In simple terms
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Evolution
Cambridge 9700 Paper 4 — Evolution (17.3). A-Level Notes diagram-backed lesson with premium structure and live visuals.
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Variation: Individuals within a population show genetic variation (due to mutation, meiosis, random fertilisation).
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Selection Pressure: The environment exerts pressure (e.g., predation, disease, resource availability).
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Differential Survival: Individuals with advantageous phenotypes are better suited to survive and outcompete others.
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Differential Reproduction: These survivors are more likely to reproduce, passing on their advantageous alleles.
What this topic covers
The official Cambridge syllabus points this lesson works through.
- 17.3.1
Outline the theory of evolution as a process leading to the formation of new species from pre-existing species over time, as a result of changes to gene pools from generation to generation
- 17.3.2
Discuss how DNA sequence data can show evolutionary relationships between species
- 17.3.3
Explain how speciation may occur as a result of genetic isolation by: • geographical separation (allopatric speciation) • ecological and behavioural separation (sympatric speciation)
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Key formulas
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Full topic notes
Formal explanation with the rigour you need for the exam.
Natural Selection: The Driving Force
Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. This mechanism drives adaptation and evolution. The process can be summarised in a sequence of observations and inferences. First, there is overproduction of offspring, leading to a struggle for existence as resources are limited. Within this struggle, there is variation among individuals. Some of this variation is heritable. Individuals with variations that make them better suited to their environment (i.e., they possess advantageous adaptations) are more likely to survive and reproduce. This is differential survival and reproduction. Over many generations, this leads to an increase in the frequency of the advantageous alleles in the population, and the population becomes better adapted to its environment.
There are three main types of natural selection:
Variation: Individuals within a population show genetic variation (due to mutation, meiosis, random fertilisation).
Selection Pressure: The environment exerts pressure (e.g., predation, disease, resource availability).
Differential Survival: Individuals with advantageous phenotypes are better suited to survive and outcompete others.
Differential Reproduction: These survivors are more likely to reproduce, passing on their advantageous alleles.
Inheritance: Over generations, the frequency of these advantageous alleles increases in the population, leading to adaptation.
Directional Selection: Favours one extreme phenotype over others, shifting the population mean towards that extreme (e.g., increasing body size in response to colder climates, or antibiotic resistance in bacteria).
Stabilising Selection: Favours intermediate phenotypes and acts against extreme variations, reducing phenotypic variance (e.g., human birth weight, where very low or very high weights are associated with higher mortality).
Disruptive Selection: Favours individuals at both extremes of the phenotypic range over intermediate phenotypes, potentially leading to two distinct sub-populations (e.g., birds with either very small or very large beaks, but not medium, in an environment with two distinct seed sizes).
Genetic Variation: The Raw Material
Evolution cannot occur without genetic variation within a population. This variation provides the different phenotypes upon which natural selection can act. The primary sources of this variation are:
Mutation: Random changes in the DNA sequence, creating new alleles. This is the ultimate source of all new genetic variation.
Meiosis: Independent assortment of homologous chromosomes during Meiosis I and crossing over between non-sister chromatids generate new combinations of alleles on chromosomes and in gametes.
Random Fertilisation: The chance combination of any male gamete with any female gamete further increases the number of possible allele combinations in offspring.
Speciation: The Formation of New Species
Speciation is the evolutionary process by which new biological species arise. A species is typically defined as a group of organisms that can interbreed and produce fertile offspring. Speciation often involves reproductive isolation and divergent evolution driven by natural selection or genetic drift.
For speciation to be complete, populations must become reproductively isolated. This means they can no longer interbreed to produce fertile offspring. Reproductive isolating mechanisms can be pre-zygotic (preventing fertilisation) or post-zygotic (acting after fertilisation).
Allopatric Speciation: Occurs when populations are geographically isolated (e.g., by a mountain range, river, or ocean). The isolated populations experience different selection pressures and accumulate genetic differences through mutation and natural selection. Over time, they become reproductively isolated even if the barrier is removed.
