Human Variation, Adaptation, and Evolutionary Behavior Lecture Flashcards

Human Adaptations and Adaptive Plasticity

• Adaptation: Defined as a result of natural selection. It involves a change in allele frequencies within a population, making that population more fit for survival and reproduction in a specific environment.

• Acclimation: A lifetime developmental response. This involves physiological changes in an individual's body to suit a local environment. These changes are generally non-reversible.

• Acclimatization: The process by which an individual's body adjusts to environmental changes using short-term, reversible responses.

• Major Drivers of Local Adaptation in Humans: Human local adaptations are typically driven by three primary processes:     1. Migrations and environment.     2. Archaic admixture.     3. Agriculture.

Adaptations Driven by Migration: UV Light and Skin Pigmentation

• The Challenge of UV Light: UV irradiation is essential for normal biological functions, including:     * Vitamin D synthesis.     * Folate metabolism.     * Bone health.     * Neural tube formation.

• The Risk of UV Light: Excessive UV can damage DNA, leading to somatic mutations and cancer (melanoma).

• Acclimatization to UV Light (Tanning):     * Melanogenesis: A temporary and reversible increase in melanin production in response to UVB exposure.

• Selective Pressures on Skin Pigmentation:     * Vitamin D metabolism: Sufficient UV is needed to synthesize Vitamin D.     * Folate metabolism: UV can break down folate, which is essential for reproduction and development.     * Melanoma: Protection against skin cancer.

Adaptations to High Altitude

• The Challenge: At high altitudes, atmospheric pressure drops, leading to a decrease in oxygen concentration.

• Hypoxia: A condition characterized by a deficiency in the amount of oxygen reaching the tissues.

• Acclimatizations to High Altitude:     * Heavy breathing.     * Increased production of Red Blood Cells (RBCs).

• Physical Adaptation: Shorter stature is observed in some high-altitude populations.

Adaptations to Agriculture and Disease

• The Challenge: Malaria:     * Agriculture and Irrigation: Swidden (slash-and-burn) agriculture and irrigation created stagnant water, leading to increased mosquito populations, particularly in tropical climates.     * Infection: Malaria is a mosquito-borne infection caused by the Plasmodium parasite. Symptoms include fever, lethargy, vomiting, headaches, and can lead to death.

• Adaptation: Sickle-Cell Anemia:     * Mutation: A mutation in the hemoglobin gene replaces glutamic acid with valine.     * Mechanism: This results in the "sickling" of RBCs.     * Genetics:         * Individuals homozygous for the sickle cell allele develop Sickle Cell Disease.         * Individuals heterozygous for the sickle cell allele confer resistance to malaria.

Adaptations to Dietary Challenges

• The Challenge: Dairying:     * Dairy contains lactose, a sugar indigestible by most adult mammals.     * Fermentation: A process used to break down and preserve food using bacteria, yeasts, and microbes. In dairy, lactose is converted into glucose, galactose, and lactic acid.

• The Challenge: Limited Food Resources:     * In high latitudes, the number and variety of species available for food diminishes.     * Adaptation: Mutation in the fatty acid desaturase gene ($FADS$) allows for the metabolism of specific fats.

Case Studies in Free Diving and Environmental Acclimation

• The Moken:     * Austronesian-speaking people of the Andaman Sea archipelago (west coast of Thailand).     * Traditionally sea-faring, living off marine resources.     * Unique Trait: Children exhibit extraordinary underwater vision as an acclimation.

• The Haenyeos:     * Female divers in the Jeju province of South Korea.     * Livelihood involves harvesting marine resources up to a depth of $30\,m$.     * Acclimatizations: Heightened Basal Metabolic Rate (BMR) and non-shivering thermogenesis.

• The Bajau:     * "Sea Nomads" of Indonesia who have fished Southeast Asian seas from houseboats for over $1,000$ years.     * Adaptation: Enlarged Spleen: The spleen contracts during diving to release additional oxygenated RBCs into the bloodstream.     * Genetics: A mutation in the $PDE10A$ gene controls levels of the thyroid hormone $T_4$, influencing spleen size.

The Thrifty Gene and Developmental Origins Hypotheses

• Thrifty Gene Hypothesis:     * Proposes that hunter-gatherers endured cycles of "feast and famine."     * Natural selection favored genes for rapid nutrient storage.     * In the context of modern Western diets, these genes are now maladaptive (genetic anachronism).

• Problems with the Thrifty Gene Hypothesis:     * No empirical support; no specific "thrifty gene" has been found.     * Ignores that not all indigenous people were hunter-gatherers and not all Europeans were agro-pastoralists.     * Relies on genetic essentialism and fails to account for racial inequalities.

