Natural Selection and Evolution

Natural selection is the mechanism by which evolution occurs. It was proposed independently by Charles Darwin and Alfred Russel Wallace in 1858 and remains one of the most important unifying concepts in biology. For A-Level Biology, you need to understand the process in detail, distinguish between different types of selection, and explain how selection leads to adaptation and evolution.

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Charles Darwin and the Theory of Natural Selection

Darwin's Observations

Charles Darwin developed his theory after years of observation, most famously during his voyage on HMS Beagle (1831–1836). His key observations were:

1. Overproduction — Organisms produce far more offspring than can survive to reproduce
2. Variation — Individuals within a population show variation in their characteristics
3. Struggle for existence — Resources (food, space, mates) are limited, so individuals must compete
4. Survival of the fittest — Individuals with characteristics better suited to their environment are more likely to survive and reproduce
5. Inheritance — Advantageous characteristics are passed to offspring, becoming more common in the population over generations

The Modern Understanding

Darwin did not know the mechanism of inheritance (he predated Mendel's work on genetics). We now understand that:

• Variation arises from mutations — random changes in DNA sequences
• Variation is also generated by meiosis (independent assortment, crossing over) and sexual reproduction (random fusion of gametes)
• Advantageous alleles increase in frequency in the gene pool because individuals carrying them have higher reproductive fitness — they leave more offspring

Exam Tip: When explaining natural selection, always use precise language. State that the environment selects for individuals with advantageous phenotypes, that these individuals have higher reproductive success, and that the alleles responsible are passed to offspring and increase in frequency. Avoid saying organisms "choose" to evolve or that they "develop" new features because they "need" them.

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The Process of Natural Selection — Step by Step

1. Within a population, there is genetic variation due to mutations and sexual reproduction
2. Environmental conditions create selection pressures (e.g., predation, disease, competition, climate)
3. Individuals with phenotypes better suited to the environment have a selective advantage — they are more likely to survive to reproductive age
4. These individuals reproduce and pass on the alleles responsible for their advantageous phenotype to their offspring
5. Over many generations, the frequency of the advantageous allele increases in the gene pool
6. The population becomes better adapted to its environment — this is evolution by natural selection

The following diagram summarises the process of natural selection from genetic variation to evolution:

flowchart TD
    A["Genetic Variation<br/>in Population"] --> B["Environmental<br/>Selection Pressure"]
    B --> C["Individuals with<br/>Advantageous Alleles<br/>Survive"]
    C --> D["Reproduce and Pass<br/>On Alleles"]
    D --> E["Allele Frequency<br/>Changes Over Time"]
    E -->|"Over many<br/>generations"| F["Evolution /<br/>Speciation"]

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Types of Natural Selection

Natural selection does not always work the same way. The pattern of selection depends on which phenotypes are favoured.

Stabilising Selection

Stabilising selection favours the intermediate phenotype and selects against extreme phenotypes. It reduces variation in the population but does not change the mean.

Feature	Detail
Effect on mean	No change — the average remains the same
Effect on variation	Decreased — extreme phenotypes are selected against
When it occurs	In stable environments where the current average phenotype is optimal
Example	Human birth weight — babies of intermediate weight (~3.5 kg) have the highest survival rate. Very low or very high birth weights increase mortality risk

Directional Selection

Directional selection favours one extreme phenotype over others, causing the mean phenotype to shift in one direction.

Feature	Detail
Effect on mean	Shifts towards the favoured extreme
Effect on variation	May decrease initially as the distribution shifts
When it occurs	When environmental conditions change, creating new selection pressures
Example	Antibiotic resistance in bacteria — when antibiotics are present, resistant bacteria are strongly favoured. The mean resistance level of the population increases over generations

Another classic example: industrial melanism in the peppered moth (Biston betularia). Before the Industrial Revolution, the light-coloured form was camouflaged on lichen-covered tree bark and was more common. When soot darkened tree bark during industrialisation, the dark (melanic) form became better camouflaged and increased in frequency — directional selection shifted the population towards the dark phenotype.

Disruptive Selection

Disruptive selection favours both extreme phenotypes at the expense of the intermediate phenotype, potentially splitting the population into two distinct groups.

Feature	Detail
Effect on mean	May remain the same, but the distribution becomes bimodal (two peaks)
Effect on variation	Increased — the population diversifies
When it occurs	When the environment has two distinct niches and the intermediate phenotype is disadvantaged in both
Example	African seedcracker finches (Pyrenestes ostrinus) — birds with large beaks are efficient at cracking hard seeds; birds with small beaks are efficient at cracking soft seeds. Intermediate-sized beaks are less efficient at both, so both extremes are favoured

Exam Tip: You must be able to draw and interpret frequency distribution graphs for each type of selection. Practise sketching the "before" and "after" curves: stabilising narrows the curve without shifting it; directional shifts the peak; disruptive creates two peaks.

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Adaptation

Adaptation is the process by which populations become better suited to their environment over time through natural selection. An adaptation can also refer to a specific feature that enhances survival and reproduction.

