Apply Natural Selection - IB Biology

SECTION 1

Historical Context & Motivation

For centuries, naturalists struggled with a deceptively simple question: why do species seem so perfectly suited to their environments, and how do new species arise? Early thinkers proposed ideas ranging from divine creation to the inheritance of acquired characteristics, but none could explain the breathtaking diversity of life on Earth with a testable, mechanistic framework. The breakthrough came in the mid-nineteenth century when Charles Darwin and Alfred Russel Wallace independently arrived at the theory of natural selection — a process by which organisms with traits better suited to their environment tend to survive and reproduce at higher rates than others.

1809 • Lamarck's Inheritance of Acquired Characteristics

Jean-Baptiste Lamarck proposed that organisms could pass on traits they developed during their lifetime. While incorrect in mechanism, his work was among the first to suggest species change over time.

1831–1836 • Darwin's Voyage on HMS Beagle

Darwin observed immense variation among species on the Galápagos Islands and across South America, gathering evidence that would later form the backbone of his theory of evolution by natural selection.

1858 • Darwin–Wallace Joint Presentation

Papers by both Darwin and Wallace were presented to the Linnean Society of London, co-introducing the theory of natural selection to the scientific community.

1859 • On the Origin of Species Published

Darwin published his landmark book, providing extensive evidence from biogeography, paleontology, and comparative anatomy to support evolution by natural selection.

1930s–1940s • The Modern Synthesis

Scientists including Theodosius Dobzhansky and Ernst Mayr merged Darwin's natural selection with Mendelian genetics, creating a unified framework explaining how heritable variation fuels evolutionary change.

The central question that natural selection answers is this: given that individuals within a population vary, how do certain traits become more or less common over generations? Understanding this mechanism is essential for explaining adaptation, the emergence of new species, antibiotic resistance, and many other biological phenomena you encounter in the IB Biology syllabus.

SECTION 2

Core Principles of Natural Selection

Natural selection operates through a set of conditions that must all be present in a population. When these conditions are met, the inevitable result is that certain heritable traits increase in frequency over successive generations. Darwin's insight was that no conscious "designer" is needed — the environment itself acts as the selective agent, filtering which organisms survive and reproduce.

Variation

Individuals within a population differ in their traits — body size, coloration, enzyme efficiency, disease resistance, and countless other features. Without phenotypic variation, natural selection has no raw material to act upon.

Heritability

At least some of the variation must be heritable, meaning it is encoded in DNA and can be passed from parent to offspring. Traits acquired purely through environment (e.g., a scar) are not subject to natural selection.

Overproduction of Offspring

Populations tend to produce more offspring than the environment can support. This leads to a struggle for existence in which not all individuals survive to reproduce.

Differential Survival & Reproduction

Individuals with traits that provide an advantage in the current environment are more likely to survive and leave offspring. This is often summarized as "survival of the fittest", where "fittest" means best adapted, not necessarily strongest.

Accumulation Over Time

Over many generations, advantageous alleles increase in frequency, leading to adaptation — a population-level shift in traits that improves the match between organism and environment.

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KEY TAKEAWAY

Think of natural selection like a filter on a social media feed. Imagine millions of posts (organisms) are generated, but only those that match the algorithm's criteria (environmental pressures) get promoted (survive and reproduce). Over time, the feed (population) becomes dominated by content that "fits" the algorithm. The algorithm doesn't design the posts — it simply filters which ones persist. Similarly, natural selection doesn't create variation; it acts on variation that already exists.

SECTION 3

Natural Selection in Action — Visual Explanation

The diagram below illustrates how natural selection operates across three generations in a beetle population. In the initial generation, beetles vary in color from light green to dark green. Birds prey more easily on light-colored beetles against dark foliage, so darker beetles survive at higher rates. Because color is heritable, the next generation has a higher proportion of dark beetles. After several generations, the population has shifted noticeably toward darker coloration — an example of directional selection.

The beetle population shifts from predominantly light-colored (Generation 1) to mostly dark-colored (Generation 3). The bar chart at the bottom tracks the changing allele frequencies as bird predation selectively removes lighter individuals each generation.

Notice that natural selection did not create the dark coloration — mutation and sexual reproduction introduced the variation in the first place. What natural selection did was change the proportion of alleles in the population by removing less-fit individuals before they could reproduce. This distinction is critical in IB Biology: natural selection is a mechanism of evolution, not a source of new genetic information.

