Molecular Evolution

Learning Objectives

  • Briefly define evolution, natural selection and fitness

  • Describe the modern synthesis of genetics and evolution with respect to the frequencies of genetics variants being affected by selection, mutation, migration and genetic drift

  • Describe sequence conservation, where specific levels of conservation occur in genes and what we can use it for

  • Describe the basis aspects of phylogenetics, what a phylogenetic tree is and how it can be used

  • Describe processes of molecular evolution with respect to the globin gene clusters

  • Describe what a pseudogene is

Definitions: Evolution, Natural Selection, and Fitness

  1. Evolution:
    Evolution refers to the change in the heritable characteristics of biological populations over successive generations. These changes result from processes like mutation, genetic drift, migration, and natural selection. Think of it as the gradual transformation of life forms as species adapt to their environments over millions of years.

  2. Natural Selection:
    Natural selection is the process by which individuals with traits better suited to their environment have higher survival and reproductive success. Imagine a population of rabbits: faster rabbits escape predators and have more offspring, gradually making the population faster over generations.

  3. Fitness:
    Fitness is a measure of an organism's ability to survive and reproduce in its environment. In simpler terms, it’s like asking: How good is this organism at leaving behind offspring that also survive to reproduce?

The Modern Synthesis: Genetics and Evolution

The modern synthesis integrates Darwin's natural selection with Mendelian genetics, providing a unified framework for understanding evolution. Here's how genetic variant frequencies are shaped:

  • Selection: Traits that increase reproductive success (fitness) become more common over generations, e.g., positive selection increases advantageous alleles.

  • Mutation: DNA errors introduce new genetic variations. Most are neutral or harmful, but rare beneficial mutations can be amplified by natural selection.

  • Migration: Movement of individuals between populations introduces or removes alleles, altering genetic diversity.

  • Genetic Drift: Random fluctuations in allele frequencies occur, especially in small populations, leading to chance loss or fixation of alleles.

Sequence Conservation

  • Definition: Sequence conservation refers to DNA or protein sequences that remain largely unchanged across different species due to strong functional or structural importance.

  • Location:

    • High conservation: Active enzyme sites, structural protein regions, coding exons.

    • Moderate conservation: Promoters and untranslated regions.

    • Low conservation: Intronic and intergenic DNA.

  • Uses: Highly conserved sequences help identify critical functions and evolutionary relationships between species.

Phylogenetics and Phylogenetic Trees

  • Basics of Phylogenetics:
    Phylogenetics studies evolutionary relationships by analyzing genetic similarities and differences.

  • Phylogenetic Tree:
    A tree-like diagram showing evolutionary divergence between species. The closer two species are on the tree, the more recently they shared a common ancestor.

  • Applications:
    Used in classifying organisms, tracing disease origins (e.g., HIV), and studying evolutionary timelines.

Molecular Evolution of Globin Gene Clusters

  • Gene Duplication:
    The ancestral globin gene duplicated and evolved into two clusters: α-globin (chromosome 16) and β-globin (chromosome 11). These genes adapted to fulfill roles in different life stages: embryonic, fetal, and adult hemoglobins.

  • Fetal Hemoglobin (HbF):
    HbF (α2γ2) has a higher oxygen affinity than adult hemoglobin (HbA) to facilitate oxygen transfer in the womb. This arises from specific mutations and promoter evolution.

  • Pseudogenes:
    Some duplicated genes, like Ψβ, lose functionality due to disabling mutations. They no longer produce functional proteins but retain sequence homology with active genes.

Pseudogenes

  • Definition:
    Pseudogenes are remnants of once-functional genes rendered inactive by mutations like premature stop codons or regulatory sequence changes.

  • Role in Evolution:
    They illustrate how gene duplication can lead to redundancy, allowing one copy to evolve new functions while the other may decay.

Additional Concepts:

  • Creationism and Science:
    Creationism posits that life originates from a supernatural creator. While untestable by scientific methods, evolution makes falsifiable predictions and is supported by experimental and observational evidence.

  • Heterozygote Advantage:
    Explains the persistence of harmful alleles like those causing sickle cell anemia, as carriers (heterozygotes) gain resistance to diseases like malaria.

Let me know if you'd like to dive deeper into any concept! We can explore prerequisites such as molecular biology, genetics, or evolutionary principles further.