IB Biology Guide: Understanding Natural Selection Through Industrial Melanism
Master the foundations of biological evolution with these definitive revision notes on the IB Biology Guide: Understanding Natural Selection Through Industrial Melanism updated for the latest IB Biology Diploma Programme (DP) Syllabus under Theme D: Unity and Diversity.
Whether you are preparing for your Paper 1A MCQs, mastering Paper 1B data-based questions, or developing concepts for your Internal Assessment (IA), this comprehensive guide breaks down complex Natural selection into easy milestones in an exam-ready format.
Before diving into the IB Biology Guide: Understanding Natural Selection Through Industrial Melanism ensure you have gone through comprehensive guide on Fossil Evidence for Evolution: Paleontological Proof & Evolutionary Lineages (IB Biology & A-Level Guide)
Table of content
- Introduction: Natural Selection in Action in Two Phenotypes of Biston betularia
- Historical Context: Pre-Industrial vs. Post-Industrial Era
- IB Key Concepts: Mechanism of Directional Selection
- Pre-Existing Genetic Variation
- Anthropogenic Environmental Change
- Differential Survival and Reproduction (Fitness)
- Shift in Allele Frequencies
- Reverse Selection: Clean Air Acts & Lichen Recovery
- Multiple Choice Question for paper 1A
- Data Analysis & Graph Questions for Paper 1B
- Extended Response Questions for paper 2
- Diagram-Based/Structure Identification Questions for paper 2
- HL extension question for Paper 3
Introduction Natural selection in Action in Two Phenotypes of Biston betularia
- Natural selection is often conceptualized as an agonizingly slow process operating across geological timescales.
- However, industrial melanism in the peppered moth (Biston betularia) serves as one of the most compelling, empirically documented examples of rapid evolution observed in real time.
- In IB Biology (Topic 5.2: Natural Selection), this case study demonstrates how anthropogenic environmental alterations directly drive changes in gene pools and allele frequencies within a wild population.
The Two Distinct Phenotypes
- Within Biston betularia populations, natural genetic variation produces two primary morphs or phenotypes:
Form typica (Pale / Light-colored Morph):
- Phenotype: White or cream-colored wings peppered with dark, speckled markings.
- Genotype: Homozygous recessive (cc).
- Ecological Niche: Exceptionally well-camouflaged against pale, lichen-encrusted tree trunks in unpolluted environments.
Form carbonaria (Melanic / Dark-colored Morph):
- Phenotype: Fully dark or melanic wings resulting from an overproduction of melanin pigment.
- Genotype: Controlled by a dominant allele (C).
- Ecological Niche: Originally a rare genetic mutation, but highly advantageous on dark, soot-covered bark devoid of lichens in industrial zones.
Why IB Biology Emphasizes This Model
- For IBDP Biology students, understanding Biston betularia is crucial because it cleanly illustrates four core evolutionary requirements defined by the IB curriculum:
- Inheritable Variation: Phenotypic traits are genetically determined, not acquired during an organism's lifespan.
- Environmental Stressors: Industrial emissions (sulfur dioxide and soot) altered the physical background of the ecosystem.
- Differential Mortality: Visual predators (birds) acted as the primary selective agent, preferentially consuming whichever morph lacked camouflage.
- Shift in Allele Frequency: Fitness advantages led to directional selection, altering the ratio of C (melanic) to c (pale) alleles across successive generations.
💡Related study to understand about the Oparin-Haldane Theory of Chemical Evolution: Origin of Life (IB Biology Notes)
- To understand how natural selection operates on Biston betularia, we must examine the dramatic ecological shifts brought about by the Industrial Revolution in Great Britain during the 18th and 19th centuries.
Pre-Industrial Era (Before the 1850s)
- Prior to widespread industrialization, woodland ecosystems across rural England were pristine and unpolluted.
- Environmental Baseline: Tree trunks, particularly native birch (Betula pendula), were covered with pale, crustose epiphytic lichens.
- Selection Dynamics: The light, speckled form (typica) blended seamlessly with lichen-covered bark. Avian predators (such as robins and thrushes) easily spotted the rare melanic mutant (carbonaria).
