🦍Biological Anthropology Unit 6 – Human Variation and Adaptation

Key Concepts and Terminology

• Human variation refers to the differences in physical, genetic, and behavioral traits among individuals and populations
• Adaptation is the process by which organisms become better suited to their environment over time through natural selection
• Genetic drift is the random fluctuation of allele frequencies in a population due to chance events
• Gene flow is the transfer of genetic material between populations through migration and interbreeding
• Founder effect occurs when a small group of individuals establishes a new population, leading to reduced genetic diversity
• Bottleneck effect is a sharp reduction in population size that can lead to decreased genetic variation
• Polymorphism is the presence of two or more distinct forms or alleles of a gene within a population
• Cline is a gradual change in a trait or allele frequency across a geographic area

Genetic Basis of Human Variation

• Human genetic variation arises from mutations, recombination during meiosis, and the shuffling of alleles during sexual reproduction
• Single nucleotide polymorphisms (SNPs) are the most common type of genetic variation, involving a change in a single DNA base pair
• Copy number variations (CNVs) involve the duplication or deletion of large segments of DNA, contributing to genetic diversity
• Epigenetic modifications, such as DNA methylation and histone modifications, can alter gene expression without changing the underlying DNA sequence
• Heritability is the proportion of phenotypic variation in a population that is attributable to genetic variation
  • Heritability estimates can help determine the relative contributions of genes and environment to a trait
• Polygenic traits are influenced by multiple genes, each with a small effect on the phenotype (height, skin color)
• Pleiotropy occurs when a single gene influences multiple seemingly unrelated traits (sickle cell anemia and malaria resistance)

Environmental Influences on Human Adaptation

• Climate and temperature can drive adaptations in body size and shape (Bergmann's rule, Allen's rule)
• Altitude influences physiological adaptations, such as increased hemoglobin levels in high-altitude populations (Tibetans, Andeans)
• Diet and nutrition can lead to adaptations in digestive enzymes and metabolic pathways (lactase persistence, amylase gene copy number)
• Pathogen exposure can drive adaptations in immune function and disease resistance (sickle cell anemia, CCR5 mutation)
• Cultural practices, such as clothing and shelter, can buffer environmental pressures and influence the direction of adaptation
• Niche construction is the process by which organisms modify their environment, creating new selective pressures for adaptation
• Gene-environment interactions occur when the effect of a gene on a trait depends on the environment in which it is expressed

Types of Human Adaptations

• Morphological adaptations involve changes in physical features, such as body size, shape, and color (skin pigmentation, nose shape)
• Physiological adaptations are changes in the functioning of organs and systems to maintain homeostasis (sweating, shivering)
• Behavioral adaptations are changes in behavior that improve an individual's chances of survival and reproduction (tool use, social cooperation)
• Genetic adaptations involve changes in the frequency of alleles that confer a selective advantage in a particular environment (lactase persistence, sickle cell anemia)
• Developmental adaptations are changes in the timing or trajectory of development in response to environmental cues (plasticity, acclimatization)
• Acclimatization is a short-term physiological response to environmental changes that occurs within an individual's lifetime (high-altitude acclimatization)
• Adaptation can occur at different levels of biological organization, from molecules to populations

Case Studies in Human Variation

• Skin pigmentation varies across human populations in response to UV radiation levels, balancing the need for vitamin D synthesis and protection against folate degradation
  • Populations near the equator have darker skin, while those at higher latitudes have lighter skin
• Lactase persistence, the ability to digest lactose in adulthood, has evolved independently in several populations with a history of dairy farming (Europeans, East Africans)
• High-altitude adaptations, such as increased oxygen uptake and delivery, have evolved in populations living in the Andes, Himalayas, and Ethiopian highlands
  • Tibetans have a unique genetic variant (EPAS1) that helps them maintain high blood oxygen levels
• Sickle cell anemia, a genetic disorder caused by a mutation in the hemoglobin gene, confers resistance to malaria in heterozygous individuals (West Africans)
• The Duffy null allele, which confers resistance to Plasmodium vivax malaria, is nearly fixed in many African populations
• The CCR5 mutation, which provides resistance to HIV infection, is more common in European populations
• Inuit populations have adaptations for a high-fat diet and cold tolerance, including a more compact body shape and increased brown adipose tissue

Measuring and Analyzing Human Diversity

• Anthropometric measurements, such as height, weight, and body proportions, can be used to quantify human physical variation
• Genetic markers, such as SNPs and microsatellites, can be used to estimate genetic diversity within and between populations
• F-statistics (FST) measure the degree of genetic differentiation among populations
  • FST values range from 0 (no differentiation) to 1 (complete differentiation)
• Principal component analysis (PCA) is a statistical method used to visualize patterns of genetic variation in multidimensional space
• Admixture analysis can be used to estimate the proportions of ancestry from different source populations in admixed individuals
• Genome-wide association studies (GWAS) can identify genetic variants associated with specific traits or diseases
• Ancient DNA analysis can provide insights into the genetic diversity and population history of past human groups

Evolutionary Perspectives on Adaptation

• Natural selection is the primary mechanism driving adaptation, favoring traits that increase an individual's fitness in a given environment
• Balancing selection maintains genetic variation in a population when heterozygotes have a fitness advantage (sickle cell anemia)
• Directional selection shifts the mean value of a trait in a population over time (skin pigmentation)
• Stabilizing selection reduces variation in a trait by favoring intermediate values (birth weight)
• Disruptive selection favors extreme values of a trait, leading to a bimodal distribution (beak size in finches)
• Genetic drift can lead to the fixation or loss of alleles by chance, especially in small populations
• Gene flow can introduce new alleles into a population or homogenize genetic variation between populations
• Adaptation is a continuous process, as environments and selective pressures change over time

Implications for Modern Human Health

• Understanding the evolutionary history of human populations can inform the diagnosis and treatment of genetic diseases (sickle cell anemia, cystic fibrosis)
• Identifying genetic variants associated with disease risk can lead to personalized medicine approaches (BRCA mutations and breast cancer)
• The mismatch hypothesis suggests that some modern health problems arise from a discordance between our evolutionary adaptations and current environments (obesity, diabetes)
  • For example, the thrifty gene hypothesis proposes that genes that promoted energy storage in times of feast or famine may now contribute to obesity in environments with abundant food
• Studying the microbiome and its co-evolution with human hosts can provide insights into the role of gut bacteria in health and disease
• Investigating the genetic basis of drug metabolism can help optimize dosing and reduce adverse reactions (cytochrome P450 enzymes)
• Evolutionary medicine applies evolutionary principles to understand the origins and treatment of human diseases (antibiotic resistance, cancer)
• Integrating evolutionary perspectives into public health interventions can improve their effectiveness and sustainability (HIV prevention, vaccination strategies)