Types of Adaptation

Type	Definition	Examples
Anatomical (structural)	Physical features of the body	Thick fur in Arctic mammals; long roots in desert plants; streamlined body in fish
Physiological (biochemical)	Internal body processes	Production of antifreeze proteins in Antarctic fish; ability to concentrate urine in desert mammals; venom production in snakes
Behavioural	Actions or behaviours	Migration in birds; courtship displays; nocturnal activity to avoid heat in desert animals

Co-adaptation

Co-adaptation occurs when two or more species evolve in response to each other. This includes:

• Co-evolution between predators and prey — as prey evolve better defences, predators evolve better means of overcoming them (an "evolutionary arms race")
• Co-evolution between parasites and hosts — parasites evolve to exploit hosts; hosts evolve resistance
• Mutualistic co-evolution — e.g., flowering plants and their pollinators evolve together, with flower shape and pollinator mouthparts becoming increasingly matched

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Fitness

Fitness in evolutionary biology has a precise meaning: it is the reproductive success of an individual — the number of offspring it produces that survive to reproductive age. Fitness is always relative — an individual's fitness is measured compared to others in the same population.

• High fitness — the individual has alleles that confer a selective advantage, allowing it to survive and produce more offspring
• Low fitness — the individual has alleles that are disadvantageous, reducing its survival or reproduction

Natural selection acts on phenotypes (which are determined by genotypes and the environment). Over time, alleles associated with higher fitness increase in frequency.

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Genetic Drift

Not all evolutionary change is due to natural selection. Genetic drift is the random change in allele frequencies in a population due to chance events. It is most significant in small populations.

Key Features of Genetic Drift

• It is random — unlike natural selection, it does not favour beneficial alleles
• It can lead to the fixation (100% frequency) or loss (0% frequency) of alleles
• Its effects are strongest in small populations and negligible in very large populations
• It can reduce genetic diversity

Examples

• Bottleneck effect — a population crash (e.g., natural disaster) randomly eliminates alleles. The surviving population has a different allele frequency from the original, purely by chance.
• Founder effect — a small group colonises a new area. The founders carry only a sample of the original population's alleles, and random drift can rapidly change allele frequencies in the new population.

Exam Tip: Be careful to distinguish between natural selection (non-random — favours advantageous alleles) and genetic drift (random — allele changes are due to chance). Both cause evolution (changes in allele frequency), but the mechanisms are fundamentally different.

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Gene Pool and Allele Frequency

The gene pool is the total set of all alleles of all genes in a population at a given time. Evolution can be defined as a change in allele frequencies within a gene pool over generations.

Allele frequencies can change due to:

1. Natural selection — non-random; favours alleles that increase fitness
2. Genetic drift — random changes, especially in small populations
3. Gene flow — migration of individuals between populations introduces or removes alleles
4. Mutation — creates new alleles
5. Non-random mating — e.g., sexual selection or inbreeding changes genotype frequencies

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Evidence That Natural Selection Occurs

Evidence	Explanation
Antibiotic resistance	Bacteria evolve resistance to antibiotics within years — directional selection in action
Pesticide resistance	Insects and weeds evolve resistance to pesticides — the same mechanism
Industrial melanism	Peppered moth colour frequency changed with industrialisation and changed back after clean air legislation
Darwin's finches	Different beak sizes evolved on different Galapagos islands to exploit different food sources
Artificial selection	Humans mimic natural selection by selectively breeding organisms — demonstrates that selection can cause major changes in populations over relatively few generations

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Artificial Selection

Artificial selection (selective breeding) occurs when humans choose which organisms to breed based on desirable characteristics. The process is fundamentally the same as natural selection, but the selection pressure is imposed by humans rather than the environment.

Process

1. Select individuals with the desired phenotype from the population
2. Breed these individuals together
3. From the offspring, select those that best express the desired trait
4. Repeat over many generations

Examples

Organism	Selected Trait	Result
Dairy cattle	High milk yield	Modern Holsteins produce ~10,000 litres per year (wild cattle produce ~1,000)
Wheat	High grain yield, disease resistance, short straw	Modern wheat varieties produce far more grain than ancestral wild grass
Dogs	Various traits (size, temperament, appearance)	Over 400 breeds with enormous phenotypic variation
Crop plants	Resistance to specific diseases	Reduces crop losses and need for pesticides

Risks of Artificial Selection

• Reduced genetic diversity — by selecting for specific traits, many alleles are lost from the population
• Inbreeding depression — closely related individuals are often bred together, increasing homozygosity and the expression of harmful recessive alleles
• Vulnerability to disease — genetically uniform populations can be devastated by a single pathogen

Exam Tip: Exam questions may ask you to compare natural selection with artificial selection. Key similarities: both involve selection of individuals with advantageous phenotypes and the inheritance of those traits. Key difference: in natural selection, the environment determines which phenotypes are advantageous; in artificial selection, humans make the choice.

