Natural Selection

Natural selection is the central explanatory mechanism of evolutionary biology. It is the process by which differential survival and reproduction of individuals — driven by heritable phenotypic variation and environmental selection pressures — generates adaptive change in populations over generations. Unlike mutation (which is random with respect to fitness) and drift (which is statistical noise), natural selection has direction: it consistently favours alleles that increase reproductive success in the prevailing environment. The result, accumulated over many generations, is a population whose genotype distribution is biased toward configurations that work in that environment — adaptation.

Spec mapping: This lesson sits in AQA 7402 Section 3.7.2 (genetic diversity and adaptation — the principles of natural selection in the evolution of populations; the three types of selection: directional, stabilising, disruptive). Refer to the official AQA specification document for exact wording. It builds on Hardy-Weinberg (lesson 1, which sets the null model that selection violates) and on the gene-mutation content in Section 3.4.3 (course 4).

Connects to: Hardy-Weinberg equilibrium (Section 3.7.2, lesson 1); gene mutation as the source of new alleles (Section 3.4.3, course 4 DNA, Genes and Inheritance); selection on enzyme alleles (Section 3.1.4, course 1 Biological Molecules); speciation (Section 3.7.3, lesson 4 of this course).

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The Logical Structure of Darwin's Argument

Charles Darwin's 1859 work On the Origin of Species set out the argument for evolution by natural selection. His framework was developed independently by Alfred Russel Wallace, who reached strikingly similar conclusions during fieldwork in the Malay Archipelago; the joint Darwin-Wallace papers were read to the Linnean Society of London in 1858. The argument can be deconstructed into six logical premises:

1. Overproduction (Malthusian premise). Organisms produce far more offspring than the environment can sustain. A single oak produces tens of thousands of acorns per year; a cod can release millions of eggs in a single spawning. Most do not survive to reproduce.
2. Variation. Individuals within a population vary in their phenotypes — size, shape, colouration, physiology, behaviour. Variation is partly heritable.
3. Struggle for existence. Limited resources (food, mates, space, light) lead to competition, both interspecific and intraspecific.
4. Differential survival. Phenotypic differences mean some individuals are better equipped than others to obtain resources, avoid predators, or attract mates. These individuals are more likely to survive and reproduce.
5. Heritability. Because variation has a genetic component, the alleles underlying advantageous phenotypes are transmitted preferentially to the next generation.
6. Accumulation. Over many generations, the frequency of advantageous alleles rises. The population becomes increasingly well adapted to its environment.

Darwin lacked a mechanism of inheritance — he wrote before Mendel's 1866 paper was widely understood. The integration of Mendelian genetics with natural selection occurred during the modern synthesis of the 1930s–1940s, in the work of Sewall Wright, R. A. Fisher, J. B. S. Haldane, Theodosius Dobzhansky, and Ernst Mayr. We will return to the modern synthesis explicitly in lesson 7.

Key Definition — Fitness: In biology, fitness is an organism's ability to survive and reproduce in a particular environment, measured by the number of viable offspring contributed to the next generation that themselves survive to reproduce. Fitness is always relative: a trait advantageous in one environment may be deleterious in another (e.g. dark colour in the peppered moth, discussed below).

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Variation — Sources and Heritability

Selection cannot act on a uniform population; it requires variation. The sources of heritable phenotypic variation are:

• Random mutation at the DNA level — single base substitutions (point mutations), insertions, deletions, gene duplications, chromosomal rearrangements. Mutation rate per locus per generation is typically 10⁻⁸ to 10⁻⁵.
• Independent assortment in meiosis I, which shuffles maternal and paternal chromosomes into gametes — 2ⁿ combinations from n chromosome pairs.
• Crossing over in prophase I, which produces recombinant chromosomes by exchanging chromatid segments between homologues.
• Random fertilisation — the random union of gametes in sexual reproduction generates further variation.

A core principle, foundational to A-Level evolution: mutation is random with respect to fitness. The environment does not direct which mutations arise; it determines only which mutations are favoured once they exist. This is the most-misunderstood point in the entire topic and the basis of the most common 9-mark exam discriminator.

