Simulating Natural Selection (VCE SSCE Biology): Revision Notes

Simulating Natural Selection

Introduction to natural selection

Natural selection is the process by which species evolve and change over time. This fundamental concept in biology explains how organisms become better adapted to their environments across generations.

Natural selection occurs when organisms that are better suited to their environment have a greater chance of surviving and reproducing. These organisms pass on their advantageous alleles (versions of genes) to their offspring. Over time, favourable alleles become more common in the population, whilst less beneficial ones become rarer.

Why use simulations?

Natural selection typically occurs over many generations and long time periods, making it difficult to observe directly. Computer simulations allow us to study the mechanisms of natural selection on a much shorter timescale, helping us visualise and understand how populations change in response to different environmental pressures.

Aim of the investigation

To observe how natural selection works by using a computer simulation to study rabbit populations under different environmental conditions and genetic variations.

Materials required

• Computer, laptop, iPad, or other internet-connected device
• Access to the PhET natural selection simulation: phet.colorado.edu/sims/html/natural-selection
• Select the 'Lab' option when opening the simulation

Understanding the simulation interface

Before beginning the investigation, it's essential to familiarise yourself with the simulation controls. The interface includes several important features:

Generation clock: Shows which generation your rabbit population has reached at any point in the simulation.

Climate control: Allows you to toggle between warm and cold environments to see how temperature affects survival.

Add mutations: Lets you introduce different phenotypes (observable traits) into your population. You can choose whether each new trait is controlled by a dominant or recessive allele. For example, to add brown fur as a recessive trait, you would select the recessive button for the brown fur mutation.

Environmental factors: These are your selection pressures. You can toggle them on or off at any time during the simulation. Options include:

• Limited food
• Tough food
• Wolves (predators)

Method overview

This investigation consists of eight separate simulations, each testing how different selection pressures affect populations with varying genetic traits. Each simulation begins with default settings (warm climate, no mutations, no environmental factors) unless otherwise specified.

Simulation 1: Baseline population growth

This simulation establishes a baseline by observing how quickly rabbits reproduce without any selection pressures.

Procedure: Start with default conditions and record how many generations it takes for white rabbits to overrun the planet.

Purpose: This control simulation shows natural population growth without limiting factors.

Simulation 2: Limited food resources

Key change: Limited food is introduced as a selection pressure from the start.

Purpose: Demonstrates how resource scarcity affects population growth and survival rates.

Simulation 3: Tough food introduction

Key change: Tough food is introduced after Generation 5.

Purpose: Shows how populations respond when a new selection pressure appears after the population has already established itself.

Simulation 4: Dominant long teeth with tough food

Key changes:

• Long teeth mutation added as a dominant trait
• Tough food introduced after Generation 5

Purpose: Demonstrates how a dominant beneficial allele spreads rapidly through a population when it provides a survival advantage (long teeth help rabbits eat tough food).

Simulation 5: Recessive long teeth with tough food

Key changes:

• Long teeth mutation added as a recessive trait
• Tough food introduced after Generation 5

Purpose: Shows how recessive alleles spread more slowly through populations compared to dominant ones, even when they provide the same survival advantage.

Simulation 6: Wolf predators

Key change: Wolves introduced as predators after Generation 5.

Purpose: Examines how predation affects rabbit populations without any defensive mutations.

Simulation 7: Brown fur (recessive) with wolves

Key changes:

• Brown fur added as a recessive trait
• Wolves introduced after Generation 5

Purpose: Investigates whether brown fur provides camouflage that helps rabbits avoid wolf predation.

Simulation 8: Brown fur in cold climate with wolves

Key changes:

• Brown fur added as a recessive trait
• Climate changed to cold
• Wolves introduced after Generation 5

Purpose: Examines how multiple environmental factors (cold temperature and predation) interact with a genetic trait.

Recording results

Results should be recorded in a table format:

Simulation	Climate	Mutations	Selection pressure	Result
1	Warm	None	None	The rabbits override the planet in ___ generations
2	Warm	None	Limited food
3	Warm	None	Tough food introduced after Generation 5
4	Warm	Long teeth (dominant)	Tough food introduced after Generation 5
5	Warm	Long teeth (recessive)	Tough food introduced after Generation 5
6	Warm	None	Wolves introduced after Generation 5
7	Warm	Brown fur (recessive)	Wolves introduced after Generation 5
8	Cold	Brown fur (recessive)	Wolves introduced after Generation 5

Key concepts and learning points

Selection pressure

A selection pressure is any environmental factor that affects an organism's ability to survive and reproduce. Examples include:

• Food availability (limited or tough food)
• Predators (wolves)
• Climate conditions
• Competition for resources

Selection pressures don't affect all individuals equally. Organisms with traits that help them cope with specific pressures have a selective advantage and are more likely to survive and reproduce.

Dominant versus recessive alleles

The simulations clearly demonstrate the difference in how dominant and recessive alleles spread through populations:

Dominant alleles (like long teeth in Simulation 4): Individuals only need one copy of the allele to show the trait. This means the beneficial trait appears quickly and spreads rapidly through the population when it provides a survival advantage.

Recessive alleles (like long teeth in Simulation 5): Individuals need two copies of the allele to show the trait. Initially, the allele can be present in the population as "hidden" carriers (heterozygotes with one copy). The trait only appears when two carriers reproduce and their offspring inherits two copies of the recessive allele. This makes recessive beneficial traits spread more slowly than dominant ones.

How recessive traits appear

Even though you might not see any rabbits with long teeth in early generations when long teeth is recessive, the allele can still be present in the population. When two rabbits that both carry one copy of the recessive allele (but don't show the trait themselves) reproduce, some of their offspring can inherit two copies and display long teeth.

This is why in Simulation 5, rabbits with long teeth start appearing in Generations 3 or 4, even though none were visible earlier. The recessive allele was being passed through the population by carrier individuals.

Camouflage and survival

Simulation 7 demonstrates how brown fur can provide a selective advantage against wolf predation. Brown rabbits may be harder for wolves to spot against brown earth or vegetation, giving them better camouflage than white rabbits. This means brown rabbits are more likely to avoid predation, survive, and reproduce, passing the brown fur allele to their offspring.

Environmental interactions

Simulation 8 shows how multiple environmental factors can interact. In a cold climate, brown fur might not provide the same advantage it does in warm climates (where white fur stands out more). This illustrates that the same trait can be beneficial, neutral, or even harmful depending on the complete environmental context.

Limitations and improvements

When conducting this investigation, consider these limitations:

Improvements could include:

• Running simulations multiple times to see if results are consistent
• Testing different combinations of selection pressures simultaneously
• Recording more detailed data about allele frequencies over time
• Comparing results between warm and cold climates more systematically