Natural Selection, Genetic Drift & Speciation – Comprehensive Exam Notes

Migration & Gene Flow

• Gene flow = movement of genetic material between populations

  • Occurs through migration of individuals or gametes.

  • Introduces new alleles, altering allele frequencies.

• Historical illustrations

  • Rh factor in China

  • Pre-contact: nearly all individuals $Rh^{+}$.

  • Arrival of European sailors introduced the $Rh^{-}$ allele ➜ new polymorphism established.

  • ABO blood group – $I^{B}$ allele

  • Historically common in Asia.

  • Spread westward with the Mongol Empire in the 12th–13th C., increasing its frequency in Western Europe.

Barriers to Gene Flow

• Geographical barriers

  • Oceans, mountain ranges, lake systems, deserts, ice sheets.

• Sociocultural barriers

  • Economic status, educational background, social class, religion, language.

  • Limit or prevent interbreeding even when geographic contact is possible.

Evolution & Roots of Natural Selection

• Evolution: gradual heritable change in species’ characteristics over time.

• Darwin & Wallace (H.M.S. Beagle; Galápagos)

  • Noted similarities/differences among organisms separated by time & geography.

  • Readings (pp. 244-246) led to "On the Origin of Species" (1859).

Darwin’s Three Core Observations

• Variation – Individuals differ, and traits are heritable.

• Birth rate – More offspring produced than environment can support.

• Nature’s balance – Despite high birth rates, population sizes remain relatively stable.

Darwin’s Interpretation

• Struggle for existence – Competition arises from high birth rate + limited resources.

• Survival of the fittest

  • Variability means some individuals possess traits better suited to local conditions.

  • Those individuals survive longer, reproduce more, and pass advantageous traits onward.

• Modern wording: Natural selection is the non-random differential survival of alleles that confer a survival/reproductive advantage.

  • The environmental factor imposing the advantage/disadvantage = selective agent.

• Key reminder: Individuals do not adapt; populations adapt over generations.

Alleles, Gene Pools & Natural Selection

• Natural selection viewed as changing allele frequencies in the gene pool.

• Summary of Principles

  • Heritable variation exists within populations.

  • Overproduction of offspring.

  • Competition/struggle for resources.

  • Differential survival & reproduction (fitness).

  • Inheritance of advantageous alleles ➜ increased frequency in next generation.

  • Over time, populations exhibit descent with modification.

• Some alleles provide a reproductive advantage, becoming increasingly common ($\uparrow$ frequency).

Adaptive Body Structure – Climate Example

• Hot climates (e.g.0° N Africa)

  • Long limbs + short torsos.

  • Larger surface-area-to-volume ratio ➜ more efficient heat dissipation.

  • Alleles for short body/long limbs favored; others wane.

• Cold climates (e.g.0° N Arctic Inuits)

  • Short limbs + long torsos.

  • Reduced surface-area-to-volume ratio ➜ heat conservation.

• Selective pressure = ambient temperature; repeated selection produces population-level morphologies.

Disease-Related Natural Selection

Sickle-Cell Anaemia (Hb^S)

• Selective agent: Malaria parasite (Plasmodium spp.).

• Genotypes & fitness

  • $Hb^{S}Hb^{S}$ (homozygous sickler): severe disease ➜ early mortality; removed from gene pool.

  • $Hb^{A}Hb^{A}$ (normal): healthy in absence of malaria but higher malaria mortality.

  • $Hb^{A}Hb^{S}$ (heterozygote): mild/no anaemia unless O$_2$ scarce plus resistance to malaria ➜ highest fitness in endemic regions.

• Result: Hb^S allele maintained at high frequency where malaria persists (balancing selection).

Tay-Sachs Disease (Hex-A deficiency)

• Lethal recessive; degeneration of nervous system, early childhood death for $ts\,ts$.

• Heterozygote advantage: $T!s$ carriers show increased resistance to tuberculosis (TB).

• Historical context – Jewish populations in WWII Europe:

  • $ts\,ts$ individuals died young.

