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Year 11 Science

Evolution and Natural Selection

Understand how populations change over time through variation, selection pressures and adaptation -- the driving forces behind the diversity of life on Earth.

Darwin's Theory of Natural Selection

Charles Darwin proposed the theory of evolution by natural selection in 1859. He observed that individuals within a population show variation in their traits, and those with traits better suited to the environment are more likely to survive and reproduce. Over many generations, beneficial traits become more common in the population.

The Four Steps of Natural Selection

1. Variation

Individuals in a population differ in their traits (due to mutations, sexual reproduction)

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2. Competition

Resources (food, mates, territory) are limited; not all survive

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3. Selection

Individuals with advantageous traits are more likely to survive and reproduce

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4. Inheritance

Favourable traits are passed to offspring; allele frequency changes over generations

Key distinction: Evolution acts on populations, not individuals. An individual organism cannot evolve during its lifetime -- it is the frequency of alleles in the population gene pool that changes over generations.

Evidence for Evolution

Multiple lines of evidence support the theory of evolution from diverse fields of biology and geology.

Fossil Record

Fossils show a progression of forms over geological time. Transitional fossils (e.g. Tiktaalik) bridge gaps between major groups, such as fish and tetrapods.

Comparative Anatomy

Homologous structures (e.g. human arm, whale flipper, bat wing) share underlying skeletal layout despite different functions, indicating a common ancestor.

Molecular Biology

DNA and protein sequence comparisons reveal evolutionary relationships. More closely related species have more similar DNA sequences.

Biogeography

The distribution of species across continents (e.g. marsupials in Australia) reflects their evolutionary history and geographical isolation.

Australian example: Australia's unique marsupial fauna (kangaroos, koalas, wombats) evolved in isolation after the continent separated from Gondwana ~45 million years ago, filling ecological niches occupied by placental mammals on other continents.

Adaptation and Speciation

An adaptation is a heritable trait that increases an organism's fitness (ability to survive and reproduce) in its environment. Over long periods, accumulating adaptations can lead to speciation -- the formation of new, distinct species.

Allopatric Speciation

A physical barrier (river, mountain range, ocean) geographically isolates populations. Over time, they diverge genetically and can no longer interbreed.

Sympatric Speciation

New species arise within the same geographic area, often due to polyploidy (in plants) or ecological/behavioural differences that reduce gene flow.

Types of Adaptation

Structural: Physical features (e.g. the thick fur of a polar bear for insulation, or the sharp beak of an eagle for tearing meat).

Physiological: Internal body processes (e.g. the ability of camels to conserve water, or antibiotic resistance in bacteria).

Behavioural: Actions that improve survival (e.g. migration of birds, nocturnal hunting to avoid daytime predators).

Key Vocabulary

Natural Selection

The process by which organisms with favourable traits are more likely to survive and reproduce, passing those traits to the next generation.

Allele Frequency

The proportion of a particular allele in a population's gene pool. Changes in allele frequency over time is the measurable basis of evolution.

Speciation

The evolutionary process by which populations diverge to become distinct species that can no longer interbreed.

Fitness

An organism's ability to survive and reproduce in its environment. Higher fitness means greater reproductive success.

Worked Examples

1

Explain how antibiotic resistance in bacteria is an example of natural selection.

Step 1: Within a bacterial population, random mutations create variation -- some bacteria have alleles that make them resistant to the antibiotic.

Step 2: When the antibiotic is applied (selection pressure), non-resistant bacteria are killed. Resistant bacteria survive.

Step 3: The resistant survivors reproduce, passing the resistance allele to offspring. Over generations, the allele frequency for resistance increases, and the population becomes largely antibiotic-resistant.

2

How does geographic isolation lead to speciation?

Step 1: A physical barrier (e.g. a new river) splits a population into two isolated groups.

Step 2: Each group experiences different environmental conditions and selection pressures. Mutations accumulate independently.

Step 3: Over many generations, genetic differences become so great that the two populations can no longer interbreed even if reunited. They are now separate species (allopatric speciation).

3

The peppered moth population in industrial England shifted from mostly light-coloured to mostly dark-coloured. Explain this using natural selection.

Step 1: Before industrialisation, light-coloured moths were camouflaged on lichen-covered trees. Dark moths were easily spotted by birds (predation = selection pressure).

Step 2: Pollution killed the lichen and darkened tree bark. Now dark moths were camouflaged and light moths were conspicuous.

Step 3: Dark moths survived and reproduced more. The allele frequency for dark colouration increased in the population -- a classic example of directional selection.

Knowledge Check

Select the correct answer for each question. Click "Check Answer" to see if you are right.

Question 1

Which of the following is NOT a requirement for natural selection to occur?

Question 2

Homologous structures are evidence for evolution because they suggest:

Question 3

Allopatric speciation requires:

Question 4

A camel's ability to conserve water through concentrated urine is an example of:

Question 5

Evolution is best defined as:

Key Concepts Summary

Year 11: Genetics and Inheritance Year 11: Intermolecular Forces