- Allele Frequencies: Form typica accounted for over 99% of the population, while the dominant melanic allele (C) was maintained at extremely low frequencies (<1%) strictly due to recurrent spontaneous mutations.
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| Industrial melanism diagram showing Biston betularia peppered moth - light morph typica and dark morph carbonaria |
Post-Industrial Era (Late 1800s – Mid 1900s)
- The rapid expansion of coal-powered manufacturing plants transformed urban and rural landscapes across Northern England (e.g., Manchester and Birmingham).
- Lichen Depletion: Industrial emissions released massive amounts of sulfur dioxide gas (SO2), which is toxic to lichens, killing them off completely.
- Soot Deposition: Heavy industrial soot and fly ash coated exposed tree bark, turning pale tree trunks dark and grime-covered.
- Reversal of Selective Advantage: On dark, soot-blackened trunks, form typica stood out clearly and suffered severe mortality rates due to heavy visual predation by birds. Form carbonaria gained a massive camouflage advantage, significantly reducing its predation risk.
- Allele Frequency Shift: In urban regions, the dark phenotype rose from less than 1% to over 95% of the localized peppered moth population within a span of roughly 50 generations (1848–1895).
IB Key Concepts: Mechanism of Directional Selection
- Directional selection is a mode of selection in which an extreme phenotype is favored over other phenotypes, causing the allele frequency to shift continuously in one direction.
- In Biston betularia, the transition from a predominantly light-colored population to a dark-colored population follows a textbook four-step evolutionary sequence defined by the IB curriculum.
Step 1: Pre-Existing Genetic Variation
- Natural selection cannot occur without pre-existing genetic variation within a population.
- Genetic Basis: Variation in wing pigmentation in peppered moths arises from spontaneous genetic mutations in the cortex gene.
- Allelic Interaction: The allele for melanism (C) is dominant over the allele for pale coloration (c).
- Important IB Distinction: Environmental pollution did not induce or cause the mutation for dark wings. The melanic allele existed in the gene pool at very low frequencies prior to the Industrial Revolution due to rare, random germline mutations.
Step 2: Anthropogenic Environmental Change
- For natural selection to alter a population, an environmental change must introduce a selective pressure.
- Habitat Modification: Coal combustion during the Industrial Revolution released heavy atmospheric particulates (soot) and sulfur dioxide (SO2).
- Loss of Camouflage Medium: Toxic SO2 levels decimated epiphytic lichen communities on tree trunks, while soot deposits turned pale birch bark dark and reflective-deficient.
- Altered Background: The physical background against which moths rested during daylight hours shifted from pale-speckled to uniform charcoal-black.
Step 3: Differential Survival and Reproduction (Fitness)
- When the environment changes, different phenotypes experience unequal rates of mortality based on their adaptive value (fitness).
- Selective Agent: Daytime visual predators, primarily avian species like sparrows (Passer domesticus) and robins (Erithacus rubecula), act as the primary selective agent.
- Form typica (Light): Highly visible against dark, soot-stained trunks. High predation rates led to low survival and reduced reproductive success.
- Form carbonaria (Dark): Well-camouflaged against dark trunks. Low predation rates led to high survival, allowing them to reach reproductive maturity.
- Evolutionary Fitness: Dark moths passed their dominant melanic alleles (C) to a significantly higher proportion of offspring in the next generation.
Step 4: Shift in Allele Frequencies
- Cumulative differential reproduction over successive generations results in microevolutionary change across the gene pool.
- Directional Shift: Because melanic moths consistently produced more surviving offspring, the relative frequency of the dominant melanic allele (C) increased rapidly in polluted urban regions.
- Gene Pool Alteration: Within 50 generations, the phenotype frequency of form carbonaria rose from <1% to >95% in industrial cities like Manchester.
- Mathematical Representation: If p represents the frequency of allele C and q represents allele c, the selective pressure caused p to approach 1.0 in industrial environments.
💡Related study to understand about the IB Biology Notes: Evidence for Evolution (Homology & Organs)
Reverse Selection: Clean Air Acts & Lichen Recovery
- A common query in IB Biology Paper 2 data analysis questions is whether evolutionary changes driven by natural selection are permanent or reversible.
