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:
- No differences in survival or reproductive success among genotypes (no natural selection).
- No change in population size (no founder or bottleneck effects).
- No mutation in the locus being studied.
- Large population size (eliminates sampling error / genetic drift).
- 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
- Pre-existing genetic variation in the ancestral (parent) population.
- New ecological opportunity or environmental shift within the habitat (e.g., newly introduced resource).
- Behavioral or ecological divergence exploiting that opportunity (often affects mating timing, habitat choice, or food source).
- 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).