11. Genome Evolution

Genetics and Human Adaptive Traits

Learning Objectives

  1. Terminology

    1. Selection Pressure -

    2. Heterozygous Advantage -

    3. Fixation of an Allele -

    4. Non-coding DNA Enhancer -

    5. Transcription Factor -

    6. Convergent Evolution -

    7. Locus Control Region -

  2. Diseases/Conditions

    1. Lactase Persistence

    2. High Altitude Adaptations

    3. Red-Green Color-Blindness

    4. Photoreceptor Degeneration

  3. Fitness Advantage of Mutations

Overview

  • Fundamental concepts of genetics and evolution.

  • Examples of human genome evolution including adaptations to environments and diets.

  • Key themes: regulatory changes in the genome and their implications for human traits.

  • Learning goals include key concepts relevant for exams.

Genome Evolution

  • Definition: Changes in genetic material (genome) over time.

  • Mutation Rate: Mutations occur at a relatively fixed rate within populations.

  • Hardy-Weinberg Equilibrium: p² + 2pq + q² = 1, where population allele frequencies remain constant in the absence of evolutionary pressures.

  • Evolutionary Success: Measured by the increased representation of a mutation in future generations.

Hardy-Weinberg Assumptions

  1. No mutation.

  2. Random mating among individuals.

  3. No migration into or out of the population.

  4. Large population size.

  5. No selection advantage for any allele.

Natural Selection

  • Concept: "Survival of the fittest" - organisms best adapted to their environment are more likely to survive and reproduce.

  • Genetic Level: Traits providing advantages are linked to increased allele frequencies in populations over generations.

  • Selection Pressures: Include factors like predation, disease, resource availability, climate, and competition.

Evidence of Selection in the Genome

  • Selection causes local changes in neutral genetic variation relative to neutral regions shaped by genetic drift.

  • Observations: More synonymous mutations and less recombination than expected in advantageous alleles.

Genetic Drift

  • Definition: Changes in allele frequencies due to random sampling over generations.

  • Influenced by random mating and mutations.

  • Perturbations:

    • Founder Effect: When a small group starts a new population.

    • Population Bottleneck: Dramatic reductions in population size.

    • Gene Flow: Introduction of alleles from outside groups.

Selection Pressure Variability

  • Selection pressure is not uniform across the genome; different regions experience various pressures.

  • Distinctions between: Autosomes, Sex Chromosomes, Organelle DNA (e.g., mtDNA), Coding vs. Non-coding DNA.

Adaptive Genetic Features

Lifespan of Alleles

  • Lethal alleles may persist due to post-reproductive phenotype emergence, Hardy-Weinberg violations, or heterozygote advantages (e.g., Sickle-cell trait, Cystic Fibrosis).

Example of Dietary Adaptations: Lactase Persistence

  • Lactase Gene (LCT): Most mammals express lactase early and reduce it later, but some populations retain lactase into adulthood (lactase persistence).

  • Mutation Impact: Mutation in enhancer regions linked to lactase gene expression.

  • Geographic Variation: Lactase persistence frequency varies globally and reflects cultural dietary practices.

Human Evolution: High-Altitude Adaptations

  • Key physiological changes for life at high altitudes include increased red blood cell (RBC) production, improved lung capacity, and vascular adaptations.

  • Case Studies: Andean and Tibetan populations exhibit different adaptations to low oxygen environments (hemoglobin variations).

  • HIF2A Gene Modifications: Involves specific non-coding mutations influencing gene expression under hypoxic conditions.

Color Vision Evolution

  • Red-Green Colorblindness: An X-linked recessive trait affecting primarily males while females can be carriers.

  • Opsins Mechanism: Opsins undergo conformational changes in response to light, crucial for color detection.

  • Trichromatic Vision: Enhanced vision in some primates through opsin mutations, showing evolutionary adaptations for color perception.

  • Different mechanisms like unequal crossing over can lead to color vision impairments, including colorblindness.

Locus Control Region (LCR) Role in Opsin Expression

  • LCR ensures that only one type of opsin is expressed in cone cells for clarity in vision.

  • The deletion of LCR can have detrimental effects on opsin expression leading to vision issues, such as degeneration of photoreceptors.

Key Concepts for Review

  • Terminology: Selection pressure, heterozygous advantage, fixation of an allele, non-coding DNA functionality.

  • Conditions and Diseases: Lactase persistence, adaptations to high altitudes, and X-linked disorders related to color vision.

Important Theories and Definitions

  • Mutations are preserved favorably when they provide beneficial advantages, influenced by cultural and physiological contexts.

  • Identification of similar mutations across populations demonstrates evolution acting via non-coding regulatory DNA, minimizing disruption of essential gene functions.

  • MCAT Relevant Terms: Biometry, natural selection, evolutionary metrics in genome changes, genetic drift, population bottlenecks, and founder effects.