• Developmental Origins Hypothesis (Barker Hypothesis):     * Proposed by David Barker, an English epidemiologist.     * Observed that the poorest areas in the UK had high rates of infant mortality and late-onset coronary heart disease.     * Suggested that poor prenatal nutrition and low birth weight are primary causes.     * DOHaD (Developmental Origins of Health and Disease): Establishes a link between early life experiences, development, and later health outcomes.

The Dutch Hunger Winter ($1944\times 1945$)

• Event: Toward the end of WWII, Germany blockaded the Netherlands, cutting off food and fuel.

• Findings: Children born to mothers pregnant during the famine had:     * Lower birth weights.     * Higher rates of diabetes, obesity, and cardiovascular disease.

• Intergenerational Impact: The grandchildren of these women also exhibited lower birth weights and negative health effects, demonstrating "intergenerational biological memory."

Race, Health, and Epigenetics in the United States

• Social Context: In the context of colonialism, race is a major factor influencing social inequalities and poverty.

• Pre-term Birth Statistics:     * African American mothers have pre-term birth rates of $15\times 18\%$, which is more than double the rate of white mothers.     * Mothers immigrating from Africa to the US show normal pre-term birth levels in their home countries, but the rate rises upon entering the US.     * Post-$9/11$ Study: Women from the Middle East showed a $31\%$ increase in rates of low infant birth weight.

• Epigenetics:     * Chemical modifications of DNA or associated proteins (e.g., Cytosine Methylation) that regulate gene expression without changing the DNA sequence.     * Dutch Hunger Winter Epigenetics: Altered regulation of genes related to growth, nutrition, and lipid metabolism.     * Trauma: Children who experienced early life trauma or institutionalization show increased methylation of stress-related genes.

The Biological Impacts of Colonialism

• Settler Colonialism:     * A distinct form of colonialism where the settler intends to stay.     * Defined as a "structure, not a single event" (Patrick Wolf, $2006$).     * Involves land theft, genocidal violence, and enslavement.

• The "Wake":     * Concept by Christina Sharpe ($2016$) regarding life after slavery.     * Refers to the track behind a ship, keeping watch with the dead, and coming to consciousness.

• Historical Trauma Hypothesis:     * Cumulative emotional and psychological wounding across generations emanating from massive group trauma.     * Historical Trauma Response: A constellation of features (unresolved grief, Depression, PTSD, addiction) resulting from group trauma. Involves genetic and epigenetic effects.

Non-genetic Effects: Boarding Schools in North America

• Context: Late $19^{th}$ to mid-$20^{th}$ centuries; removal of Indigenous children for cultural assimilation ("Kill the Indian, Save the Man").

• Impact: Negatively impacted family structures and involved common physical and sexual abuse.

• Health Outcomes: Institutionalized children were more likely to have tuberculosis, arthritis, diabetes, anemia, high cholesterol, gall bladder disease, and cancer (Running Bear et al., $2019$).

Genetic Impact of Colonization in the Americas

• Population Decline: Massive decline due to European-borne pandemics.

• Evidence of Stress:     * Wilson Bands: Internal enamel micro-growth defects in teeth resulting from nutritional deficiencies or interruptions.

• Natural Selection on Immune Genes: Pandemics were so severe they drove selection on genes such as $MUC19$, $IL1R1$, and $HLA\text{-}DQA1$.

• Genetic Variation Patterns: Geographic variation reflects colonization history (e.g., an East-to-West gradient of European ancestry).

The Transatlantic Slave Trade and Genetic Impacts

• Scope: Approximately $12.5\,million$ people forcibly moved and enslaved from the $16^{th}$ to $19^{th}$ centuries.

• Regional Differences: Gene flow was shaped by colonial legal contexts (e.g., Spanish vs. English models).

• Sex Bias: African ancestry in the Americas is primarily from the maternal line.

• Pathogens: Genomic data recovered from early enslaved individuals includes Treponema pallidum sub. pertenue (syphilis) and Hepatitis B virus ($1,500\text{X}$ genome), both clustering with present-day African pathogens.

Human Variation and the Concept of Sex

• The Binary Sex Concept Assumptions:     1. Two discrete sexes: males and females.     2. Clear genetic, hormonal, developmental, reproductive, anatomical, physiological, and behavioral distinctions.

• Layers of Biological Sex: At birth, a baby has five layers of biological sex which may not all correspond.

• Intersex Variation:     * Defined as individuals born with sex characteristics that are not typically male or female.     * Up to $2\%$ ($1$ in $50$) of humans are intersex (Fausto-Sterling, $2000$), similar to the frequency of red hair or green eyes.     * Humans are "sexually polymorphic."

Sex Chromosome Structure and Inheritance

• Anatomy of Chromosomes:     * X chromosome: Larger, approx. $5\%$ of the haploid genome.     * Y chromosome: Shorter, approx. $2\times 3\%$ of the haploid genome.