Sympatric Speciation: Occurs within the same geographical area. This can happen through various mechanisms, such as polyploidy (especially in plants, where changes in chromosome number prevent interbreeding) or disruptive selection (where extreme phenotypes are favoured, leading to reproductive isolation without a physical barrier).
Temporal isolation: Breeding at different times of the day or year.
Ecological isolation: Living in different habitats within the same area.
Behavioural isolation: Differences in courtship rituals or mating calls.
Mechanical isolation: Incompatible reproductive structures.
Gametic isolation: Sperm and egg are incompatible.
Hybrid inviability: The hybrid zygote fails to develop or reach maturity.
Hybrid sterility: The hybrid offspring is sterile (e.g., a mule).
The Hardy–Weinberg Principle
The Hardy–Weinberg principle describes a theoretical population that is not evolving. It provides a baseline to compare real populations against, allowing us to detect if evolutionary forces are at play. It assumes five conditions are met: a large population size, random mating, no mutation, no gene flow (migration), and no natural selection.
p + q = 1 p^2 + 2pq + q^2 = 1
Where:
- = frequency of the dominant allele
- = frequency of the recessive allele
- = frequency of homozygous dominant genotype
- = frequency of homozygous recessive genotype
- 2pq = frequency of heterozygous genotype
Genetic Drift
Genetic drift is the change in allele frequencies in a population due to random chance. Unlike natural selection, which is driven by fitness differences, genetic drift is completely random. It has a much more significant impact on small populations, where chance events can quickly lead to the loss or fixation of alleles. The primary consequence of genetic drift is a reduction in genetic variation within a population, as alleles can be lost entirely. This can be particularly harmful if the lost alleles were beneficial under different environmental conditions. Unlike natural selection, which consistently leads to adaptation, the changes from genetic drift are random and not necessarily adaptive.
Bottleneck Effect: Occurs when a population undergoes a drastic reduction in size (e.g., due to a natural disaster). The surviving individuals may not represent the original population's genetic diversity, leading to altered allele frequencies.
Founder Effect: Occurs when a small group of individuals colonises a new area. The new population's gene pool may be very different from the source population due to the small, unrepresentative sample of alleles.
Worked examples
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In a population of 10,000 students, 1% are homozygous recessive for a particular genetic condition. Calculate the frequency of: (a) the recessive allele. (b) the dominant allele. (c) heterozygous individuals.
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We use the Hardy-Weinberg equations: p + q = 1 and p^2 + 2pq + q^2 = 1. Given: frequency of homozygous recessive genotype = 1% = 0.01.
In a population of 500 insects, 20 are susceptible to a new pesticide. Susceptibility is a recessive trait (r). Calculate: (a) The frequency of the homozygous recessive genotype (rr). (b) The frequency of the recessive allele (r). (c) The frequency of the dominant allele (R) for resistance. (d) The number of heterozygous insects (Rr) in the population.
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We use the Hardy-Weinberg equations. The total population size is 500.
How it all connects
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Glossary
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Revision flashcards
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What is natural selection?
The process where organisms with phenotypes better adapted to their environment tend to survive and reproduce more successfully, passing on their advantageous alleles to the next generation.
Key takeaways
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Variation: Individuals within a population show genetic variation (due to mutation, meiosis, random fertilisation).
- ✓
Selection Pressure: The environment exerts pressure (e.g., predation, disease, resource availability).
- ✓
Differential Survival: Individuals with advantageous phenotypes are better suited to survive and outcompete others.
- ✓
Differential Reproduction: These survivors are more likely to reproduce, passing on their advantageous alleles.
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Inheritance: Over generations, the frequency of these advantageous alleles increases in the population, leading to adaptation.
Practice — then mark it
The whole point: a real Cambridge question, marked mark-by-mark.
9700/42 · Q2(a)
Outline the processes that may affect allele frequencies in wildlife populations.
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