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Forces of Evolution - Comprehensive Study Notes

What is Inheritance?

  • Inheritance refers to how traits are passed from parents to offspring.
  • Microevolution looks at changes in allele frequencies from generation to generation.
  • Key idea: evolution is measured as changes in the genetic makeup (allele frequencies) of a population over time.

Law of Segregation

  • Particles (genes) for traits appear separately in the sex cells of parents.
  • These discrete particles are reunited in the offspring.
  • Concept: traits are inherited as discrete units that segregate and recombine.

Forces of Evolution

  • Mutation
  • Natural Selection
  • Gene flow
  • Genetic drift

Levels at which forces act

  • Mutation acts at DNA and chromosome level.
  • Natural selection acts at the level of the individual organism.
  • Genetic drift and gene flow act at the population level.

What Evolution Is Not

  • Not Lamarkian; not a linear progression from simple to complex; not goal-oriented or human-directed; not social.

Definition of Evolution

  • A change in allele frequency in a population over time.

Ferns as a Case Study

  • Demonstrates microevolution in real organisms across generations.

Microevolution

  • Small-scale evolution: changes in allele frequency from one generation to the next.
  • Example shows generation-to-generation shifts in allele frequencies (illustrative data in slides).
  • Generational shifts can be described as first generation vs. second generation allele frequencies (e.g., 75% vs 25% in different variants) demonstrating changing allele frequencies.

Macroevolution

  • Large-scale evolution such as speciation events occurring after hundreds or thousands of generations.
  • Example: horse evolution over 60 million years (60 mya to 1 mya).
  • Height progression example:
    • Eohippus: height ≈ 0.4 m
    • Mesohippus: height ≈ 0.6 m
    • Miohippus / Merychippus: height ≈ 1.0 m
    • Pliohippus: height ≈ 1.0 m
    • Modern horse: height ≈ 1.6 m (≈ 5.25 ft)

Micro vs Macro: Key Difference

  • Microevolution: short-term changes at genetic level within populations.
  • Macroevolution: long-term changes at the population level that can lead to speciation.

Population Concepts

  • Population: all inter-breeding individuals of a species in a local area.
  • Key processes shaping populations:
    • Births
    • Deaths
    • Immigration
    • Emigration
  • Gene pool: all the alleles present in the population.

Population Genetics (Integrated View)

  • Population Genetics studies how evolutionary forces change allele frequencies within a population.
  • It combines:
    • Natural selection (differential survival/reproduction)
    • Mendelian genetics (inheritance patterns)
    • Molecular genetics and population biology (genomic data, etc.)

Traits: Discrete vs Continuous; Polygenic Inheritance

  • Mendelian traits: discrete, controlled by alleles at a single genetic locus; phenotypes do not overlap.
  • Polygenic traits: continuous, controlled by multiple loci; each locus contributes additively to the phenotype.
  • Genotype + Environment = Phenotype
    • Important concept: the phenotype is the product of genotype and environmental influences.

Polygenic Inheritance (Conceptual Overview)

  • Phenotypic variation results from additive effects of alleles across multiple loci.
  • Environmental factors influence the expressed trait values.
  • Example visuals (noting that actual slide figures show genotype distributions across a trait scale).

Mutation

  • A random change in a gene or chromosome.
  • Creates a new trait that can be advantageous, deleterious, or neutral in its effects on the organism.

Natural Selection

  • Traits that improve an organism's fit to its environment tend to be passed on more frequently to the next generation.
  • Over time, advantageous traits become more common in successive generations.

Balanced Polymorphism

  • A situation where selection maintains two or more phenotypes for a specific gene in a population.

Patterns of Natural Selection

  • Directional selection
  • Stabilizing selection
  • Disruptive selection
  • No selection

Directional Selection

  • Selection for one allele over the other, causing allele frequencies to shift in one direction.
  • Classic example: Peppered moths in Great Britain.

Directional Selection (Illustrative Example)

  • Original vs. post-selection population shows a shift where light-colored moths were favored in a pristine environment, and dark-colored moths were favored under soot-darkened environments.
  • Industrial Revolution in 19th-century England accelerated the shift toward darker moths due to better camouflage on soot-covered trees.

