Biology | Natural selection

Welcome to the Chapter: Natural Selection!

Hi everyone! You are diving into one of the most fundamental concepts in all of Biology—Natural Selection. This chapter sits within the theme of Continuity and Change because it explains how life changes (evolves) over massive stretches of time, yet still maintains links to its ancestors.

Don't worry if this seems abstract at first. Natural selection is simply nature's way of "filtering" traits. Once you understand the four key steps, the entire process clicks into place. This is crucial for understanding evolution, biodiversity, and even modern issues like disease resistance.

1. Defining Natural Selection

Natural selection is the primary mechanism by which evolution occurs. It is often summarized as "survival of the fittest," but we need a more precise definition:

Natural Selection is the process where organisms better adapted to their environment tend to survive longer and produce more offspring than those less adapted.

Key Distinction: Natural Selection vs. Evolution

• Natural Selection acts on individuals (determining who lives and reproduces).
• Evolution is the cumulative change in the heritable characteristics of a population over many generations, driven by natural selection.

2. The Four Pillars of Natural Selection (The Mechanism)

For natural selection to occur, four specific conditions must be met. If you can memorize these four steps, you can explain any example of evolution!

Memory Aid: V C S A (Variation, Competition, Selection, Adaptation)

Step 1: Variation Exists Within a Species

If every individual in a species were identical, natural selection could not happen. It requires differences!

• Detail: Individuals in a population show a wide range of inherited traits (characteristics). For example, some giraffes have slightly longer necks, some beetles are slightly darker, and some bacteria are slightly more resistant to toxins.
• Importance: This variation must be heritable (passed down genetically) for selection to have a long-term effect.

Step 2: Overproduction and Competition (The Struggle for Survival)

Most organisms produce more offspring than the environment can support. This leads to intense competition.

• Overproduction: Populations tend to produce far more young than the available resources (food, water, shelter) can sustain.
• Competition: This leads to a "struggle for existence" between individuals for those limited resources. Individuals also compete to avoid predators, fight diseases, and resist harsh weather.

Analogy: Imagine a lottery. If only one winning ticket exists, thousands of people compete intensely for that single prize.

Step 3: Selection (Differential Survival and Reproduction)

Due to competition, not all individuals survive long enough to reproduce. This is where the "filtering" happens.

• The Filter: Individuals whose traits give them an advantage in the current environment (e.g., a longer neck to reach higher food, camouflage to avoid predators) are more likely to survive, find a mate, and reproduce.
• Differential Reproduction: Organisms with the advantageous traits pass those traits on more frequently than those with disadvantageous traits.
• Crucial Point: The selection pressure (the force driving the selection, like a drought or a new predator) dictates which traits are favorable.

Step 4: Adaptation and Change in Allele Frequency

Over generations, the frequency of the advantageous allele (the gene variant responsible for the favorable trait) increases in the gene pool of the population.

• Adaptation: The favorable trait becomes more common, meaning the population as a whole becomes better suited, or adapted, to the environment.
• Evolution: Since the genetic makeup of the population has changed over time, evolution has occurred.

3. Sources of Variation (Prerequisite for Selection)

Since selection requires variation (Step 1), where does that genetic diversity come from?

The three main sources of variation, which ensure that no two non-twin siblings are exactly alike, are:

1. Mutation: Random changes in the base sequence of DNA (new alleles are created). This is the original source of all new genetic characteristics.
2. Meiosis: Generates new combinations of existing alleles through crossing over (exchange of genetic material between homologous chromosomes) and independent assortment (random orientation of homologous pairs).
3. Sexual Reproduction (Fertilization): The random fusion of two gametes (egg and sperm) from two different parents creates a unique diploid zygote, mixing parental alleles in new combinations.

4. Real-World Examples of Natural Selection

The IB syllabus expects you to apply the four pillars (VCSA) to specific, observable evolutionary changes.

Example A: Antibiotic Resistance in Bacteria

This is a rapid, powerful example of natural selection in action.

1. Variation: Within a large bacterial population, there is natural variation. Due to random mutation, some bacteria possess an allele that provides low-level resistance to a specific antibiotic.
2. Competition/Overproduction: Billions of bacteria are produced, competing for resources within the host body.
3. Selection (Application of Antibiotics): When the antibiotic is applied, it acts as the selection pressure. Most non-resistant bacteria die (they are "selected against"). Only the few resistant bacteria survive.
4. Adaptation/Evolution: The surviving resistant bacteria reproduce rapidly, passing on their resistant allele. Over generations, the entire population shifts, and the antibiotic is no longer effective. The population has evolved to be resistant.

Did you know? The misuse or overuse of antibiotics accelerates this process, giving the resistant bacteria a massive selective advantage.

Example B: Changes in the Beaks of Galápagos Finches

Charles Darwin observed finches with different beak shapes depending on the island and the local food source (a classic case of directional selection).

• Background: Beak size is a variable, heritable trait in finch populations. Large beaks are better for cracking large, tough seeds; small beaks are better for small seeds.
• Selection Pressure (Drought): A severe drought hits the island. The only seeds left are large, hard ones that require significant force to break open.
• Differential Survival: Finches with smaller beaks cannot feed efficiently and starve (selected against). Finches with naturally larger, stronger beaks can utilize the remaining food source and survive.
• Result: These large-beaked survivors breed, passing the genes for larger beaks to the next generation. After the drought, the average beak size in the population has increased significantly.

5. Adaptive Features and Consequence of Selection

What is an Adaptation?

An adaptation is any characteristic that helps an organism survive and reproduce in its environment. Adaptations develop over time through natural selection, not within a single generation.

• Adaptations can be structural (e.g., camouflage), physiological (e.g., producing venom), or behavioral (e.g., migratory patterns).

The Consequences of Selection

Natural selection can lead to several observable consequences within a population's traits:

1. Favored traits (adaptations) become more frequent.
2. Unfavorable traits become less frequent or disappear.
3. A species becomes better fit to its environment.
4. Over vast time scales, accumulated changes can lead to speciation (the formation of new species). (This links to the next major topic in "Continuity and Change".)