A-Level misconception watch: Students routinely write "the antibiotic causes bacteria to mutate to become resistant". This is wrong twice over: the antibiotic does not induce the mutation, and the resistance mutation is not for anything — it just happens to be useful. Marks are lost reliably here.

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The Three Modes of Selection

Natural selection can act on a population in three structurally distinct ways, depending on which part of the phenotypic distribution is favoured. The diagrams below describe the effect on a continuous trait's frequency distribution.

flowchart LR
  A["Original phenotype distribution"] --> B["Directional: shift toward one extreme"]
  A --> C["Stabilising: narrower around mean"]
  A --> D["Disruptive: bimodal, two peaks"]

1. Directional Selection

Definition. Individuals at one extreme of the phenotypic range have higher fitness than the rest of the population. Over generations, the mean phenotype shifts toward the favoured extreme.

Characteristics.

• Frequency-distribution curve shifts to one side.
• Mean phenotype changes.
• Genetic variation in the favoured trait often decreases as alleles at the disfavoured end are removed.

Example — Antibiotic resistance in bacteria.

1. A bacterial population contains spontaneous variation in susceptibility to a given antibiotic. Most cells are susceptible; rare mutants carry resistance alleles (e.g. β-lactamase production, modified ribosomal RNA, drug efflux pumps).
2. When the antibiotic is administered, susceptible cells are killed — the antibiotic acts as a selection pressure.
3. Resistant cells survive and reproduce, passing the resistance allele to descendants.
4. Within a few bacterial generations (which can be 20 minutes in E. coli), the resistance allele can rise to fixation in the surviving population.
5. The mean level of resistance has shifted upwards — classic directional selection.

The clinical consequence is that the antibiotic becomes ineffective. This is a public-health crisis: methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant enterococci (VRE), and multi-drug-resistant Mycobacterium tuberculosis (MDR-TB) are direct consequences of selection imposed by clinical antibiotic use.

Example — Pesticide resistance.

Pesticide resistance in insect populations follows the same logic. Random mutations produce variation in pesticide susceptibility; the pesticide kills susceptible individuals; resistant insects reproduce; resistance allele frequency rises over generations; the pesticide becomes less effective; new chemicals or integrated pest management are required. This is evolution in action — observable, measurable allele-frequency change within human timescales.

2. Stabilising Selection

Definition. Individuals with the intermediate (average) phenotype have higher fitness than those at either extreme. Variation is reduced; the mean does not change.

Characteristics.

• Frequency-distribution curve becomes narrower and taller around the mean.
• Mean phenotype is unchanged.
• Genetic variance decreases over time.

Example — Human birth weight.

Babies of very low birth weight (< 2.5 kg) have reduced neonatal survival due to poor thermoregulation, immune immaturity, and underdeveloped lungs. Babies of very high birth weight (> 4.5 kg) face elevated risks of obstructed delivery and birth trauma. Babies of intermediate weight (~3.0–3.5 kg) have the highest survival rate. Stabilising selection has maintained mean human birth weight in this window for much of human evolutionary history. (Modern obstetric intervention partially relaxes the selection pressure at the heavy end — caesarean section makes large-baby deliveries survivable in a way they were not historically. This is itself an interesting case of medical intervention altering selection pressures.)

3. Disruptive Selection

Definition. Individuals at both extremes have higher fitness than those with the intermediate phenotype. The population can develop a bimodal distribution and may, given reproductive isolation between the two morphs, be a precursor to sympatric speciation (lesson 4).

Characteristics.

• Frequency-distribution curve becomes bimodal — two peaks at the extremes.
• Genetic variance increases.
• Mean phenotype is unchanged or only slightly altered.

Example — Beak size in African seed-crackers (Pyrenestes ostrinus).

Two distinct seed sizes are available in this finch's habitat. Birds with large beaks can crack the harder, larger seeds; birds with small beaks process small seeds efficiently; birds with intermediate beaks handle neither well. Disruptive selection maintains a bimodal beak-size distribution within a single interbreeding population, with discrete large-beak and small-beak morphs.

