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This lesson explores natural selection, evolution and speciation through the lens of temperate deciduous woodland. This is one of the most species-rich biomes in the mid-latitudes, and the organisms that live within it provide clear, compelling examples of how evolution works. Every oak tree, every warbler, and every woodland mouse is the product of billions of years of natural selection acting on genetic variation.

Natural selection: the engine of evolutionary change

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3.1.4 Natural selection is the mechanism driving evolutionary change.

Natural selection operates continuously and can take place over billions of years, resulting in the biodiversity of life on Earth.

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Look at any temperate deciduous woodland and you will see extraordinary diversity: hundreds of species of trees, shrubs, ground-layer plants, fungi, invertebrates, birds and mammals, all coexisting in a single habitat. How did this diversity arise?

The answer is natural selection, a process that has been operating continuously since life first appeared on Earth approximately 3.8 billion years ago. Natural selection is not a one-off event. It operates every day, in every population, in every generation. It is the mechanism that drives evolutionary change and has, over immense timescales, produced the biodiversity of life we see today.

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Consider the European robin (Erithacus rubecula), a year-round resident of temperate deciduous woodland. Robins face the full seasonal challenge of this biome: cold winters with short days, reduced invertebrate availability, and competition for limited food and territories. Within any population of robins, there is natural genetic variation: differences in body size, metabolic rate, fat storage capacity and territorial behaviour.

Robins with slightly higher metabolic efficiency or better capacity to store fat reserves survive harsh winters more successfully. They are more likely to be alive in spring to breed. Their offspring inherit these advantageous traits. Over many generations, the frequency of these advantageous genes increases in the population. This is natural selection in action.

1. In your own words, explain why natural selection is described as "continuous".

2. Identify two selection pressures acting on robins during winter in a temperate deciduous woodland.

3. Explain why natural selection can only act on traits that are heritable (passed from parent to offspring through genes).

The four components of natural selection

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3.1.5 Evolution by natural selection involves variation, overproduction, competition for limited resources, and differences in adaptation that affect rates of survival and reproduction.

Natural selection occurs because genetic diversity gives rise to variation within a population. Individuals with variations that provide an advantage in a given environment are more likely to survive and reproduce than others. Variation is heritable; therefore, individuals with advantageous genes can pass them on to their offspring. As a result, the frequency of advantageous genes will increase over many generations.

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Evolution by natural selection can be broken down into four interconnected components. All four must be present for natural selection to operate. Each is illustrated here with examples from temperate deciduous woodland.

1. Variation

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Genetic diversity gives rise to variation within a population. In a woodland population of great tits (Parus major), individuals vary in bill depth, body mass, clutch size and the timing of egg-laying. These differences are not random; they arise from genetic diversity produced through sexual reproduction. Some great tits have genes that cause them to begin nesting earlier in the season than others.

2. Overproduction

Organisms produce far more offspring than the environment can support. A single pedunculate oak (Quercus robur) can produce over 70,000 acorns in a mast year. Of these, only a tiny fraction will germinate, survive herbivory, competition and disease, and grow into mature trees. Blue tits (Cyanistes caeruleus) typically lay clutches of 7 to 13 eggs, but most chicks will not survive their first winter.

3. Competition for limited resources

Because more offspring are produced than can survive, individuals must compete for limited resources. In spring, caterpillar abundance in the woodland canopy peaks for just two to three weeks. Woodland birds such as great tits and blue tits compete intensely for this brief food pulse to feed their nestlings.

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On the woodland floor, light is a critical limiting resource. Bluebells (Hyacinthoides non-scripta) must complete their entire above-ground life cycle, from leaf emergence to flowering to seed set, in the narrow window before the canopy closes in late April and blocks sunlight from the forest floor.

4. Differential survival and reproduction

Individuals with variations that provide an advantage in a given environment are more likely to survive and reproduce. Great tits that time their egg-laying to coincide with the peak in caterpillar abundance raise more surviving chicks than those that lay too early or too late. Oak seedlings that germinate in a canopy gap where a mature tree has fallen receive more light and are more likely to survive than those growing beneath a closed canopy.

Because variation is heritable, individuals with advantageous genes pass them on to their offspring. Over many generations, the frequency of advantageous genes increases in the population. This gradual shift in the genetic composition of a population is evolution.

1. Copy and complete the table below:

1. A single oak tree produces over 70,000 acorns in a mast year, yet an ancient woodland may contain only a few hundred mature oaks. Use the four components of natural selection to explain this difference.

2. Bluebells flower in April before the canopy closes. Explain how natural selection could have led to this timing.

3. Explain why variation within a population is essential for natural selection to occur.

Card sort: natural selection in the woodland