- The post-industrial trajectory of Biston betularia provides empirical proof that when environmental selective pressures reverse, allele frequencies shift in the opposite direction.
- In the mid-20th century, severe smog crises (such as the Great Smog of London in 1952) prompted parliamentary and legislative action across the United Kingdom and Western Europe.
- Clean Air Act of 1956 (UK): Introduced smoke control areas, relocated power stations away from urban centers, and mandated the transition from heavy coal combustion to smokeless fuels and electricity.
- Atmospheric Quality Shift: Over subsequent decades, atmospheric sulfur dioxide (SO2) levels dropped dramatically, and heavy soot particulates cleared from rural and industrial woodlands.
Biological Lichen Recolonization & Habitat Restoration
- As atmospheric toxicity decreased, tree bark ecosystems began undergoing rapid ecological recovery:
- Epiphytic Lichen Return: Sensitive crustose and foliose lichens recolonized birch (Betula pendula) and oak (Quercus robur) tree trunks.
- Substrate Lightening: Soot layers were naturally washed away by precipitation, restoring tree trunks to their natural pale, textured background.
Dynamics of Reverse Selection (Directional Selection Reversal)
- The restoration of pre-industrial forest characteristics fundamentally flipped the selective advantage (fitness value) between the two phenotypes.
- Fitness Deficit for Form carbonaria: On lichen-covered bark, dark moths lost their camouflage. Avian predators easily identified melanic moths, driving their mortality rate significantly higher.
- Fitness Advantage for Form typica: Pale, speckled moths once again blended seamlessly into the lichen background, experiencing decreased predation pressure and achieving higher reproductive success.
- Allele Frequency Shift: The frequency of the dominant melanic allele (C) steadily declined across industrial centers like Manchester—dropping from over 90% in the 1960s to under 5% by the early 2000s.
Q1. Which condition is an essential prerequisite for natural selection to occur within a population of Biston betularia?
A. A constant environmental temperature across all seasons
B. Pre-existing genetic variation within the gene pool
C. Equal survival rates among all phenotypes
D. An acquired immunity to environmental pollutants
Q2. During the late 19th century in polluted industrial regions of England, the dark morph (carbonaria) of the peppered moth became more prevalent than the light morph (typica). What was the primary selective agent driving this change?
A. High atmospheric concentrations of sulfur dioxide gas (\text{SO}_2)
B. Visual predation by avian predators
C. Direct toxic effects of coal soot on adult moths
D. Increased ambient air temperatures in urban centers
Q3. The allele for dark wing coloration (C) in peppered moths is dominant over the allele for pale coloration (c). Following the implementation of Clean Air Acts, soot pollution decreased and lichens recovered on tree trunks. Which graph correctly illustrates the expected change in allele frequencies over time?
A. An exponential increase in the frequency of allele C
B. A steady decrease in the frequency of allele C
C. Equal distribution of alleles C and c at 0.50
D. A complete elimination of allele c from the population
Q4. Which statement best explains why individual light-colored moths (typica) did not change their wing color to dark gray during their lifetime in soot-stained forests?
A. Natural selection alters allele frequencies of populations, not individual phenotypes during a lifespan.
B. The mutation rate of the cortex gene was suppressed by coal combustion products.
C. Recessive alleles cannot express phenotypic changes in response to environmental stressors.
D. Birds preferentially preyed upon moths that attempted somatic chromatic adaptation.
Q5. In a rural, unpolluted habitat where birch trees are covered in light-colored lichens, what is the relative evolutionary fitness of the light morph (typica) compared to the dark morph (carbonaria)?
A. Lower fitness, because typica absorbs less heat energy from sunlight.
B. Higher fitness, because typica experiences lower rates of visual predation.
C. Equal fitness, because both morphs belong to the same breeding population.
D. Zero fitness, because typica lacks the dominant allele required for reproductive survival.
Q6. How did the introduction of industrial emissions (sulfur dioxide and soot) directly influence the forest ecosystem inhabited by Biston betularia?