• Pseudoautosomal Regions (PAR): Homologous regions where X and Y align and recombine during meiosis. Genes here are not sex-specific.

• Non-Recombining Region (NRY):     * Contains repeat sequences and "gene deserts."     * Key genes: $AZF1$ (azoospermia factor $1$), $SRY$ (sex-determining region), and $TSPY$ (testis-specific protein).

• $SRY$ and Sex Determination:     * Historically thought to be the sole sex-determining gene for males.     * Counter-genes for ovary development include $WNT4$ and $RSPO1$ (both on Chromosome $1$).

Variation in Sex Chromosomes and Cellular Sex

• Numeric Variations:     * Turner Syndrome ($45,X$).     * Klinefelter Syndrome ($47,XXY$).     * XYY Syndrome ($47,XYY$).     * Trisomy X ($47,XXX$).     * Tetrasomy X ($48,XXXX$).

• Mosaicism: When an individual has two or more genetically different populations of cells.     * $82\%$ of human embryos show some mosaicism.     * In adults over $50$, it is found in approx. $2\%$ of cases.

• Chimerism: Being composed of cells with more than one distinct genotype.     * Macrochimerism: Fusion of two zygotes.     * Microchimerism: Integration of cells from a fetus or partner (found in $50\times 70\%$ of parous women).

Gonadal and Hormonal Sex

• Gonadal Plasticity: Stability requires maintenance via DNA-binding proteins. Deactivating $FoxI2$ can turn oogenic cells spermatogenic; deactivating $Dmrt1$ can turn testicular cells ovarian.

• Hormonal Distribution: Testosterone and Estrogen distributions are bimodal, not binary. Everyone has both.

• Social Influence on Hormones:     * Testosterone rises in women in power and decreases in men in subordinate roles.     * Fatherhood: Testosterone decreases significantly after a child's birth in proportion to paternal investment.

• Clinical Variations:     * Complete Androgen Insensitivity Syndrome (CAIS): Individuals are $XY$ with $SRY$ but cells don't respond to testosterone; it is converted to estrogen.     * Congenital Adrenal Hyperplasia (CAH): Excess androgen production in $46,XX$ or $46,XY$ individuals.     * $5\text{-}\alpha$ Reductase Deficiency ($5\text{-}αRD$): Mutation in $SRD5A2$ (Chromosome $2$). Prevents conversion of testosterone to dihydrotestosterone. $46,XY$ individuals may be born without an external penis.     * Mayer-Rokitansky-Küster-Hauser (MRKH): $46,XX$ individuals with functional ovaries/breasts but absent/small vagina/uterus. Linked to $WNT4$ mutations.

Evolution of Sexual Behavior

• Muller's Ratchet: In asexual reproduction, harmful mutations accumulate. Sexual reproduction and recombination restore fit genotypes.

• Darwin's Theory of Sexual Selection:     * Intrasexual selection: Competition within one sex (usually males).     * Intersexual selection: Mate choice (usually females choosing attractive traits).     * Critiques (Huxley/Wallace): Argued many features stimulate reproduction or signal quality rather than being strictly for competition.

• Bateman's Principle ($1948$):     * Experiment on fruit flies (Drosophila melanogaster).     * Found higher variability in male reproductive success ($21\%$ of males had no offspring vs. $4\%$ of females).     * Concluded males are "promiscuous/eager" and females are "choosy/passive."

• Parental Investment Theory (Trivers, $1972$):     * The sex investing more in offspring (usually females) becomes choosy.     * Critiqued by Patricia Gowaty: Bateman's data had sampling biases and miscalculations; fly variances matched expected results of random mating.

• Sarah Hrdy's Findings: Focused on Hanuman Langurs. Showed females are promiscuous (to confuse paternity and prevent infanticide) and build alliances.

Sexual Dimorphism in Apes

• Gibbons: Little dimorphism; pair-bonded.

• Orangutans: Large dimorphism; solitary. Males exhibit bimaturism (two puberties: one for fertility, one for size/cheek flanges).

• Gorillas: Large dimorphism; single male, multi-female.

• Chimpanzees: Little dimorphism; multi-male, multi-female.

Genetics and Sexual Orientation

• Kin Selection: Behaviors promote survival of genetic relatives.     * Hamilton's Rule: rB > C (where $r$ = relatedness, $B$ = benefit, $C$ = cost).

• GWAS Study (Ganna et al., $2019$):     * Sample size: $477,522$ (UK Biobank) and $15,142$ (23andMe).     * ID'd $5$ autosomal loci related to olfaction and hormone regulation.     * Variants accounted for $8\times 25\%$ of variation in same-sex sexual behavior.