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Summary

Key Concept	Detail
Natural selection	Individuals with advantageous phenotypes survive and reproduce more; alleles increase in frequency
Stabilising selection	Favours intermediate phenotype; reduces variation
Directional selection	Favours one extreme; shifts the mean
Disruptive selection	Favours both extremes; increases variation
Adaptation	Anatomical, physiological, or behavioural features enhancing survival
Fitness	Reproductive success relative to others in the population
Genetic drift	Random allele frequency changes; significant in small populations
Artificial selection	Humans select for desired traits; reduces genetic diversity

Exam Tip: Natural selection is perhaps the most important concept in A-Level Biology. You must be able to explain the mechanism clearly, distinguish between the three types of selection, and provide specific named examples for each.

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A-Level Deep Dive: Natural Selection and Evolution

Spec mapping

The Edexcel 9BI0 specification places natural selection within Topic 4: Biodiversity and Natural Resources, with synoptic ties forwards to Lesson 7 (Speciation) — selection plus reproductive isolation drives speciation — and Lesson 8 (Evidence for Evolution) — fossil, biogeographical, anatomical and molecular records of selection's outcomes. Backwards, the topic depends on Topic 8 (Modern Genetics) — DNA mutation as the source of new alleles, Topic 6 (Immunity) — antibiotic resistance as directional selection in real time, and Topic 1 — sickle-cell heterozygote-advantage balancing selection in malarial regions, with the molecular mechanism (haemoglobin polymerisation under low oxygen) tying directly to Topic 7 protein structure. Specification statements concern: variation as the substrate; the four conditions for natural selection; the three modes (stabilising, directional, disruptive); allele-frequency change as the operational definition of evolution; the Hardy-Weinberg principle as the null model; and artificial selection as a human-imposed analogue (refer to the official Pearson Edexcel 9BI0 specification document for exact wording).

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Worked example with full mark scheme

Question (8 marks):

The sickle-cell allele $\text{Hb}^S$HbS persists at high frequency in West African populations because heterozygotes ($\text{Hb}^A\text{Hb}^S$HbAHbS) are partially protected against Plasmodium falciparum malaria, while $\text{Hb}^S\text{Hb}^S$HbSHbS homozygotes suffer sickle-cell disease.

(a) In one West African sample the frequency of the recessive $\text{Hb}^S$HbS allele is $q = 0.15$q=0.15 and the population is approximately at Hardy-Weinberg equilibrium. Calculate the expected frequencies of the three genotypes and the percentage of the population expected to be heterozygous carriers. (3)

(b) The same allele is present at $q \approx 0.02$q≈0.02 in an African-descent population now living in a malaria-free region. Explain, using allele-frequency arguments, why $q$q is expected to fall further over many generations in this new environment. (3)

(c) Explain why a population at Hardy-Weinberg equilibrium is not evolving, and state which of the H-W assumptions the West African sickle-cell case most clearly violates. (2)

Solution with mark scheme:

(a) With $q = 0.15$q=0.15 and $p = 1 - q = 0.85$p=1−q=0.85:

$p^2 = 0.85^2 = 0.7225$p2=0.852=0.7225 ($\text{Hb}^A\text{Hb}^A$HbAHbA, $\sim 72.3\%$∼72.3%).

$2pq = 2 \times 0.85 \times 0.15 = 0.255$2pq=2×0.85×0.15=0.255 ($\text{Hb}^A\text{Hb}^S$HbAHbS, $\sim 25.5\%$∼25.5%).

$q^2 = 0.0225$q2=0.0225 ($\text{Hb}^S\text{Hb}^S$HbSHbS, $\sim 2.25\%$∼2.25%).

M1 (AO2.1) — correct $p = 0.85$p=0.85 from $p + q = 1$p+q=1.

M1 (AO2.1) — correct H-W expansion ($p^2 + 2pq + q^2 = 1$p2+2pq+q2=1) with all three genotype frequencies.

A1 (AO2.1) — heterozygote frequency $\sim 25.5\%$∼25.5%, with disease frequency $q^2 \sim 2.25\%$q2∼2.25% as sanity check.

(b) M1 (AO3.1a) — in the new environment the heterozygote has lost its selective advantage (no malaria), while the $\text{Hb}^S\text{Hb}^S$HbSHbS homozygote retains its disease cost.

M1 (AO3.1a) — the balanced-polymorphism equilibrium no longer applies; selection is now directional against $\text{Hb}^S$HbS.

A1 (AO3.2a) — $q$q is expected to fall over many generations, slowed by recessivity (selection acts only on the rare $q^2$q2 homozygote).

(c) M1 (AO1.2) — H-W specifies a null model: a population is at H-W equilibrium when allele frequencies do not change between generations. Evolution is, operationally, allele-frequency change; therefore an H-W population is by definition not evolving.

M1 (AO3.1a) — the West African case violates the no selection assumption — heterozygote-advantage selection actively maintains $q$q.

Total: 8 marks.

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Specimen question modelled on the Edexcel 9BI0 paper format

Question (6 marks): Outline the four conditions required for evolution by natural selection and apply them to the classic peppered moth (Biston betularia) study, evaluating the strength of the evidence that industrial melanism is an example of directional natural selection.

Mark scheme decomposition by AO: