccaleon blog from week one of class;  Dr. O'Rourke work focuses on the sampling and analysis of ancient DNA,  quantitative methods, and population and evolutionary genetics. The areas and populations that he focuses on are native America, and the North American and Siberian Arctic.

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Picture of Jake Enk cutting a Mammoth tooth.

Jake Enk is a Doctoral student at McMatser University in Hamilton, Ontario. He received his Masters at The University of Utah.

In reading this chapter of Human Biology (Chapter 4), the authors bring into focus the effects of genetics on human populations in reference to geography and how understanding these topics can be, at best, informative to the history of our species and, at worst, complicated or even convoluted. The equations aside (which are, admittedly,  of great importance in the derivation of these data), in this chapter we delve deeply into understanding the mechanics of mutation , the interplay between human migration and genetic information, and how these factors shape our, often debated, view of how human populations came into existence and continued to move around the globe.

The chapter begins with explanations of basic evolutionary factors, specifically how mutations come to prominence in populations and how not only natural selection, but population size directly affects how readily mutations take hold. In somewhat simpler terms, the smaller the population, the more readily the frequency of an advantageous allele will increase within that population (There’s a convenient figure on p.108 that lays it out for you). As interesting as data acquisition and these formulas are, the real meat of Chapter 4 lies within the application of these methods to understanding our species’ history and how this data can better serve that purpose.

To begin, we can challenge, disprove, or validate many earlier assumptions of this field through the use of genetics as evidence. One great example Chapter 4 illustrates is Hrdlička’s hypothesis of a replacement population in the Aleutian Islands roughly a thousand years ago. His 1945 work, based mainly on differences in cranial forms, hypothesized an existing Aleutian population being replaced by a wave of newcomers who would become the ancestors of modern Aleuts. While his assumptions weren’t definitive beyond a shadow of a doubt, modern genetic testing (after a few upsets with unfortunately small sample sizes) has proven his work correct, contributing even more validity to our understanding of human history.

Sometimes, however, this availability of new data can obscure what was previously seen as a clean-cut understanding of that history. The authors bring up the issue of the presence of Neanderthal genetic information in modern human and the debate as to how it got there, if it is Neanderthal DNA in the first place. Sequencing of Mitochondrial DNA from Neanderthals show incredible variation from modern humans and ancient anatomically modern humans (supporting that long-held belief that humans and Neanderthals never interbred), yet nuclear genome sequences suggest that one to four percent of our genome may be made up from Neanderthal DNA, possibly from admixture. So did early humans and Neanderthals mate, or could these traits be the result of latent genetic traits that we both received form an ancient ancestor? The answer: we don’t know. Maybe both. Perhaps you’ll be the ones to figure it out.

Finally, this chapter shows how even our understanding of genetic data in relation to prehistoric human events can be far more complicated than that data would initially suggest. In the example of the populating of Europe, differences in mitochondrial DNA and nuclear molecular DNA, which seem incompatible, illustrated a far richer story. In this case, the mitochondrial DNA suggests a movement of humans into Europe from the Middle East, roughly 10,000 years ago, coinciding with the advent of agriculture. The nuclear molecular DNA evidence, however, suggests something much different. Rather than 10,000 years ago, this evidence suggests a divergence in populations from 46,000 to 130,000 years ago. So which evidence is correct? Both. By comparing and combining these data, human biologist now surmise that while a clinal migration from the Middle East into Europe did take place around 10,000 years ago, Europe had already been populated by Homo Sapiens long before that event.

All in all, this chapter is helpful in illustrating the importance of genetic information and its contribution to our understanding of the human story. If you want to know the specifics of how DNA evidence is used to formulate new hypotheses and support or refute existing ones: this is the chapter for you... But don’t take our word for it! (Insert Reading Rainbow theme here)

1) What do you believe the current genetic distance is? Is this a good thing or bad thing?

2) What are some current examples of migration? And how have they influenced the current gene population?

1

About the Authors

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Mark L. Weiss, Ph.D.

Professor

Department of Anatomy and Physiology at Kansas State University

Professor Weiss is part of the KSU Stem Cell Biotechnology Research team where his current work has been to focus on characterizing non-embryonic stem cells that have been discovered in the umbilical cord matrix and rat embryonic stem cell.  The point of this research is to characterize the role of human and animal umbilical cord matrix stem cells to reverse the behavioral deficits in a rat model of Parkinson’s disease.  Dr. Weiss received his Ph.D in biology from the University of Pennsylvania (1986) and in his postdoctoral from Michigan State University (1986-1989).

