A Level Biology 2nd Year - Population Genetics and Evolution Study Notes

Population Genetics and Evolution

Overview of Evolution

  • Definition: Evolution is the process of change through which new forms of life emerge from pre-existing forms. It occurs over long periods as accumulated changes over generations.
  • Focus of Population Genetics: Investigates the genetic and phenotypic variation within populations over time, emphasizing that it is the population that evolves, not the individual.

Types of Evolution

  • Microevolution: Small-scale evolutionary changes that occur over a short duration, such as mutations affecting allele frequencies.
  • Macroevolution: Large-scale evolutionary changes that lead to the emergence of new species over extended periods, including the divergence from single-celled ancestors to multicellular forms.

Population Dynamics

  • Definition of a Population: A population is a group of interbreeding individuals of the same species living in a shared habitat.
  • Evolution Indication: A population is considered to evolve when the allele frequency changes over generations.
Examples
  • Initial Generation: 1 brown-eyed and 9 blue-eyed individuals.
  • Next Generation: 5 brown-eyed and 30 blue-eyed individuals.
  • Change in Ratios: Blue eyes evolved from a 1:10 to 1:7 ratio, demonstrating evolutionary change.

Variation in Populations

  • Definition: Variation refers to differences in phenotypes among organisms of the same species.
  • Importance: Variation is essential for evolution; populations with no variation cannot evolve.
  • Influence of Genotype and Environment: Phenotypes are primarily determined by genotypes but are also influenced by environmental factors.
Types of Variation
  1. Continuous Variation:
       - Exhibits infinite potential phenotypes (e.g., height).
       - Affected by multiple genes (polygenes) and environmental factors.
  2. Discontinuous Variation:
       - Exhibits distinct phenotypes with no intermediate forms (e.g., blood groups).
       - Typically controlled by one or two genes with multiple allelic forms.

Measuring Variation

  • Data collected from a large number of individuals is represented in histograms.
  • Histogram Types:
       - For Discontinuous Variables: Bar graphs are separated and display limited data sets.
       - For Continuous Variables: Touching bars showing a normal distribution with a bell curve indicating most individuals clustering around the mean, e.g., heights usually between 140 cm and 165 cm.
Environmental Influence on Variation
  • Genotypic potential is influenced by environmental factors ensuring that no two organisms have identical phenotypes, even among identical twins.

Sources of Genetic Variation

  • Genetic variation arises from:
      1. Crossing-over of genes during meiosis (Prophase I).
      2. Independent assortment of chromosomes during meiosis (Anaphase I & II).
      3. Random fusion of gametes during fertilization.
      4. Mutations causing changes in DNA.
Mutations
  • Definition: Changes in DNA that may affect genotype and be inherited (
       - Germline mutations: Passed to offspring.
       - Somatic mutations: Not inherited.
  • Types of Mutations:
       - Spontaneous Mutations: Occur naturally due to inaccuracies in DNA replication.
       - Induced Mutations: Triggered by external agents (mutagens) like UV radiation or chemicals.
       - Gene Mutations: Changes at a single locus, e.g., insertion, deletion, substitution, and inversion of base pairs.
       - Chromosome Mutations: Changes in chromosome structure (e.g., deletion, duplication, inversion, translocation) or number.

Detailed Mutation Types

Gene Mutations
  1. Substitution: Replacing one base pair with another.
       Example: ATGCATGC becomes TTGCATGC.
  2. Insertion: Adding an extra nucleotide.
       Example: ATGACATGC.
  3. Deletion: Removing a nucleotide.
       Example: ATGCATGC.
  4. Inversion: A segment of DNA is reversed.
       Example: ATCGATGC.
       - Silent mutations do not change amino acids.
       - Frameshift mutations cause misreading during translation.
Chromosome Mutations
  • Types:
       - Deletion: Loss of a chromosome segment, leading to possible lethality (e.g., Cri du chat).
       - Duplication: Extra chromosome segment leading to disorders like Fragile X syndrome.
       - Inversion: Chromosomal segment is reversed, may cause developmental issues.
       - Translocation: Part of one chromosome moves to another chromosome leading to issues like leukemia.
Changes in Chromosome Numbers
  • Aneuploidy: Loss or gain of a single chromosome due to nondisjunction.
      - Monosomy: One chromosome present (2n-1).
      - Trisomy: Three copies of a chromosome (2n+1).
      Examples include Down's syndrome (trisomy 21) and Turner syndrome (monosomy).
  • Polyploidy: Cells possessing more than two sets of chromosomes (e.g., triploid, tetraploid).
      - Beneficial for plant speciation and diversity.
      - Polyploidy can lead to new species through self-fertilization or hybridization between species.

