Natural Selection and Microevolution

Key Definitions and Concepts

• Natural selection: process whereby environmental selection pressures confer a selective advantage on a specific phenotype, enhancing its survival (viability) and reproduction (fecundity).

  • Occurs when phenotypic variation exists and is heritable.

  • Leads to differential reproductive success and therefore changes in allele frequencies within a population.

• Microevolution: small-scale genetic change within a population’s gene pool across generations; driven by mutation, gene flow, genetic drift, non-random mating and natural selection.

• Phenotypic selection may be

  • Positive → favours a phenotype, increasing its allele frequency.

  • Negative → selects against a phenotype, decreasing its allele frequency.

• Gene pool: the complete set of alleles present in a population at any one time.

  • Large gene pool → high genetic diversity → increased biological fitness / lower extinction risk.

  • Small gene pool → low diversity → elevated extinction risk.

• Allele frequency (AF): proportion of a specific allele among all alleles of a gene in a population.

  • Expressed as decimal or percentage.

  • In a simple 2-allele system, p + q = 1 where p = dominant-allele frequency, q = recessive-allele frequency.

  • General formula: \text{AF} = \frac{(2\times \text{homoz. of that allele}) + (\text{heteroz.})}{2N} where N = number of individuals.

Darwin’s Theory of Natural Selection

• Overproduction: more offspring produced than survive to reproduce.

• Variation: individuals differ in heritable traits.

• Competition: limited resources → struggle for existence.

• Survival of the fittest phenotype: individuals with favourable traits survive & reproduce.

• Increase of favourable traits in subsequent generations.

Selection Pressures

• External factors altering viability/fecundity.

• Density-dependent

  • Predation, inter/intra-specific competition, disease, sexual selection.

• Density-independent

  • Natural disasters.

• Biotic factors (living) and abiotic factors (non-living).

• Disadvantageous phenotypes ↓ in frequency under sustained pressure.

Genetic Variation and Mutations

• Mutation = ultimate source of new alleles; “raw material” for evolution.

  • Single mutations can have large effects; usually cumulative small changes.

  • Recurrent spontaneous mutations may persist if neutral or beneficial.

Gene Pool & Allele-Frequency Worked Example (16-allele beetle pool)

• 8 beetles → 16 total alleles.

• Counts: A = 9, a = 7 → p = 9/16 = 0.56, q = 0.44.

• Tracks changes across phases: natural selection eliminates two aa → immigration/emigration adjusts counts → allele frequencies recalculated after each event.

Mechanisms of Microevolution

Mutation

• Creates novel alleles; may be neutral, deleterious or advantageous.

Gene Flow (Migration)

• Movement of alleles between populations by immigration/emigration, pollen or seed dispersal (plants), human relocation, etc.

• Maintains similarity between populations; absence leads to isolation & divergence.

• Blood-group & Duffy gene examples illustrate geographic gradients.

Genetic Drift

• Random fluctuation of allele frequencies, most pronounced in small populations with limited gene flow.

• Alleles of the “lucky” survive; not necessarily fitter.

• Computer simulations show faster fixation/loss in populations of \le 20 vs \ge 2000 breeders.

• Two special forms:

  • Bottleneck effect: drastic but temporary reduction in population size (e.g.
    Navajo Long Walk → ↑ incidence of Xeroderma Pigmentosum; drought/volcano impacts on honeyeaters & possums).

  • Founder effect: small group colonises new area (e.g. Amish Ellis-van Creveld syndrome; Afrikaner & Venezuelan clusters of Huntington’s; island beetles blown offshore).

Non-random (assortative) mating

• Alters genotype frequencies; can amplify sexual-selection traits.

Natural Selection (directional, stabilising, disruptive)

• Detailed under “Types of Phenotypic Selection”.

Genetic Equilibrium (Hardy–Weinberg)

Equilibrium requires:

1. Large population size.

2. Random mating.

3. No mutation.

4. No migration (gene flow).

5. No selection pressure.

Rarely met in nature → allele frequencies change (microevolution). Violations correspond to the mechanisms above.

Types of Phenotypic Selection

Graphical summary:

• Directional: distribution moves left/right.

• Stabilising: narrower central peak (tails truncated).

• Disruptive: central trough, twin peaks at extremes.

Case Studies & Examples

Galapagos Finches (Medium Ground Finch)

• 1977 drought → seed size ↑; average beak depth shifted upward.

• Gene ALX1: “blunt” vs “pointy” alleles correspond to diet.

• Demonstrates directional selection & allele frequency change.

Sickle-Cell & Malaria (HbS)

• Hb^Hb normal; HbSHbS sickle-cell anaemia (negative selection).

• HbAHbS heterozygotes resist malaria → positive selection where malaria endemic.

• Gabon study: heterozygote frequency 22% in high-malaria villages; no adults with HbS homozygous (high childhood mortality).

Tibetan High-Altitude Adaptation

• EPAS1, EGLN1, PPARA alleles show unusually high frequencies.

