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Hardy–Weinberg Equilibrium, Speciation Mechanisms & Ecological Species Concept – Complete Review Notes

Hardy–Weinberg Principle

  • Framework for predicting allele (p, q) and genotype (p², 2pq, q²) frequencies in a population that is not evolving.
  • Serves as the null hypothesis; if real data deviate, evolution (or violation of an assumption) is inferred.
  • Key formulas to memorize and apply:
    • p^2 + 2pq + q^2 = 1 (genotype‐frequency equation)
    • p + q = 1 (allele‐frequency equation)
  • Variable assignments (must not be swapped):
    • p = frequency of the dominant allele (e.g.
      A).
    • q = frequency of the recessive allele (e.g.
      a).
    • Genotype translations:
    • AA \rightarrow p^2 (homozygous dominant)
    • Aa \rightarrow 2pq (heterozygous)
    • aa \rightarrow q^2 (homozygous recessive)

Observed vs. Expected Data

  • Observed counts = literal genotype numbers gathered in the field or lab.
    • Example (flowers): AA = 320, Aa = 160, aa = 20.
    • When observed counts are given, calculate allele frequencies directly by counting alleles (no Hardy–Weinberg assumption).
  • Expected counts = values computed from Hardy–Weinberg equations if allele frequencies are known but genotypes are not.
    • Used when only phenotype totals (e.g., “red” vs. “white” flowers) are provided.
  • Key takeaway:
    • Observed → count alleles first.
    • No genotype data → assume Hardy–Weinberg, solve for p & q, then compute genotype frequencies.

Conditions for Hardy–Weinberg Equilibrium

Population must satisfy five criteria; violation of any introduces evolutionary change:

  1. No differences in survival or reproductive success among genotypes (no natural selection).
  2. No change in population size (no founder or bottleneck effects).
  3. No mutation in the locus being studied.
  4. Large population size (eliminates sampling error / genetic drift).
  5. Random mating (no sexual selection, no assortative mating).

Practice & Preparation

  • Review worked examples from class (flowers, mice) + new practice set (≈ 5 additional HW problems promised in Week 4 folder).
  • Drill variable meanings (p vs.
    q) to avoid answer‐inversion errors.

Speciation Overview

  • Speciation = evolutionary process by which populations diverge to form distinct species, commonly via reduction/elimination of gene flow.
  • Two focal mechanisms covered: Allopatric & Sympatric.

Allopatric Speciation

  • Involves a physical/geographic barrier that splits one population into two.
  • Barrier limits movement → reduced gene flow → divergence → reproductive isolation.
  • Sub-modes:
    • Dispersal/colonization (e.g., Galápagos finches migrating from mainland).
    • Vicariance (environmental change creates new barrier: river course shift, mountain uplift).
  • Classic example: Kaibab vs. Abert squirrel populations separated by the Grand Canyon (~10 000 yr divergence).
  • Memory cue: Allopatric – populations are Apart.

Sympatric Speciation

  • Divergence within the same geographic area, no obvious physical barrier.
  • Considered conceptually challenging; ongoing area of active research.
  • Memory cue: Sympatric – populations remain in the Same place.

Four‐Step Roadmap to Sympatric Speciation

  1. Pre-existing genetic variation in the ancestral (parent) population.
  2. New ecological opportunity or environmental shift within the habitat (e.g., newly introduced resource).
  3. Behavioral or ecological divergence exploiting that opportunity (often affects mating timing, habitat choice, or food source).
  4. Reduction in gene flow → genetic divergence → potential speciation over long time spans.

Case Study 1 – Apple vs. Hawthorn Maggot Flies

  • Original population exploited only hawthorn berries.
  • 1800s: humans introduced apple trees to North America (new resource).
  • Genetic variation in timing & host preference allowed some flies to lay eggs later, switching to apples.
  • Resulting temporal (prezygotic) isolation:
    • Hawthorn race reproduces in spring.
    • Apple race reproduces months later (mid-to-late summer).
  • Continued assortative mating + time → genetic divergence.