SECTION 4

How Natural Selection Changes Allele Frequencies

While natural selection is primarily a conceptual framework, its effects on populations can be described quantitatively using allele frequency — the proportion of a specific allele in a gene pool. The Hardy-Weinberg principle provides a baseline: it predicts that allele frequencies remain constant in the absence of evolutionary forces. When natural selection is operating, however, allele frequencies shift in predictable directions.

ALLELE FREQUENCY

f(A) = (number of A alleles) ÷ (total number of alleles at that locus)

f(A) represents the frequency of allele A in the population. If a population of 100 diploid organisms has 120 copies of allele A out of 200 total alleles, then f(A) = 120 ÷ 200 = 0.60.

HARDY-WEINBERG EQUATION

p² + 2pq + q² = 1

Where p = frequency of the dominant allele, q = frequency of the recessive allele, p² = frequency of homozygous dominant genotype, 2pq = frequency of heterozygotes, and q² = frequency of homozygous recessive genotype. Deviation from expected values signals that evolution (e.g., natural selection) is occurring.

SELECTION COEFFICIENT

w = 1 − s

Where w is the relative fitness of a genotype (ranging from 0 to 1) and s is the selection coefficient (the reduction in fitness relative to the most fit genotype). A selection coefficient of s = 0.3 means that genotype has 70% the reproductive success of the fittest genotype.

In IB Biology, you are not expected to perform complex population genetics calculations, but you should understand the logic: when a genotype has lower fitness (higher selection coefficient), its allele frequency decreases over generations. When a genotype has higher fitness, its allele frequency increases. The speed of this change depends on how strong the selection pressure is, the population size, and whether the allele is dominant or recessive.

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IB Exam Tip

IB Biology questions often ask you to apply natural selection to a scenario rather than calculate allele frequencies. Be ready to identify the variation, the selective pressure, and explain how allele frequencies would change over time. Always specify that the trait must be heritable for natural selection to cause evolution.

SECTION 5

Types of Natural Selection

Natural selection does not always push a population in one direction. Depending on the environmental pressures, selection can take different forms that reshape the distribution of traits in a population. The three main patterns are directional selection, stabilizing selection, and disruptive selection. Each type produces a characteristic shift in the population's trait distribution curve.

Three patterns of natural selection compared. Directional selection shifts the trait distribution toward one extreme (e.g., larger body size). Stabilizing selection narrows the distribution around an intermediate value. Disruptive selection favors both extremes, creating a bimodal distribution that can eventually lead to speciation.

Type of Selection	What Is Favored	Effect on Distribution	Real-World Example
Directional	One extreme phenotype	Curve shifts toward the favored extreme	Peppered moth darkening during Industrial Revolution; antibiotic-resistant bacteria
Stabilizing	Intermediate phenotype	Curve becomes narrower (less variation)	Human birth weight — very small or very large babies have lower survival rates
Disruptive	Both extreme phenotypes	Curve splits into two peaks (bimodal)	African seedcracker finches with very large or very small beaks (intermediate beaks are less efficient)

Comparison of the three main types of natural selection

SECTION 6

Worked Example: Antibiotic Resistance in Bacteria

One of the most important modern applications of natural selection is explaining antibiotic resistance. Let's walk through how natural selection operates in a bacterial population exposed to an antibiotic.

How does a bacterial population develop antibiotic resistance?

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Step 1 — Identify the Variation

A population of Staphylococcus aureus contains millions of individual bacteria. Due to random mutations during DNA replication, a small number of bacteria carry an allele that produces a modified enzyme — one that the antibiotic methicillin cannot bind to effectively.

Variation exists: most bacteria are susceptible, but a few carry a resistance allele.
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Step 2 — Apply the Selection Pressure

A patient is prescribed methicillin. The antibiotic kills the vast majority of susceptible bacteria. However, the few bacteria carrying the resistance allele survive because their modified enzyme prevents methicillin from disrupting cell wall synthesis.

Differential survival: resistant bacteria survive; susceptible bacteria die.
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Step 3 — Reproduction of Survivors

The surviving resistant bacteria reproduce rapidly through binary fission. Because the resistance allele is encoded in their DNA, it is passed to all daughter cells. With the susceptible competitors removed, resources are abundant, allowing the resistant population to expand quickly.