A. By inducing germline mutations that produced melanic offspring in high proportions.
B. By accelerating the growth rate of pale epiphytic lichens on tree bark.
C. By eliminating epiphytic lichens and darkening tree trunk surfaces.
D. By eliminating avian predators from industrial urban habitats.
Q7. Which type of natural selection is illustrated by the shift from a high frequency of typica phenotypes to a high frequency of carbonaria phenotypes in response to environmental pollution?
A. Stabilizing selection
B. Disruptive selection
C. Directional selection
D. Frequency-dependent selection
Q8. A student analyzes a dataset showing the phenotypic frequency of Biston betularia across a 100\text{ km} transect from an industrial city center to a remote rural forest. What trend would be expected when moving from the city center toward the rural forest?
A. A continuous increase in the proportion of form carbonaria.
B. A continuous decrease in the proportion of form carbonaria.
C. A constant ratio of 1:1 between form typica and form carbonaria.
D. An absolute absence of form typica across all locations along the transect.
📝Data Analysis & Graph Questions for Paper 1B
Context 1 : Biologists tracked the frequency of the dark morph (carbonaria) of Biston betularia alongside atmospheric sulfur dioxide (SO2) concentrations in an industrial region of Northern England between 1960 and 2010 following the enactment of clean air legislation
Question 1 : Identify the general trend in the frequency of the melanic morph (carbonaria) between 1960 and 2010.
Answer : There is a continuous / steady decrease in the phenotypic frequency of the melanic morph (carbonaria) from 94% in 1960 to 3% in 2010.
Question 2 : Calculate the percentage decrease in atmospheric SO2 concentration from 1960 to 2000.
Answer : % Decrease = (Initial value - Final value) X 100 / Initial value
= (300 - 20) X 100 /300
= 93.33 %
Question 3 : Deduce the relationship between atmospheric SO2 concentrations and the phenotypic frequency of Biston betularia. Explain the biological mechanism driving this relationship.
Answer : There is a strong positive correlation between atmospheric SO2 levels and the frequency of the melanic morph (carbonaria)—as SO2 levels drop, the frequency of the dark morph also decreases.
Biological Mechanism : Lower SO2 levels allowed pale lichens to recolonize tree trunks and removed soot deposits.
On lichen-covered bark, dark moths (carbonaria) lost their camouflage and faced higher visual predation by birds, while light moths (typica) had higher survival and reproductive rates (reverse directional selection).
Question 4 : Predict, with a reason, what would happen to the frequency of the light morph (typica) if atmospheric SO2 levels were to rise significantly again in the future.
Answer : The frequency of the light morph (typica) would decrease.
Reason : Rising SO2 would kill lichens and darken tree bark with soot, making light-colored moths easily visible to avian predators, thereby reducing their survival rate and reproductive output.
Context 2 : The graph below illustrates the changes in the percentage frequency of the dark melanic morph (carbonaria) of Biston betularia (solid line) and atmospheric sulfur dioxide (SO2) levels (dashed line) recorded near Manchester, UK, between 1960 and 2000.
Questions 1 : State the maximum frequency of the carbonaria morph recorded and the year in which it occurred.
Answer: Maximum frequency was 94% ( 1%), recorded in 1960 (or early 1960s).
Question 2 : Describe the trend in atmospheric SO2 levels between 1960 and 1995.
Answer : Atmospheric SO2 levels decreased overall from 300 micro gm3 to under 30 micro gm3.
The steepest drop occurred between 1965 and 1980, after which the decline leveled off / plateaued toward 1995.
Question 3 : Compare the rate of decline of atmospheric SO2 levels with the rate of decline in the percentage frequency of the carbonaria morph.
Answer : Similarity: Both variables show a continuous, significant decrease across the 40-year period.
Difference: The decline in SO2 levels occurs earlier / more rapidly in the initial years (1960–1975), whereas the decline in carbonaria frequency lags behind and decreases steadily at a more delayed pace.
Question 4 : Explain the presence of a time lag (delay) between the reduction in SO2 pollution and the subsequent decline in the frequency of the carbonaria phenotype.
Answer : Ecological Delay: Lichens require time to recolonize tree trunks and grow once atmospheric SO2 drops to non-toxic levels.