  • $T!s$ carriers survived TB outbreaks, marrying within faith (endogamy) and propagating the allele.

  • $T!T$ homozygotes susceptible to TB.

Thalassaemias

• Haemoglobin composed of 4 globin chains. Mutations decrease chain production.

  • $\alpha$-thalassaemia: HBA gene (chromosome 16) ➜ $\alpha$-globin deficiency.

  • $\beta$-thalassaemia: HBB gene (chromosome 11) ➜ $\beta$-globin deficiency.

• Both autosomal recessive; spectrum: mild anaemia → cardiomegaly/hepatomegaly.

• Geographical correlation with malaria:

  • $\alpha$ common in SE Asia; $\beta$ in Mediterranean.

  • Reduced Hb lowers malaria parasite success ➜ allele maintained.

Genetic Drift

• Definition: Random, non-directional fluctuations in allele frequencies generation-to-generation.

• Key properties

  • Acts in all populations but strongest in small, isolated groups.

  • Also termed Sewall Wright effect.

• Illustrative model (Fig 9.25)

  • Start: 50 red, 50 black balls (alleles).

  • Randomly pick 20 red & 30 black → each duplicates → population = 40 red, 60 black.

  • Next random pick 15 red & 35 black → duplicates → 30 red, 70 black, etc.

Empirical study – Gulf of Carpentaria Islands

• Bentinck vs. Mornington Islands vs. Bayley Point mainland

  • Rising sea levels isolated islands.

  • Mornington retained some contact (stepping-stone islands); Bentinck remained isolated.

  • Blood group data:

  • Bentinck: very high $I^{B}$, complete absence of $I^{A}$.

  • Mainland: low $I^{B}$, moderate $I^{A}$.

  • Differences attributed to genetic drift, not selection.

Founder Effect – Extreme Drift

• Occurs when a small pioneering group forms a new, isolated population.

  • Sample of founders may be unrepresentative of source gene pool.

  • Leads to different allele frequencies + reduced genetic diversity.

• Model (Fig 9.26) demonstrates chance selection of founders → lasting impact on descendants.

• Dunkers (Pennsylvania, USA)

  • Descended from a few 18th-century immigrants from Hesse, Germany.

  • Religious prohibition of out-marriage → isolated gene pool.

  • Traits (ABO, Rh, MN, handedness, ear-lobe type) differ from both modern Hessians & surrounding Americans ➜ drift, not selection, responsible.

• Ashkenazi Jews & Tay-Sachs

  • Approx. 1 / 27 carry the allele vs 1 / 300 in non-Ashkenazi Jews.

  • Roots: small Middle-Age founding population + cultural endogamy ➜ elevated carrier frequency via drift.

Population Bottleneck

• Another extreme drift scenario: sudden population size reduction (catastrophe, disease, habitat loss).

  • Post-event allele frequencies reflect random survival, not fitness.

  • Leads to sharp loss of genetic diversity; two possible outcomes:

  • Recovery if survivors possess traits suitable to environment.

  • Extinction if not.

Speciation

• Species definition: interbreeding natural populations producing fertile offspring.

• Four-stage model (Fig 9.28)

  1. Variation – ancestral population with diverse traits shares a common gene pool.

  2. Isolation – formation of a barrier (geographic or sociocultural) splits population; gene flow ceases.

  3. Selection – distinct selective pressures act on each isolated population; allele frequencies diverge; subspecies stage.

  4. Speciation – accumulated differences become so great that, even if contact resumes, reproductive isolation prevents fertile offspring ➜ two distinct species.

Integrative Insights & Real-World Relevance

• Medical genetics: Understanding heterozygote advantage guides public-health planning in malaria, TB, and haemoglobinopathies.

• Anthropology & Forensics: Morphological adaptations aid in reconstructing past climates & migrations.

• Conservation biology: Bottlenecks & drift inform strategies to maintain genetic diversity in endangered species.

• Bioethics & social policy

  • Sociocultural barriers (religious endogamy, caste) impact gene flow and disease prevalence.

  • Awareness can influence genetic counseling & community health initiatives.