Biology (0610) | Selection

Chapter 18.3: Selection - The Engine of Evolution

Hello Biologists! This chapter is one of the most exciting topics in IGCSE Biology. We're going to explore how species change over time. This isn't just theory; selection is happening all around us, right now, from the crops we eat to the superbugs doctors fight!

Don't worry if the vocabulary seems tough—we will break down the two main types of selection: Natural Selection and Artificial Selection. Let's dive in!

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1. Natural Selection (Core and Supplement)

What is Natural Selection?

Natural Selection is the process where organisms better adapted to their environment tend to survive and produce more offspring. Think of it as nature deciding who gets to pass their genes on.

This idea was famously described by Charles Darwin. It explains how populations of organisms gradually become better suited, or adapted, to their environment over many generations.

The Four Key Steps of Natural Selection

Natural selection is a step-by-step process that requires three main ingredients: variation, competition, and inheritance.

1. Genetic Variation Exists (18.3 Core 1a)
   

   In any population (e.g., all the rabbits in a field), there are differences in features (phenotypes). These differences are caused by different alleles (alternative forms of a gene) and are the result of mutation and genetic mixing (meiosis, random fertilisation).
   Example: Some rabbits are naturally faster, some have thicker fur, some have better camouflage.

2. Overproduction and Struggle for Survival (18.3 Core 1b & 1c)
   

   Organisms produce many more offspring than the environment can support. This leads to a struggle for survival. They must compete for resources like food, water, shelter, and mates, or struggle to avoid predators.

3. Differential Survival (Survival of the Fittest) (18.3 Core 1d)
   

   Individuals with features (alleles) that make them better adapted to the environment have a greater chance of surviving this struggle than others.
   Example: The fastest rabbits are more likely to escape the fox and survive.

4. Inheritance: Passing on Alleles (18.3 Core 1e)
   

   The surviving, better-adapted individuals are more likely to reproduce. When they reproduce, they pass their beneficial alleles onto the next generation.

Key Takeaway: Over many generations, these beneficial alleles increase in frequency in the population, and the whole population becomes better suited to the environment.

Adaptation (Supplement)

The result of natural selection is adaptation.

• Describe adaptation (18.3 Supplement 4): Adaptation is the process, resulting from natural selection, by which populations become more suited to their environment over many generations.
• An adaptive feature is an inherited characteristic that increases an organism's chance of survival and reproduction in its environment.
  Example: The thick, oily cuticle on xerophytes (desert plants) is an adaptation to reduce water loss.

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2. Natural Selection in Action: Antibiotic Resistance (Supplement)

The development of antibiotic-resistant bacteria is a perfect, worrying real-world example of natural selection in action today (18.3 Supplement 5).

Imagine a colony of bacteria causing an illness.

1. Initial Variation: Within the huge bacterial population, there is natural variation. Due to random gene mutation, a few bacteria might have an allele that gives them slight resistance to an antibiotic.
2. The Selection Pressure: A patient takes the antibiotic. The antibiotic acts as the selection pressure.
3. Differential Survival: The vast majority of non-resistant bacteria are killed by the drug. However, the few naturally resistant bacteria survive.
4. Inheritance and Reproduction: The survivors reproduce rapidly (bacteria divide very quickly). Since only the resistant ones are left, the next generation is primarily resistant.
5. Result: The antibiotic is now much less effective because the bacterial population has quickly evolved (adapted) to resist the chemical.

Did you know? Superbugs like MRSA are dangerous because they have adapted to resist multiple types of antibiotics, making them very difficult to treat. This is why doctors emphasize only using antibiotics when absolutely necessary.

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3. Artificial Selection (Selective Breeding) (Core)

While natural selection is guided by the environment, Artificial Selection (or Selective Breeding) is guided by human choices and desires.

Defining Artificial Selection (18.3 Core 2)

Selective breeding is a process where humans intentionally choose organisms with desirable traits and breed them together to produce offspring that also possess those traits.

The goal is not survival in the wild, but utility or aesthetic value to humans.

How Artificial Selection is Carried Out (18.3 Core 3)

Selective breeding is a continuous process carried out over many generations to enhance specific features in domesticated animals and crop plants.

1. Selection of Individuals: Humans identify and choose individuals that possess the desired features (e.g., a cow that produces lots of milk, or a wheat plant that gives high yield).
2. Controlled Crossing: These chosen individuals are deliberately crossed (bred) together.
3. Selection of Offspring: The offspring are evaluated. Only those young showing the most desirable features are selected to become the parents for the next generation.
4. Repeat: This process is repeated over many generations until the desired trait is strongly expressed and the population is mostly pure-breeding for that trait.

Examples of Artificial Selection

• Agriculture: Improving crop plants for high yield, disease resistance, or better nutritional value (e.g., making wild grasses into modern maize).
• Livestock: Breeding cattle for maximum meat or milk production, or chickens for fast growth or high egg output.
• Pets: Creating the huge variety of domestic dog breeds (from tiny Chihuahuas to huge Great Danes) all came from the wild wolf through selection for different characteristics.

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4. Comparing Natural and Artificial Selection (Supplement)

It is important to clearly understand the differences between these two selection processes (18.3 Supplement 6).

Feature	Natural Selection	Artificial Selection
Selection Pressure	Environmental factors (predators, climate, food availability, disease).	Humans decide (economic value, appearance, yield).
Goal	Increased survival and adaptation of the species in the wild.	To produce offspring with specific traits desirable to humans.
Outcome (Adaptation)	Occurs over a very long period (many generations).	Can be much faster due to intense human control.
Genetic Variation	Maintains genetic variation (if beneficial).	Reduces genetic variation (creates pure-breeding lines).