Justin Tackney

Ph.D. Graduate Research Assistant

Department of Anthropology at the University of Utah

Justin Tackney is a Ph.D. graduate student studying social and behavioral sciences at the University of Utah

Basic Genetics

  • Basic concepts formed by Gregor Mendel (1822-1884)
  • Mendel conducted series of experiments on the passage of traits from generation to generation of pea plants.
  • Key Terms created by Mendel:
    • Gene- The basic unit of inheritance; each parent contributes on copy
    • Diploid State- cells that contain two genes for each trait
    • Mendel’s Principle of Segregation- States that each sexually reproducing organism has two genes per trait, but only one pair of each gene is passed on to the offspring of the parents.
    • Meiosis- The reproduction of haploid gametes (sex cells)
    • Mitosis- Process of cell division producing cells with same number of chromosomes as parental cell
    • Recombination- Process of forming associations of genes at different loci after chromosomal crossing over
    • Allele- Various forms of a gene
    • Homozygous- Possessing two identical alleles for a trait
    • Heterozygous- Possessing two different alleles for a trait
    • Genotype- Combination of alleles that one posses for a specific trait
    • Phenotype- Visible appearance of trait
    • Dominate Trait- When an allele masks the presence of another allele
    • Recessive Trait- The allele that is masked by the dominate trait.
    • Codominate Trait- When an allele is neither dominate or recessive. Both are expressed in the phenotype.
    • Genes are composed of nucleic acids
      • Two kinds
        • DNA (deoxyribonucleic acid)- the genetic material for most species
        • RNA (ribonucleic acid)- nucleic acid where the sugar is the backbone and substitution of uracil for thymine
          • These nucleic acids transmit information
          • In humans, DNA is the genetic material and the RNA helps the DNA carry out tasks

DNA’s Two Roles

  1. It must be able to transmit information from one generation to the next
  2. Directing the production of proteins

DNA Replication

  • DNA Replication is semiconservative. This means that one double strand of DNA serves as the basis for making two double strands; each of the new double strands contains one old and one new strand of DNA.
  • DNA is able to reproduce with few errors due to the constructing pairing
  • Mutation- Alteration in the DNA sequence

DNA Makes Protein

  • Most genes direct the production of polypeptide chains. These are then assembled into protein s
  • Proteins- molecules constructed out of amino acids
    • Twenty amino acids are arranged into protein molecules
    • Thymine pairs with adenine (T-A)
    • Cytosine pairs with guanine (C-G)
    • DNA is located in the nucleus of the cell
    • Proteins and ribosomes are assembled in the cytoplasm
    • Transcription- process by which mRNA is constructed
    • DNA is constructed in a double helix
    • Transcription- Process of converting DNA into RNA
    • Translation- Process of converting mRNA into amino acid.

Studying the DNA

  • Major developments in the study of DNA
    • Restriction enzymes- Due to their discovery the cutting of DNA is now more predictable. This allows one to produce recombinant DNA.
    • Polymerase chain reaction (PCR)- Technique that allows researchers to reproduce almost limitless copies of DNA by using only one piece of it.
      • This has been helpful to anthropologists who deal with ancient bone and tissue

Gene Structure

  • Eukaryotes- Organisms with a nucleus
    • Split into alternating DNA segments
    • Exon- DNA sequence that is expressed
    • Interons or IVSs- unexpressed sequences
  • Prokaryotes- Organisms without a nucleus

Gene Families

  • Some genes are similar to other in their functions and evolutionary history
  • Gene families help to connect one species to another throughout evolution
  • Gene families can be analyzed for gene duplication and divergence

Other sorts of DNA

  • DNA is contained inside chromatin because all of our DNA cannot fit into a nucleus
  • Purifying selection- method for identifying functional genome sequences

Pseudogenes

  • Non-functional DNA sequence that has a resemblance of a functional gene
  • May represent the relics of once- functional genes that experienced a mutation that prevented their expression

Repetitive DNA Sequences

  • Sometimes referred to as satellite DNA
  • Three types
    • a-satellite DNA
    • minisatellite DNA
    • microsatellites or short tandem repeats (STRs)

Mitochondrial DNA

  • Referred to as mtDNA
  • Maternally inherited DNA found in the mitochondria
  • Resent study of mtDNA in the mid-1980’s indicated an earlier divergence of human groups
  • Allowed for a common ancestor in Africa to be discovered
  • Used to help reconstruct the human population history
  • Used to decipher interplay between social rules and genetic phenomena

The Y Chromosome

  • Y contains few genes and is small in size compared to the X chromosome
  • The Y chromosome in chimps and gorillas does not have the same sequences as humans
    • This implies they have gone under alterations since humans diverged

Comparing the Human and Chimpanzee Genomes

  • Differences
    • Human chromosome 2 evolved from two ancestral chromosomes still present in apes
    • There are none pericentric chromosomal inversions in humans but not chimps
    • Human chromosomes have human-specific heterochromatin additions and apes do not have this

Pattering in the Genes

  • Genetic variants in human can be used to trace back lines of evolutionary relatedness between populations and other parts of human population history
  • Evidence that a small number of critical genetic changes cause significant change in a phenotype.

Rise of Genomics

  • Genomics helps give a better understanding of what defines us as Homosapiens. This can help us trace back our lineage.

Genetics and the Evolution of the Modern Human Brain

  • Researchers are hoping to eventually discover what lead to humans having distinctively large brains
    • This trait cannot be limited to protein- coding loci and must extend to proteins that regulate the expressions of proteins

two trees

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