Concepts of Population Genetics

  • Population Definition: A group of individuals of the same species in the same habitat capable of interbreeding.
  • Gene pool: Total sum of genes and alleles in a population capable of being passed on.
      - Example: In a population of 40 individuals with 80 alleles for body color, types include red (R), blue (B), and green (G).
Genetic Composition and Frequencies
  • Allele frequency: Proportion of a specific allele in relation to all alleles in the population.
      - Calculations performed for allele presence, e.g.
        - Total alleles = 80
        - Frequencies calculating using allele presence.
  • Genotype Frequency: The proportion of a specific genotype within the population.
  • Hardy-Weinberg Principle: Predicts constants in allele frequencies in a population under specific conditions (random mating, no mutations, etc.).
Conditions for Hardy-Weinberg Equilibrium
  1. Large population size.
  2. Random mating.
  3. Equal fertility of all genotypes.
  4. No mutations.
  5. No migrations.
  6. No overlapping generations.

Mechanisms of Evolution

  • Changes not driven solely by mutation or random fertilization; also involve several factors leading to evolutionary change:
      - Mutation
      - Non-random mating
      - Genetic drift
      - Gene flow
      - Selection
Natural Selection
  • Adaptive Evolution: Environmental influences drive changes in allele frequencies through selective pressures that favor certain phenotypes.
Selective Pressures
  • Various factors causing competition that lead to adaptation or extinction, driving evolution greatly, influenced by both internal and external factors.
Recent Evolution Cases
  • Notable examples of rapid evolution include antibiotic resistance in bacteria and pesticide resistance in agricultural pests, showing how quickly populations can undergo changes in response to environmental pressures.

Types of Natural Selection

  1. Stabilizing Selection: Favors average phenotypes and reduces extremes.
       - Results in reduced variability, e.g., optimal birth weight.
  2. Directional Selection: Favors one extreme phenotype, shifting the mean of the trait, e.g., antibiotic resistance.
  3. Disruptive Selection: Favors extremes and may lead to speciation, favoring diverse phenotypes in varying environmental conditions.
Polymorphism
  • The existence of multiple distinct forms (morphs) within a species, significant for natural selection.
  • Two types of polymorphism:
      1. Balanced Polymorphism: Stable coexistence of forms.
      2. Transient Polymorphism: Rapid change under strong selection pressures.

Artificial Selection

  • Human-mediated selective breeding of organisms for desired traits, leading to new breeds or varieties.
  • Involves inbreeding and outbreeding strategies to increase variation and reduce genetic load.

Speciation Concepts

  • Definition of Species: Groups with a common gene pool, reproductively isolated from others.
  • Reproductive Isolating Mechanisms: Barriers effective pre-zygotically (before fertilization) or post-zygotically (after fertilization) to prevent interbreeding.
  • Methods of Speciation:
      - Allopatric Speciation: Physical barriers lead to divergence and speciation.
      - Sympatric Speciation: Speciation occurs in shared habitats due to reproductive isolating mechanisms.
      - Polyploidy: Especially in plants, allows for instantaneous speciation through chromosomal duplication.
Evolutionary Theories
  • Gradualism vs. Punctuated Equilibrium:
       - Gradualism posits continuous adaptation and small changes leading to new species.
       - Punctuated equilibrium suggests rapid speciation and stasis between major changes.
       - Evidence suggests both processes occur across varying taxa.