🦍Biological Anthropology Unit 2 – Evolutionary Theory and Genetics

Key Concepts and Terminology

• Evolution the change in heritable characteristics of biological populations over successive generations
• Natural selection the process through which populations of living organisms adapt and change, first described by Charles Darwin
• Genetics the study of genes, genetic variation, and heredity in organisms
• Adaptation a trait that increases an organism's survival and reproduction in a specific environment
• Speciation the formation of new and distinct species in the course of evolution
• Fossil the preserved remains or traces of ancient life forms
• Hominins a taxonomic tribe that includes modern humans and our extinct ancestors and relatives
• Phylogeny the evolutionary history and relationships among groups of organisms

Historical Context of Evolutionary Theory

• Pre-Darwinian ideas about evolution existed, such as Lamarckism which proposed that organisms could pass on characteristics acquired during their lifetime to their offspring
• Charles Darwin's voyage on the HMS Beagle (1831-1836) provided him with key observations and insights that led to the development of his theory of evolution by natural selection
  • Darwin observed the diversity of life and noticed patterns of similarity and difference among species
  • He collected specimens of plants and animals from different regions, noting their adaptations to local environments
• Alfred Russel Wallace independently developed a similar theory of evolution by natural selection and corresponded with Darwin
• Darwin's "On the Origin of Species" (1859) laid out his arguments for evolution by natural selection and provided extensive evidence from his research and observations
  • The book was controversial at the time, challenging prevailing ideas about the immutability of species and the role of a divine creator
• Gregor Mendel's experiments with pea plants (1850s-1860s) laid the foundation for the field of genetics, although his work was not widely recognized until the early 20th century
• The modern synthesis (1930s-1940s) integrated Darwin's theory of evolution with Mendelian genetics, providing a more comprehensive understanding of the mechanisms of evolution

Mechanisms of Evolution

• Mutation the ultimate source of genetic variation, involving changes in DNA sequences
  • Point mutations single nucleotide changes in DNA
  • Insertions and deletions addition or removal of one or more nucleotides
  • Chromosomal mutations large-scale changes in chromosomes (duplications, deletions, inversions, translocations)
• Gene flow the transfer of alleles or genes between populations through migration or interbreeding
• Genetic drift random changes in allele frequencies over time, particularly in small populations
  • Bottleneck effect a sharp reduction in population size that can lead to loss of genetic variation
  • Founder effect occurs when a small group of individuals establishes a new population, resulting in reduced genetic diversity
• Non-random mating mating patterns that deviate from random mating, such as assortative mating (mating between individuals with similar traits) or disassortative mating (mating between individuals with dissimilar traits)
• Sexual selection a form of natural selection in which traits that enhance mating success are favored
  • Intrasexual selection competition within one sex (usually males) for access to mates
  • Intersexual selection mate choice by one sex (usually females) based on preferences for certain traits

Genetic Basics and Inheritance

• DNA (deoxyribonucleic acid) the hereditary material in organisms, consisting of four nucleotide bases: adenine (A), thymine (T), guanine (G), and cytosine (C)
• Gene a segment of DNA that codes for a specific protein or functional RNA molecule
• Allele one of the alternative forms of a gene that can occur at a particular locus
• Genotype the genetic makeup of an individual, typically referring to the specific alleles present at one or more loci
• Phenotype the observable characteristics of an individual, resulting from the interaction of its genotype with the environment
• Mendelian inheritance patterns of inheritance first described by Gregor Mendel, including:
  • Law of segregation alleles segregate during gamete formation, and each gamete carries only one allele for each gene
  • Law of independent assortment alleles for different genes assort independently during gamete formation
• Punnett square a diagram used to predict the genotypes and phenotypes of offspring based on the genotypes of the parents
• Incomplete dominance a pattern of inheritance in which the heterozygous phenotype is intermediate between the two homozygous phenotypes (snapdragons with pink flowers)