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Comparing the Three Modes

Feature	Directional	Stabilising	Disruptive
Favoured phenotype	One extreme	Intermediate	Both extremes
Effect on mean	Shifts	No change	No change (or slight)
Effect on variance	Decreases (skewed)	Decreases	Increases
Distribution shape	Skewed	Narrower, taller	Bimodal
Evolutionary role	Adaptation to new conditions	Maintenance of optimal phenotype	Can drive speciation
Canonical example	Antibiotic resistance	Human birth weight	African seed-cracker beaks

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Industrial Melanism — The Peppered Moth (Biston betularia)

The peppered moth case is the most cited British example of natural selection in the wild, and a paradigm illustration of how environmental change reverses the direction of selection.

Before the Industrial Revolution (early 19th century):

• The typical (typica) light-coloured form dominated, camouflaged against pale, lichen-covered tree bark in unpolluted woodlands.
• The rare melanic (carbonaria) form was conspicuous against the lichen and was selectively predated by birds.

During industrialisation (mid–late 19th century):

• Coal smoke killed lichen on tree trunks across northern Britain and deposited dark soot, darkening bark.
• The melanic form was now better camouflaged; the typica form became conspicuous.
• The frequency of the carbonaria allele rose rapidly through directional selection. In Manchester by 1895, over 95% of peppered moths were melanic.

After Clean Air legislation (1956 onward):

• Pollution declined; lichen recolonised; bark lightened.
• Selection reversed direction.
• Typica allele frequency rose again. By the 2000s, melanic-form frequency in formerly industrial areas had declined to single-figure percentages.

The case is unusually well documented because Kettlewell's mark-release-recapture studies in the 1950s recorded predation rates directly, and recent molecular work has identified the underlying cortex gene transposable-element insertion that confers the melanic phenotype. Modern critiques have refined some of Kettlewell's experimental design without overturning his core finding.

Key Point: Natural selection is not goal-directed. When the environment changes, the direction of selection can reverse — the same allele that was advantageous becomes deleterious. There is no "improvement" toward an absolute optimum; there is only fit to the current environment.

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Selection Coefficients and Rate of Change

The strength of selection on an allele is quantified by the selection coefficient (s):

s = 1 − (relative fitness of genotype)

For a perfectly lethal recessive (every aa individual dies pre-reproduction), s = 1. For a mild fitness reduction of 1%, s = 0.01. The rate at which an allele's frequency changes per generation depends on s, on the current allele frequency, and on whether the allele is dominant or recessive:

• Dominant beneficial alleles spread quickly because they are immediately visible to selection in heterozygotes.
• Recessive beneficial alleles spread slowly when rare because most copies are masked in heterozygotes; once they reach moderate frequency, the spread accelerates.
• Recessive deleterious alleles are removed slowly when rare because most copies are hidden in heterozygotes — this is the same logic that explains why disease alleles like CF and PKU persist at substantial carrier frequencies despite strong selection against affected homozygotes (cross-link to lesson 1).

The full mathematical treatment (Wright's equation for selection on an autosomal locus) is beyond AQA 7402 but is the natural undergraduate extension.

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Selection in Action — Modern Documented Examples

A-Level questions sometimes ask for named examples of natural selection in modern populations:

• Antibiotic resistance — MRSA, VRE, MDR-TB; directional selection imposed by clinical antibiotic use.
• Pesticide resistance — DDT resistance in mosquitoes (Anopheles), pyrethroid resistance in cotton bollworms.
• Glyphosate (Roundup) resistance in weeds — selection imposed by Roundup-Ready cropping systems.
• HIV antiretroviral resistance — selection by antiretroviral drugs within individual patients (intra-host evolution).
• Beak-size shift in Galápagos medium ground finches (Geospiza fortis) following drought years — Grant & Grant's long-term Daphne Major study documented allele-frequency shifts within a few generations.
• Sickle-cell allele frequency in malaria-endemic regions — balancing selection (heterozygote advantage; cross-link to lesson 1).

These examples are often the subject of 6- and 9-mark exam questions; commit at least three to memory with the named species and the named selection pressure.

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Sexual Selection (A-Level Extension)

Darwin's Descent of Man (1871) introduced sexual selection — selection that arises from differential mating success rather than differential survival. Two forms: