Variation, Natural Selection and Adaptation

Evolution happens when populations change over time, and the engine of change is natural selection acting on variation. This lesson ties together the origin of variation (genetic and environmental), the mechanism by which favourable variants spread (natural selection in its three modes), and the adaptations that result. OCR A-Level Biology A specification 4.2.2 (h)–(j) requires you to describe variation, explain how it relates to natural selection, and recognise the anatomical, behavioural and physiological adaptations of organisms to their environments.

Key Definitions:

• Variation — the differences between individuals of the same species.
• Interspecific variation — variation between different species.
• Intraspecific variation — variation within the same species.
• Continuous variation — traits that vary along a smooth gradient (e.g. height, mass).
• Discontinuous variation — traits falling into distinct categories (e.g. blood group).
• Natural selection — differential survival and reproduction of individuals best adapted to their environment.
• Adaptation — any feature that enhances an organism's survival and reproduction in its environment.

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Variation

Variation is the raw material of evolution. Without differences between individuals, natural selection has nothing to act on.

Interspecific vs Intraspecific

• Interspecific variation is the differences between species (e.g. a lion vs a tiger).
• Intraspecific variation is the differences within a species (e.g. two lions from the same pride).

Both are real and measurable; the two are connected because intraspecific variation is the starting point from which new species arise.

Causes of Variation

1. Genetic causes:
   • Mutation — random changes in DNA sequence generate new alleles.
   • Meiosis — independent assortment and crossing over shuffle existing alleles.
   • Random fertilisation — any sperm can fertilise any egg.
   • Sexual reproduction in general combines alleles from two parents.
2. Environmental causes:
   • Nutrition affects growth and size.
   • Temperature can alter phenotype (e.g. Himalayan rabbits have dark fur only on cooler body parts).
   • Injury and disease.
   • Light affects plant growth.
3. Gene–environment interactions:
   • The majority of traits result from both genes and environment.
   • Identical twins reared apart can differ in height or health because of environment.

Continuous and Discontinuous Variation

flowchart TD
    A[Variation] --> B[Continuous]
    A --> C[Discontinuous]
    B --> B1[Polygenic traits]
    B --> B2[Normal distribution]
    B --> B3[Strong environmental influence]
    C --> C1[Usually one or few genes]
    C --> C2[Distinct categories]
    C --> C3[Little environmental influence]

Continuous variation — traits that form a smooth gradient:

• Height, mass, skin colour, leaf length, milk yield.
• Usually polygenic (many genes contribute).
• Often strongly influenced by environment.
• Produces a normal (bell-shaped) distribution when plotted.

Discontinuous variation — traits that fall into distinct categories:

• ABO blood groups (A, B, AB, O).
• Sex (male/female in most mammals).
• Tongue rolling (yes/no).
• Flower colour in many plants.
• Usually controlled by one or a few genes with major effects.
• Little or no environmental influence.
• Produces a discrete distribution (e.g. a bar chart with separate categories).

Exam Tip: OCR mark schemes want you to state clearly: continuous = polygenic + environmental influence + normal distribution; discontinuous = one/few genes + little environment + distinct categories. Use these exact phrases.

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Natural Selection

Darwin and Wallace

Natural selection was proposed in 1858 by Charles Darwin and independently by Alfred Russel Wallace in a joint paper to the Linnean Society of London. Darwin expanded it the following year into On the Origin of Species (1859). The theory has four essential observations and two inferences:

Observations:

1. Organisms produce more offspring than can survive.
2. Populations nevertheless remain roughly constant over time.
3. Individuals show variation, much of it heritable.
4. Resources are limited (food, mates, nesting sites).

Inferences:

1. A struggle for existence takes place.
2. Individuals with favourable variations are more likely to survive and reproduce; over generations, their traits become more common.

The Modern Synthesis

In the 20th century, natural selection was combined with Mendelian genetics (Darwin did not know about Mendel's work) to produce the modern synthesis — the framework of population genetics in which evolution is defined as a change in allele frequencies over generations.

The Process of Natural Selection (Step by Step)

1. Variation exists within a population (from mutation, meiosis, random fertilisation).
2. More offspring are produced than can survive; there is competition for resources.
3. Selection pressure (predator, disease, climate) means some phenotypes are more successful.
4. Favourable alleles (those contributing to successful phenotypes) are passed to the next generation.
5. Over generations, allele frequencies change in the population.
6. The population becomes better adapted.

This sequence is the standard mark-scheme answer for "describe how natural selection works". Memorise it.

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Modes of Natural Selection

Natural selection can push a population in three different ways:

flowchart TD
    A[Natural Selection] --> B[Directional]
    A --> C[Stabilising]
    A --> D[Disruptive]
    B --> B1[One extreme favoured]
    C --> C1[Mean favoured, extremes selected against]
    D --> D1[Both extremes favoured, mean selected against]

1. Directional Selection

One extreme of the phenotype range is favoured, shifting the mean of the population over generations.