Heritability: the resistance allele is faithfully copied to offspring.
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Step 4 — Shift in Allele Frequency

After several generations, the allele frequency has shifted dramatically. Before treatment, the resistance allele might have been present in less than 1% of the population. After treatment, nearly 100% of surviving bacteria carry the resistance allele. The population has evolved — it is now a population of MRSA (methicillin-resistant Staphylococcus aureus).

Result: The population has adapted to the antibiotic environment through natural selection.
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Step 5 — Evaluate and Conclude

This example demonstrates all the requirements for natural selection: variation (some bacteria had the resistance allele), heritability (the allele was passed through DNA), overproduction (bacteria reproduce rapidly), and differential survival (the antibiotic killed non-resistant bacteria). Importantly, the antibiotic did not cause the mutation — the mutation arose randomly before treatment. The antibiotic simply acted as the selective pressure that changed allele frequencies.

Key IB point: Natural selection does not cause mutations. It acts on pre-existing genetic variation.

SECTION 7

Evidence For and Limitations of Natural Selection

Natural selection is one of the best-supported theories in all of science, backed by evidence from many fields. However, it is important to recognize that natural selection is not the only mechanism of evolution, and it has certain limitations. The table below summarizes the key lines of evidence supporting natural selection alongside important caveats.

Evidence For Natural Selection	Limitations & Misconceptions
Fossil record: Transitional fossils (e.g., Tiktaalik) show gradual changes consistent with selection over time.	Natural selection cannot create new alleles — it only acts on variation produced by mutation and recombination.
Homologous structures: Similar bone structures in different species (e.g., whale flipper, human arm) suggest common ancestry shaped by different selection pressures.	Genetic drift can also change allele frequencies, especially in small populations, independent of selection.
Direct observation: Antibiotic resistance, pesticide resistance, and the Galápagos finch beak studies by Peter and Rosemary Grant.	Natural selection does not produce "perfect" organisms — it can only work with available variation and is constrained by trade-offs.
Molecular biology: DNA sequence comparisons reveal patterns of conserved and divergent regions consistent with selective pressure.	Not all traits are adaptive — some persist due to genetic linkage, pleiotropy, or neutral drift.
Biogeography: Species distributions across islands and continents match predictions of selection in different environments.	Evolution by natural selection is not goal-directed — it does not "aim" for improvement but responds to current conditions.

Evidence supporting natural selection and important limitations

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KEY TAKEAWAY

Natural selection is a powerful mechanism of evolution, but it is not the only one. Think of evolution as a toolkit: natural selection is the most well-known tool, but genetic drift, gene flow, and mutation also contribute. In IB Biology, you should be able to distinguish scenarios where natural selection is the primary driver from scenarios where other evolutionary forces are at play.

SECTION 8

Natural Selection and Speciation

Natural selection operates on populations, but its long-term consequence can be the formation of entirely new species — a process called speciation. When populations become geographically or reproductively isolated, natural selection may push them in different directions, eventually making them so different that they can no longer interbreed. This is one of the ways the enormous diversity of life on Earth has arisen.

Concept	Natural Selection (This Lesson)	Speciation (Advanced)
Scale	Changes allele frequencies within a single population (microevolution)	Results in new species from divergence of populations (macroevolution)
Time frame	Can be observed within a few generations (e.g., bacteria)	Typically requires thousands to millions of years
Requirements	Variation, heritability, differential reproduction	Reproductive isolation (geographic, behavioral, or temporal)
Outcome	Population becomes better adapted to its environment	Two or more distinct species emerge from a common ancestor
Example	Peppered moth color change during industrialization	Darwin's finches diverging into 13+ species on the Galápagos Islands

Natural selection vs. speciation — how microevolution connects to macroevolution

As you continue in IB Biology, you will explore how reproductive isolation and adaptive radiation build upon the foundation of natural selection. Keep in mind that natural selection is the engine, but isolation is the road that can lead a single population to branch into multiple species. Understanding natural selection deeply now will make those advanced concepts much more accessible.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

A farmer notices that some of her cows produce more milk than others. She selectively breeds only the highest-producing cows for the next generation. Is this an example of natural selection? Explain why or why not, and name the correct term for this process.

Show answer

This is not natural selection — it is artificial selection. In natural selection, the environment determines which individuals survive and reproduce without human intervention. In this case, the farmer is acting as the selective agent, deliberately choosing which cows reproduce based on a desired trait (milk production). Both natural and artificial selection rely on heritable variation, but they differ in the source of the selective pressure. Natural selection is driven by environmental factors such as predation, disease, or resource availability, whereas artificial selection is driven by human choice.