• Genome comparison of 50 Tibetans vs 40 Han Chinese vs 200 Danes.

• Positive selection for efficient oxygen usage without polycythaemia.

Rock Pocket Mouse

• Light granite vs dark lava substrates.

• Mc1r gene: d = light (wild type), D = dark melanic.

• On lava: 95% melanic phenotype; D allele frequency ≈ 0.82 in dark mice vs 0 in light mice.

• Predatory owls = selecting agent.

Human Skin Colour

• Balances UV protection of folate (needs dark pigment) vs vitamin D synthesis (needs lighter skin).

• Spatial gradient: tropical latitudes → darker skin; high latitudes → lighter skin.

Lactose Tolerance

• Middle East / N. Africa cattle domestication ~7500–9000 ybp.

• Mutation enabling adult lactase persistence under strong positive selection in pastoralist societies → rapid AF increase.

Peppered Moths (Biston betularia)

• Industrial soot killed lichens → directional selection for melanic form.

• Post-1956 Clean Air Act reversed trend toward mottled form.

Coho Salmon

• Disruptive selection: small “sneakers” & large competitive males have reproductive advantage, intermediates disadvantaged.

Cane Toads in Australia

• Front-line toads evolve longer legs & narrower heads (↑ dispersal); later cohorts revert to less-dispersive, higher reproductive investment traits.

Human Genetic Drift

• Genghis Khan Y-chromosome: ≈8% of men across 16 Asian populations (≈16 million males) share his lineage.

Founder‐effect Diseases

• Amish Ellis-van Creveld, Afrikaner Huntington’s, Lake Maracaibo Huntington’s cluster.

Modelling & Mandatory Practicals

• HHMI “Beak of the Finch” simulation.

• Cambridge computer modelling of allele frequency under selection pressure.

• Worksheets: sickle-cell selection, desirable disadvantages, SparkLabs activities.

• Gene-drift simulations (population sizes 20–2000 across 140 generations).

Ethical / Philosophical / Practical Implications

• Conservation: bottlenecks in endangered species demand genetic-diversity management (e.g. helmeted honeyeater).

• Medical genetics: understanding founder effects guides carrier screening (Amish, Afrikaner, Navajo XP).

• Public-health policy: sickle-cell trait education in malaria regions; lactase persistence influences dietary guidelines.

Quick-Reference Equations & Data Tools

• p + q = 1 (allele frequencies)

• p^2 + 2pq + q^2 = 1 (Hardy–Weinberg genotype frequencies; implied though not explicitly in transcript).

• Allele frequency from counts: \frac{2(\text{homozygotes}) + (\text{heterozygotes})}{2N}.

Exam & Practice Questions

• QCAA sample Q20: identify selection type from trait-distribution graph (answer: \text{D – directional}).

• QCAA Q26: explain microevolution via mutation → stress mutation as source of new alleles that, if advantageous, rise in frequency under selection.

• “Challenge Yourself” items: classify evolutionary scenarios → (a) disruptive; (b) stabilising; (c) directional; (d) disruptive leading to incipient speciation; (e) directional.

• Natural selection: process whereby environmental selection pressures confer a selective advantage on a specific phenotype, enhancing its survival (viability) and reproduction (fecundity).

  • Occurs when phenotypic variation exists and is heritable.

  • Leads to differential reproductive success and therefore changes in allele frequencies within a population.

• Microevolution: small-scale genetic change within a population’s gene pool across generations; driven by mutation, gene flow, genetic drift, non-random mating and natural selection.

• Phenotypic selection may be:

  • Positive
    → favours a phenotype, increasing its allele frequency.

  • Negative
    → selects against a phenotype, decreasing its allele frequency.

• Gene pool: the complete set of alleles present in a population at any one time.

  • Large gene pool
    → high genetic diversity
    → increased biological fitness / lower extinction risk.

  • Small gene pool
    → low diversity
    → elevated extinction risk.

• Allele frequency (AF): proportion of a specific allele among all alleles of a gene in a population.

  • Expressed as decimal or percentage.

  • In a simple 2-allele system, p + q = 1 where p = dominant-allele frequency, q = recessive-allele frequency.

  • General formula: \text{AF} = \frac{(2\times \text{homoz. of that allele}) + (\text{heteroz.})}{2N} where N = number of individuals.

Mechanisms of Microevolution

• Mutation: ultimate source of new alleles; “raw material” for evolution. Creates novel alleles; may be neutral, deleterious or advantageous.

• Gene Flow (Migration): Movement of alleles between populations by immigration/emigration, pollen or seed dispersal (plants), human relocation, etc. Maintains similarity between populations; absence leads to isolation & divergence.

• Genetic Drift: Random fluctuation of allele frequencies, most pronounced in small populations with limited gene flow.

  • Two special forms:

    • Bottleneck effect: drastic but temporary reduction in population size.

    • Founder effect: small group colonises new area.

Types of Phenotypic Selection