Natural Selection and Adaptation

• Fitness the ability of an individual to survive and reproduce in a given environment
• Directional selection favors extreme phenotypes, shifting the population mean in one direction (beak size in Galápagos finches during droughts)
• Stabilizing selection favors intermediate phenotypes, reducing variation in the population (human birth weight)
• Disruptive selection favors extreme phenotypes over intermediate ones, leading to a bimodal distribution (beak size in African seedcracker finches)
• Convergent evolution the independent evolution of similar traits in different lineages (wings in birds and bats)
• Divergent evolution the evolution of different traits from a common ancestor due to different selective pressures (Darwin's finches)
• Coevolution the evolution of two or more species in response to each other (flowers and their pollinators)
• Vestigial structures structures that have lost their original function but are still present in an organism (human appendix)

Human Evolution and Fossil Evidence

• Hominins the taxonomic tribe that includes modern humans and our extinct ancestors and relatives
• Ardipithecus (4.4-5.8 million years ago) early hominin with a combination of ape-like and human-like features
• Australopithecus (4.2-1.9 million years ago) early hominins with bipedal locomotion and relatively small brains (Australopithecus afarensis, "Lucy")
• Homo the genus that includes modern humans and our extinct relatives, characterized by larger brains and more advanced stone tools
  • Homo habilis (2.3-1.4 million years ago) the first species in the genus Homo, known for more advanced stone tools
  • Homo erectus (1.9 million-143,000 years ago) the first hominin to migrate out of Africa, with larger brains and more complex tools
  • Homo sapiens (300,000 years ago-present) the only surviving species of the genus Homo, with advanced cognitive abilities and cultural innovations
• Out of Africa hypothesis proposes that modern humans originated in Africa and migrated to other continents, replacing earlier hominin populations
• Multiregional hypothesis suggests that modern humans evolved from earlier hominin populations in different regions, with gene flow between populations

Modern Applications and Research

• Genome sequencing the process of determining the complete DNA sequence of an organism's genome
  • Has allowed for comparative genomic studies between humans and other species, revealing insights into evolutionary history and shared genetic traits
• Ancient DNA analysis the study of DNA recovered from ancient biological samples, such as fossils or preserved remains
  • Has provided evidence for interbreeding between modern humans and other hominins (Neanderthals, Denisovans)
  • Allows for the reconstruction of past population migrations and genetic diversity
• Phylogenetic analysis the study of evolutionary relationships among organisms based on genetic and morphological data
  • Helps to reconstruct the evolutionary history of species and identify common ancestors
• Genome-wide association studies (GWAS) examine the association between genetic variants and specific traits or diseases in populations
  • Can identify genes involved in complex traits and provide insights into the genetic basis of human variation
• Evolutionary medicine applies evolutionary principles to understand the origins and treatment of human diseases
  • Antibiotic resistance the evolution of bacteria resistant to antibiotics due to selective pressure from antibiotic use
  • Mismatch hypothesis proposes that some diseases arise due to a mismatch between our evolutionary adaptations and modern environments (obesity, diabetes)

Controversies and Debates

• Evolutionary psychology the study of the evolution of human behavior and mental processes
  • Critics argue that some evolutionary psychology explanations are speculative and difficult to test empirically
• Sociobiology the study of the biological basis of social behavior in animals and humans
  • Controversial for its application of evolutionary principles to human social behaviors, which some argue can be misused to justify social inequalities
• Intelligent design the idea that some features of the universe and living things are best explained by an intelligent cause rather than an undirected process like natural selection
  • Critics argue that intelligent design is not a scientific theory, as it cannot be tested or falsified using scientific methods
• Epigenetics the study of heritable changes in gene expression that do not involve changes to the underlying DNA sequence
  • Some researchers suggest that epigenetic inheritance may play a role in evolution, challenging the traditional view of inheritance based solely on DNA
• Evolutionary rates the speed at which evolutionary changes occur in different lineages
  • Punctuated equilibrium proposes that evolution occurs in rapid bursts followed by long periods of stasis, challenging the idea of gradual change over time
  • Molecular clock hypothesis suggests that the rate of molecular evolution is relatively constant over time, allowing for the estimation of divergence times between species