Case Study 2 – Cichlid Fish in Lake Victoria

  • Lake Victoria = 2nd largest lake by surface area (after Lake Superior).
  • ~500 cichlid species inhabit one continuous water body – a showcase of adaptive radiation.
  • Key ecological gradient: light penetration with depth
    • Surface water: blue wavelengths dominate.
    • Deeper water: red wavelengths dominate (blue absorbed).
  • Genetic variation in visual pigment sensitivity + male color (blue vs. red)
    • Blue-sensitive / blue-colored fish → surface, higher mating success there.
    • Red-sensitive / red-colored fish → deeper zones, higher mating success there.
  • Depth-linked mate choice diminishes gene flow → divergence over thousands of years.

Reproductive Isolation Types (Context)

  • Prezygotic barriers: prevent zygote formation (temporal, behavioral, habitat, mechanical, gametic).
  • Postzygotic barriers: hybrid zygotes formed but exhibit reduced viability/fertility.
  • Maggot fly example = temporal prezygotic isolation.

Species Concepts Reviewed

Multiple frameworks exist; no single concept suffices in all cases.

  • Biological species concept (reproductive isolation).
  • Morphological species concept (shared form).
  • Phylogenetic/Genetic species concept (common ancestry on a phylogeny).
  • Ecological species concept (focus of today).

Ecological Species Concept (ESC)

  • Defines species by ecological niche—how organisms exploit biotic & abiotic resources.
  • Useful for microbes & asexual organisms where mating data are unavailable.
  • Criteria may include: resource type, habitat parameters (temperature, pH, salinity), predator/parasite interactions.

Example – Nitrogen-Cycling Bacteria

  • Nitrosomonas: \text{NH}3 \rightarrow \text{NO}2^-
  • Nitrobacter: \text{NO}2^- \rightarrow \text{NO}3^-
  • Both handle nitrogen but occupy distinct ecological steps → classified as separate species under ESC.

Example – Galápagos Finches

  • Morphologically similar Geospiza species differentiated by seed size preference:
    • Medium ground finch (G. fortis) → large seeds.
    • Small ground finch (G. fuliginosa) → small seeds & insects.
  • Resource partitioning supports species delimitation via ESC.

Strengths & Caveats of ESC

  • Strengths:
    • Applies to sexual & asexual taxa.
    • Captures adaptive differences ignored by morphology alone.
  • Limitations:
    • Ecological similarity does not guarantee gene flow (two lineages could share a niche yet be genetically isolated).
    • Requires detailed ecological data; may miss cryptic species or seasonal niche shifts.

Connections & Broader Implications

  • Founder & bottleneck effects (from earlier lectures) directly violate Hardy–Weinberg by altering population size.
  • Human activities (e.g., apple introduction) can trigger sympatric divergence—illustrates anthropogenic roles in evolution.
  • Conservation angle: understanding speciation mechanisms helps manage biodiversity (e.g., protecting depth-specific cichlid habitats).
  • Philosophical note: No single species concept perfectly captures nature’s continuum; using multiple lines of evidence yields the most robust classifications.

Numerical & Statistical References

  • Hardy–Weinberg genotype counts (flower example): 320,\ 160,\ 20.
  • Approximate divergence time across the Grand Canyon: 10\,000 years.
  • Lake Victoria cichlid radiation: \sim500 species.
  • Additional practice set: \approx5 HW problems to be posted.

Study Tips & Action Items

  • Memorize Hardy–Weinberg formulas & variable meanings; practice inversion checks.
  • Work through posted practice questions (Week 4 folder).
  • Use the A-apart / S-same mnemonic for speciation types.
  • When analyzing speciation scenarios, map them to the four-step sympatric / allopatric frameworks provided.
  • Cross-apply multiple species concepts for any real-world classification problem.
  • Relate ecological examples back to foundational evolutionary principles (gene flow, selection, drift, mutation).
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