Biology · 12th Grade · Evolutionary Dynamics · Weeks 19-27

Darwin and the Theory of Natural Selection

Explore the historical context of Darwin's theory and the core principles of natural selection.

• Common Core State Standards
• HS-LS4-2

About This Topic

Natural selection and adaptation are the central mechanisms of evolutionary change, explaining how populations evolve over time in response to environmental pressures. 12th grade students analyze the four requirements for natural selection: overproduction of offspring, genetic variation, struggle for existence, and differential reproductive success. This topic aligns with HS-LS4-2 and HS-LS4-4, emphasizing how the interaction of these factors leads to the adaptation of populations to their environments.

Students explore different types of selection (stabilizing, directional, and disruptive) and investigate how rapid environmental shifts can lead to speciation or extinction. The curriculum also highlights the importance of genetic variation as the raw material for evolution. This topic comes alive when students can physically model the patterns of selection through simulations and engage in collaborative investigations of real-world evolutionary case studies, such as antibiotic resistance or the beak shapes of Darwin's finches.

Key Questions

1. Explain the key tenets of Darwin's theory of natural selection.
2. Analyze how genetic variation serves as the raw material for natural selection.
3. Differentiate between natural selection and artificial selection.

Learning Objectives

• Explain the four core tenets of Darwin's theory of natural selection: overproduction, variation, struggle for existence, and differential survival and reproduction.
• Analyze how genetic variation within a population provides the raw material upon which natural selection acts.
• Compare and contrast the mechanisms and outcomes of natural selection and artificial selection, providing specific examples of each.
• Evaluate the role of environmental pressures in driving adaptive evolutionary change within a population.

Before You Start

Introduction to Genetics and Heredity

Why: Students must understand basic principles of inheritance, genes, and alleles to grasp how variation is passed down and acted upon by selection.

Population Dynamics

Why: A foundational understanding of populations, including concepts like population size and growth, is necessary to discuss overproduction and struggle for existence.

Key Vocabulary

Natural Selection	The process whereby organisms better adapted to their environment tend to survive and produce more offspring. It is a key mechanism of evolution.
Genetic Variation	The diversity of gene frequencies within a population. This variation is essential for natural selection to act upon.
Adaptation	A trait that increases an organism's survival and reproductive success in its specific environment. Adaptations arise through natural selection.
Fitness	In evolutionary terms, fitness refers to an organism's ability to survive and reproduce in a particular environment, passing on its genes to the next generation.
Differential Reproductive Success	The idea that individuals with certain traits are more likely to reproduce and pass those traits on than individuals without those traits.

Watch Out for These Misconceptions

Common Misconception

Students often think that individuals evolve during their lifetime to 'fit' their environment.

What to Teach Instead

Teachers must emphasize that evolution happens to populations over generations, not individuals. Using a simulation where students see that 'unfit' individuals simply don't reproduce helps clarify that the population's average trait is what changes.

Common Misconception

There is a common belief that evolution has a 'goal' or is moving toward 'perfection.'

What to Teach Instead

It is important to teach that natural selection only acts on existing variation and is limited by environmental context. A structured debate on 'The Imperfections of Human Anatomy' (like the lower back or the eye's blind spot) can help debunk the idea of perfect design.

Active Learning Ideas

Simulation Game: The 'Beaks and Seeds' Lab

Students use different tools (tweezers, spoons, clips) to represent bird beak variations and attempt to 'eat' different types of seeds. They collect data on survival and reproduction rates over several 'generations' to see how the population's traits shift.

50 min·Small Groups

Inquiry Circle: Antibiotic Resistance Modeling

Using colored beads to represent bacteria with different levels of resistance, students simulate the effect of an antibiotic treatment. They observe how 'missing a dose' allows the most resistant bacteria to survive and repopulate, illustrating natural selection in real-time.

40 min·Pairs

Think-Pair-Share: Selective Pressures in Urban Environments

Pairs are given examples of 'urban evolution' (e.g., moths in industrial areas or lizards in cities). They identify the specific selective pressures and predict how the population might change over the next 50 years, then share their ideas with the class.

20 min·Pairs

Real-World Connections

• Medical researchers study antibiotic resistance in bacteria to develop new treatment strategies. Understanding natural selection helps predict how pathogens will evolve in response to drugs.
• Conservation biologists use principles of natural selection to manage endangered species. They might identify traits that enhance survival in changing habitats or select for genetic diversity to improve resilience.
• Agricultural scientists practice artificial selection when breeding livestock or crops for desirable traits, such as increased yield or disease resistance, a process that mirrors natural selection but with human direction.

Assessment Ideas

Quick Check

Present students with a scenario: 'A population of rabbits lives in a snowy environment. Some rabbits have white fur, and others have brown fur. Predators can see brown fur more easily.' Ask students to identify which trait is likely to increase in frequency and explain why, referencing at least two tenets of natural selection.

Discussion Prompt

Pose the question: 'How does the presence of genetic variation in a population influence the rate and direction of evolutionary change under natural selection?' Facilitate a class discussion where students articulate the relationship between variation and adaptation.

Exit Ticket

Ask students to write a brief comparison between natural selection and artificial selection. They should name one example of each and identify the key difference in the driving force behind the selection process.

Frequently Asked Questions

What is the difference between 'fitness' in biology and everyday life?

In everyday life, 'fitness' often means physical strength or health. In biology, 'evolutionary fitness' specifically refers to an organism's ability to survive and, most importantly, pass its genes on to the next generation. A long-lived organism that never reproduces has a fitness of zero.

How does genetic variation arise in a population?

Genetic variation primarily arises through random mutations in DNA and the shuffling of genes during sexual reproduction (meiosis and fertilization). Without this variation, natural selection would have no 'choices' to act upon, and the population could not evolve.

How can active learning help students understand natural selection?

Active learning strategies like 'natural selection simulations' allow students to see the mathematical shift in traits over time. By physically acting as predators or prey, students experience the 'struggle for existence' and can more easily grasp how small advantages lead to large-scale changes in a population's gene pool.

What is the role of environmental change in evolution?

Environmental change acts as the 'filter' for natural selection. A trait that is beneficial in one environment may become a disadvantage if the environment changes. This shift in selective pressure is often what drives the emergence of new species or the extinction of others.