4.3 It’s All in the Genes! The Foundation of Evolution

Genetics plays a crucial role in human evolution, shaping our diversity and adaptability. Key concepts like genes, alleles, and DNA form the foundation for understanding how traits are inherited and expressed in populations.

Evolutionary forces like mutation, gene flow, and natural selection drive genetic variation. These processes, along with population genetics, explain how humans have evolved over time and continue to adapt to different environments.

Genetics and Human Evolution

Key genetic terms in human evolution

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• Genes are segments of DNA that code for specific traits and humans have approximately 20,000-25,000 genes
• Alleles are alternative forms of a gene that determine variations in inherited characteristics (eye color, blood type, hair texture)
• Genotypes refer to the genetic makeup of an individual, which is the combination of alleles inherited from parents and determines the potential traits an individual can express
• Phenotypes are the observable characteristics of an individual resulting from the interaction between genotype and environment (physical appearance, behavior, disease susceptibility)
• DNA (deoxyribonucleic acid) is the molecule that carries genetic information and is organized into structures called chromosomes

Mitosis vs meiosis in genetic variation

• Mitosis is cell division that produces identical daughter cells and maintains the genetic stability of somatic cells but does not contribute to genetic variation
• Meiosis is cell division that produces gametes (eggs and sperm), reduces the chromosome number by half (from diploid to haploid), and contributes to genetic variation through:
  1. Independent assortment of chromosomes
  2. Crossing over which is the exchange of genetic material between homologous chromosomes
• Meiosis generates new combinations of alleles in offspring

Mendel's laws and human diversity

• Law of Segregation states that each individual possesses two alleles for each gene and during gamete formation, alleles segregate, with each gamete receiving only one allele, explaining the inheritance of single-gene traits (widow's peak, attached earlobes)
• Law of Independent Assortment states that alleles for different genes assort independently during gamete formation, allowing for the independent inheritance of multiple traits and contributing to the wide range of human genetic diversity
• Punnett squares are diagrams used to predict the probability of offspring genotypes and phenotypes by illustrating the inheritance of single-gene traits based on parental genotypes
• Heredity is the passing of traits from parents to offspring through genetic material

Evolutionary forces on genetic variation

• Mutation refers to changes in DNA sequence and is a source of new alleles and genetic variation (sickle cell anemia, lactase persistence)
• Gene flow is the transfer of alleles between populations through migration and interbreeding, which can introduce new alleles or change allele frequencies in a population (spread of the Duffy null allele in African populations)
• Genetic drift refers to random changes in allele frequencies due to chance events and is more pronounced in small populations (founder effect, bottleneck effect)
• Natural selection is the differential survival and reproduction of individuals based on their genetic traits, favoring the propagation of advantageous alleles (resistance to malaria, adaptation to high altitudes)

Population Genetics and Evolution

• Evolution is the change in allele frequencies within a population over time
• Population genetics studies the distribution and change of allele frequencies within populations
• Genetic variation within populations provides the raw material for evolution through natural selection
• The combination of evolutionary forces acting on populations can lead to changes in allele frequencies and potentially the emergence of new species