Generational Shift: Allele frequency shifts occur over successive generations, not within a single generation of moths.
Recessive Allele Persistence: The pale color must gradually increase in homozygosity (cc) before the light phenotype (typica) becomes visually expressed and selected for by avian predators.
📝Extended Response Questions for paper 2
Question : Explain the role of pre-existing genetic variation and environmental changes in driving allele frequency shifts in a population.
Answer : Source of Variation: Genetic variation within a population arises from random gene mutations (e.g., in the cortex gene for melanism), sexual reproduction, and random assortment of alleles.
Non-Inductive Mutation: Environmental change does not cause or induce mutations; the mutant allele (e.g., dominant C for dark wings) exists at low frequencies in the gene pool prior to environmental change.
Selective Force: Environmental changes (such as sulfur dioxide emissions killing epiphytic lichens and soot darkening bark) modify the selective value (fitness) of existing phenotypes.
Differential Reproduction: Individuals possessing traits aligned with the altered environment survive predation, reproduce, and pass their advantageous alleles to offspring.
Microevolutionary Shift: Cumulative differential reproduction alters the ratio of dominant to recessive alleles across successive generations within the gene pool.
📝Diagram-Based/Structure Identification Questions for paper 2
Context : The diagram below shows two different environmental conditions (unpolluted lichen-covered bark and soot-polluted bark) containing two phenotypes of the peppered moth (Biston betularia), labeled X, Y, W, and Z
Question : Identify the phenotypes represented by labels X and Y on the lichen-covered tree trunk. .
Answer : X - Light-colored / Pale / Speckled morph (form typica).
Y - Dark-colored / Melanic morph (form carbonaria).
Question : State which moth label (Y or W) has the lower evolutionary fitness in its respective environment. Justify your answer based on visual predation pressure.
Answer : Both Y and W have low fitness in their respective environments. (Accept either Y on pale bark or W on dark bark).
They lack camouflage against their respective backgrounds, making them highly visible to visual predators (birds), leading to higher mortality rates and reduced reproductive success.
Question : Explain how the selective pressure differs between the unpolluted environment (left) and the polluted environment (right).
Answer : Unpolluted Environment (Left): Pale lichen-covered bark exerts selective pressure against the dark morph (Y).
Polluted Environment (Right): Soot-covered, lichen-free bark exerts selective pressure against the light morph (W).
Question : Deduce what will happen to the allele frequency responsible for phenotype Z over several generations in the polluted environment.
Answer : Deduction: The frequency of the dominant melanic allele (C) responsible for phenotype Z will increase.
Explanation: Phenotype Z is well-camouflaged against soot-covered bark, experiencing lower predati1on rates, surviving longer, and passing on the dominant allele to offspring (directional selection).
📝 HL extension question for Paper 3
Question : Discuss how industrial melanism in Biston betularia provides empirical evidence for both rapid microevolution and the reversibility of evolutionary processes. [6 Marks]
Answer : Evidence for Rapid Evolution: Phenotype frequency of form carbonaria shifted from <1% to over 95% in urban industrial areas (e.g., Manchester) in less than 50 generations (1848–1895), demonstrating that evolutionary adaptation can occur rapidly on an ecological timescale.
Selection Pressure Mechanism: Avian predators acted as the primary selective agent, consuming non-camouflaged moths and establishing a direct link between environmental change and differential mortality.
Clean Air Legislation Catalyst: Implementation of Clean Air Acts in the 1950s–1970s reduced SO2 and soot pollution, leading to ecological recovery (recolonization of lichens and clearing of tree bark).
Reverse Selection Mechanism: Habitat restoration reversed the selective advantage; melanic moths lost camouflage on light lichen backgrounds and suffered higher mortality rates.
Evidence for Reversibility: Frequency of the dark allele (C) steadily declined from >90% in the 1960s back to <5% by the 2000s, proving that natural selection is a dynamic, reversible process dependent on environmental conditions.
Scientific Signficance: Confirms that evolution is not an irreversible, linear trajectory, but rather a fluid equilibrium between gene pools and fluctuating environmental selective forces.
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