Question 2: BASIC CALCULATION

In a population of 200 beetles, 32 are homozygous recessive (dark-colored, genotype aa). Assuming Hardy-Weinberg equilibrium, calculate the frequency of the recessive allele (q) and the dominant allele (p).

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The frequency of the homozygous recessive genotype (q²) = 32 ÷ 200 = 0.16. To find q, take the square root: q = √0.16 = 0.4. Since p + q = 1, p = 1 − 0.4 = 0.6. This means the dominant allele has a frequency of 60% and the recessive allele has a frequency of 40% in this population. If natural selection later favors the dark phenotype, we would expect q to increase over subsequent generations.

Question 3: INTERMEDIATE

A species of lizard lives on an island with two types of terrain: dark volcanic rock and light sandy beaches. Lizards with intermediate coloration are easily spotted by hawks on both surfaces. Over time, the population develops two distinct color morphs — very dark and very light — with few intermediate individuals. Identify the type of natural selection occurring, explain the mechanism, and predict what might happen if one terrain type disappeared.

Show answer

This is an example of disruptive selection. The selective pressure (hawk predation) acts against intermediate phenotypes because they are conspicuous on both terrain types. Lizards at either extreme — very dark on volcanic rock or very light on sandy beaches — have better camouflage and survive at higher rates. Over generations, the population splits into two phenotypic groups. If one terrain type disappeared (e.g., volcanic rock was covered), the selective pressure would shift: dark lizards would lose their camouflage advantage. The selection would then become directional, favoring light coloration, and the dark morph would decline in frequency.

Question 4: APPLIED

Hospital data show that infections caused by methicillin-resistant Staphylococcus aureus (MRSA) have increased steadily over the past 30 years. A news reporter writes: "Bacteria are learning to resist antibiotics." Using your understanding of natural selection, write a scientifically accurate correction of this statement in 3–4 sentences. Include the terms "allele frequency," "variation," and "selective pressure" in your answer.

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The reporter's statement is inaccurate because bacteria do not "learn" resistance — they lack the ability to intentionally modify their DNA. A more accurate explanation is that random mutations create variation in the bacterial population, with some individuals possessing alleles that confer resistance. When antibiotics are applied as a selective pressure, susceptible bacteria are killed while resistant individuals survive and reproduce. Over many generations, the allele frequency of resistance genes increases in the population, making resistant infections more common. The antibiotic does not cause the mutation; it simply selects for bacteria that already carry the resistance allele.

Question 5: CRITICAL THINKING

A student claims: "Natural selection always leads to organisms becoming more complex and better adapted over time." Evaluate this claim using at least two counterarguments, referencing specific biological examples. Consider whether natural selection can lead to loss of traits, and discuss at least one other evolutionary mechanism that can change allele frequencies.

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The student's claim is an oversimplification. First, natural selection does not always increase complexity — it can also lead to the loss of traits. For example, cave-dwelling fish (such as the Mexican blind cavefish, Astyanax mexicanus) have lost functional eyes over generations because, in complete darkness, eyes provide no selective advantage and may even be energetically costly to maintain. Second, adaptation is relative to the current environment, not an absolute measure of improvement. A trait that is advantageous in one environment may become disadvantageous if conditions change — arctic hares' white fur is excellent camouflage in snow but a liability on bare ground. Third, genetic drift — random changes in allele frequency due to chance events, especially in small populations — can alter a population's genetic makeup regardless of whether the changes are adaptive. The founder effect and population bottlenecks are specific examples. Therefore, evolution is not an inevitable march toward greater complexity; it is a response to changing selective pressures and random events.

SUMMARY

Lesson Summary

Summary

Natural selection is the process by which organisms with heritable traits better suited to their environment survive and reproduce at higher rates, causing allele frequencies to shift over generations. It requires four conditions: phenotypic variation within a population, heritability of that variation, overproduction of offspring, and differential survival and reproduction. Depending on the selective pressure, natural selection can be directional (favoring one extreme), stabilizing (favoring the average), or disruptive (favoring both extremes).

Real-world applications include antibiotic resistance in bacteria, pesticide resistance in insects, and camouflage adaptations such as the peppered moth. Remember that natural selection does not create new variation — mutation and sexual reproduction generate the raw material, while natural selection acts as the filter. It is not the only mechanism of evolution; genetic drift and gene flow also play important roles, especially in small or connected populations.