Stabilizing Selection

  • Selection against phenotypic extremes, reducing genetic diversity for that trait.
  • Classic example: human birth weight.

Disruptive Selection

  • Selection for the extreme phenotypes at both ends of the distribution.
  • May lead to speciation events (e.g., California Salamanders – ring species).

Genetic Drift

  • Random changes in allele frequencies, especially impactful in small populations.
  • Includes:
    • Bottleneck effect: substantial loss of genetic diversity due to random events.
    • Founder effect: genetic drift when a small group establishes a new population.
  • Consequences: reduced genetic variation, potential shifts in allele frequencies due to chance.

Bottleneck Effect: Details

  • Populations experience a drastic reduction in size due to events such as volcanic eruptions, earthquakes, over-hunting, or radiation.
  • The remaining population's gene pool may differ markedly from the original population.
  • Visual representation often shows substantial loss of genetic diversity.

Founder Effect

  • A subset of a larger population establishes a new population, carrying only a fraction of the original population's genetic variation.
  • Often leads to distinctive genetic signatures in the new population.
  • Example themes: founder effects can influence health-related genetic variants in specific populations (illustrated by haplogroup and variant data across populations).

Migrations and Spread (Out of Africa)

  • Individuals from different populations can be genetically more similar to those in distant locations than to individuals from their own local population.
  • The Out of Africa model describes early human migration events: Africa as the origin, with subsequent dispersal to Europe and Asia.
  • Estimates show major exit points and varying genetic diversity across regions; higher genetic diversity in Africa indicates deeper ancestry.
  • Key takeaway: gene flow and migrations shape global genetic diversity.

Endogamy and Exogamy

  • Endogamous population: individuals breed only with other members of the population.
  • Exogamous population: individuals breed only with nonmembers of their population.

Inbreeding and Assortative Mating

  • Inbreeding: mating between related individuals, increasing homozygosity and potentially exposing deleterious recessive alleles.
  • Assortative mating: non-random mating where individuals with certain traits mate with similar others.
  • Note: Assortative mating can affect genotype frequencies and genetic variation over time.
  • Example: Habsburgs (historical illustration) show how non-random mating can concentrate certain traits/variants.
  • A cited reference: Pisanski et al., 2022 (non-random mating and its impact on trait distribution).

Epigenetics

  • Study of heritable changes in gene expression that do not involve changes to the DNA sequence.
  • Epigenetic mechanisms can influence phenotype and can be influenced by the environment.

Forces of Evolution: Levels of Influence (Recap)

  • Mutation affects DNA and chromosomes.
  • Natural selection acts on the individual.
  • Genetic drift and gene flow operate at the population level.

Practice and Reflection Prompts

  • Can you think of examples of the forces of evolution discussed so far?
  • What is the difference between microevolution and macroevolution?
    • Microevolution: level of influence is the population; short-term genetic changes; may involve allele frequency changes.
    • Macro_evolution: level of influence includes speciation events; longer time scales and broader patterns.
  • Which type of gene flow would have occurred if Romeo and Juliet had survived? (Prompts to think about population mixing and gene exchange.)
  • Which forces of evolution are considered microevolution? (Typically mutation, gene flow, genetic drift, and natural selection can operate at various scales; microevolution often emphasizes allele frequency changes within populations.)
  • Which forces of evolution are considered macroevolution? (Typically speciation and large-scale pattern changes over long time frames.)

Population Genetics Formulas (Key Tools)

  • Allele frequencies in a two-allele system:
    • Let p be the frequency of allele A and q be the frequency of allele a, with p+q=1.
    • Genotype frequencies under Hardy-Weinberg equilibrium: p^2+2pq+q^2=1.
  • Genotype-phenotype relationship with environment:
    • ext{Phenotype} = ext{Genotype} + ext{Environment}.

Next Steps / Check-in Prompts

  • Next Class: Organizing Species
  • Check-in Question #2 due at 3pm on Thursday
  • Next class reminder: Exam 1 next week; no in-person class on exam day