Population Genetics and Evolution

Population Genetics and Evolution

Introduction to DNA and its Role in Genetics

  • DNA Structure: All genetic information in cells is contained in the sequence of four subunits, called nucleotides, along the molecule deoxyribonucleic acid (DNA). The nucleotides are:

    • Guanine (G)

    • Adenine (A)

    • Thymine (T)

    • Cytosine (C)

  • Template for Protein Synthesis: DNA strands serve as templates from which cells manufacture proteins.

Coding in DNA

  • Amino Acids and Codons:

    • Proteins consist of chains of up to 20 different types of amino acids.

    • Each amino acid is encoded by a sequence of three nucleotides, known as a codon.

    • There are 64 different possible codons due to the combinations of four nucleotides taken three at a time, providing redundancy in the genetic code.

  • Substitutions and Mutations:

    • A change in one of the nucleotides of a DNA codon may or may not alter the amino acid coded for, depending on the nature of the codon change.

    • Substitutions: A specific type of mutation where one nucleotide is replaced with another.

    • Mutations: Can arise from random copying errors during DNA replication, via chemical agents, or exposure to ionizing radiation.

    • Many mutations have neither harmful nor beneficial effects, particularly if they occur in non-coding DNA sequences.

    • Changes occurring at the third codon position exhibit higher frequencies than changes at the first two positions, resulting in less likelihood of amino acid alteration.

Insights into Mutations

  • Effects of Mutations:

    • Most mutations are thought to be deleterious, as natural selection typically eliminates harmful alleles over evolutionary time.

    • New variant alleles from mutations may disrupt existing beneficial interactions between organisms and their environments.

    • Protein Properties Alteration: Mutant gene products may exhibit different properties than their wild-type counterparts, potentially affecting fitness.

Genotype and Phenotype Definitions

  • Genotype: The complete set of genetic characteristics determining the structure and functioning of an organism.

  • Phenotype: The physical expression of the genotype, encompassing both morphological and physiological characteristics.

  • Alleles: Different versions of a given gene may influence phenotype through variations in protein products.

    • Example: Variations leading to blue vs. brown eyes or enzyme efficiency differences.

Genetic Variation in Populations

  • Diploid Organisms:

    • Have two copies of all genes (excluding sex-linked genes).

    • Homozygote: An individual with identical alleles.

    • Heterozygote: An individual with two different alleles.

  • Dominant and Recessive Alleles:

    • In heterozygotes, one allele may completely mask the expression of the other, defining the masking allele as dominant and the masked allele as recessive.

Hardy-Weinberg Equilibrium Theory

  • Assumptions for Equilibrium:

    • Large population size

    • Random mating

    • No selection

    • No mutations

  • Equilibrium Proportions:

    • The relative frequencies of homozygotes and heterozygotes will stabilize under these conditions according to the equation:
      p^2 + 2pq + q^2 = 1.0
      and subsequently,
      p + q = 1.0

  • Importance: Deviations from these expected proportions indicate influences such as selection, nonrandom mating, or genetic drift.

Natural Selection and Fitness

  • Variation in Survival and Reproduction:

    • Different phenotypes interact with their environment variably, affecting survival rates and fecundity.

    • Fecundity is described as the rate at which an individual produces offspring.

    • The reproductive rate of a phenotype measures its fitness.

  • Natural Selection and Evolution:

    • Alleles conferring higher fitness tend to increase in frequency in the population.

    • Evolution: The change in genotype frequencies resulting from natural selection.

Case Study: English Peppered Moth

  • Color Morphs:

    • Two widely known morphs: light and dark.

    • Historical dominance of the light morph; however, industrial areas saw a shift towards dark morph prevalence due to environmental changes.

  • Selection by Predation:

    • Light morphs were favored in less polluted environments, while dark morphs had a selective advantage in polluted areas with darker tree trunks, making them less visible to predators.

Correlated Responses to Selection

  • Integrative Trait Selection:

    • Evolutionary responses often also affect other traits interconnected with those targeted by selection, illustrating an interdependence of phenotypic traits.

    • Example: Breeding for body weight in mice also influences skull width and tail length due to trait linkage.

Inbreeding and Outcrossing

  • Inbreeding Defined: Mating between closely related individuals, such as siblings.

  • Negative Consequences of Inbreeding:

    • It can lead to an increase in homozygosity for deleterious alleles, reducing overall fitness.

    • Typically, half of the offspring from inbred matings may express deleterious traits.

  • Outcrossing: Mating with individuals from the broader population mitigates the risk of expressing rare recessive deleterious traits.