Combined Science (9204) | Variation and Evolution

Welcome to Variation and Evolution!

Hello future Biologist! This chapter is one of the most exciting parts of science. We are going to explore two big questions: Why are we all different? And how did life on Earth change from simple beginnings to the incredible variety we see today?

Don't worry if words like 'evolution' sound complex. We will break down the process of change—called natural selection—into easy, memorable steps. Understanding this chapter helps you see the whole world through a biological lens!

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Section 1: Understanding Variation

Variation simply means the differences that exist between individuals of the same species. Without variation, evolution cannot happen.

What Causes Variation?

All the differences we see—from eye colour to height to having a scar—come from two main sources:

1. Genetic Variation (Inherited)

This variation is passed down from your parents. It is determined by the different versions of genes (called alleles) you inherit. This is the only type of variation that can be passed on to the next generation.

• Examples: Natural eye colour, blood group, inherited diseases, ability to roll your tongue.
• Analogy: Think of genetic variation like the recipe book you inherit. It determines your potential physical traits.

2. Environmental Variation (Acquired)

This variation is caused by your surroundings, diet, lifestyle, and experiences. It is not passed on to your children.

• Examples: Scars, speaking a foreign language, changes in weight due to diet, plants growing taller because they receive more sunlight.

3. Combined Variation

Most characteristics are influenced by both genes and the environment.

• Example: Your height. Your genes set your potential maximum height, but if you have a poor diet (environment), you might not reach that potential.
• Quick Tip: Only Genetic Variation matters for evolution!

Types of Variation

We classify characteristics based on how they are measured:

1. Continuous Variation

This refers to characteristics that can take any value within a range. There are no fixed categories, and measuring these traits often results in a smooth curve (like a bell curve) when plotted on a graph.

• Key Feature: Range of values, often controlled by many genes (polygenic).
• Examples: Height, weight, foot size, hand span, leaf length.

2. Discontinuous Variation

This refers to characteristics that fall into distinct, separate categories, with no intermediate values.

• Key Feature: Distinct categories, often controlled by one or a few genes.
• Examples: Blood group (A, B, AB, O), eye colour (blue, brown, green), ability to roll the tongue.

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Section 2: The Mechanism of Evolution – Natural Selection

Evolution is the change in the inherited characteristics of biological populations over successive generations. The main driver of evolution is Natural Selection, often called "survival of the fittest."

Don't worry if this seems like a large process; it happens over thousands or millions of years, guided by the environment.

The Theory of Natural Selection (Proposed by Charles Darwin)

Natural Selection works in four simple, crucial steps:

Step 1: Variation Exists

As we learned, individuals within a species are not identical due to genetic variation. Some animals in a population are slightly faster, stronger, better camouflaged, or more resistant to disease than others.

Example: In a population of rabbits, some have thicker fur than others.

Step 2: Overproduction and Competition

Organisms produce far more offspring than can possibly survive. This leads to overproduction. Since resources (food, water, shelter) are limited, a constant "struggle for existence" or competition occurs.

Analogy: If 100 people try to get 10 tickets, 90 people will fail.

Step 3: Selection Pressure and Survival of the Fittest

The environment puts selection pressure on the population (e.g., a drought, a new predator, cold weather). Individuals with traits that give them an advantage in that specific environment are more likely to survive and reach adulthood.

Example: During a sudden cold snap, the rabbits with thicker fur survive, while those with thin fur freeze.

The Key Idea: "Fittest" means best suited to survive and reproduce in the current environment.

Step 4: Inheritance and Evolution

The survivors reproduce and pass their advantageous genetic traits (like thick fur) on to their offspring. Over many generations, the advantageous characteristic becomes more common in the population, and the species gradually changes (evolves).

Result: After many generations, the entire rabbit population has significantly thicker fur than their ancestors.

A Classic Example: The Peppered Moth

• Before the Industrial Revolution: Most moths were light-coloured (camouflaged against pale tree bark). Dark moths were easily seen and eaten by birds.
• During the Industrial Revolution: Factory soot darkened the tree trunks. The light moths were now easily seen, while the rare dark moths were camouflaged and survived.
• Outcome: The dark moths reproduced, passing on their genes. Within a few decades, the population evolved, and most peppered moths were dark. The selection pressure (soot/predation) drove the change.

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Section 3: Consequences of Evolution

Adaptation

An adaptation is any inherited feature that makes an organism well-suited to survive and reproduce in its environment. Adaptations are the result of natural selection over time.

Adaptations can be grouped into three main categories:

1. Structural Adaptations (Physical Features)

These are physical body parts or features.

• Examples: A camel’s long eyelashes to keep out sand; thick, insulating fur on a polar bear; the streamlined shape of a fish.

2. Behavioural Adaptations (Actions)

These are actions or patterns of activity that help survival.

• Examples: Migration of birds during winter; animals hunting in packs; hibernation during cold months.

3. Physiological Adaptations (Internal Processes)

These relate to internal body processes and chemistry.

• Examples: Snakes producing venom; plants making toxins to deter herbivores; animals controlling their body temperature (e.g., shivering or sweating).

Speciation

If two groups of the same species become geographically separated (e.g., by a new river or mountain range), they experience different selection pressures. Over long periods, they evolve so differently that they can no longer interbreed to produce fertile offspring. When this happens, a new species has been created—this process is called speciation.

Key Definition: A species is a group of organisms that can interbreed and produce fertile offspring.

Speciation in brief:

1. Isolation: A barrier separates populations (e.g., geographic barrier).
2. Selection: Different selection pressures act on each isolated group.
3. Evolution: Each group evolves different characteristics.
4. Reproductive Isolation: They can no longer mate successfully, meaning they are now two distinct species.

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Section 4: Evidence for Evolution

The theory of evolution is supported by a large amount of evidence collected across different fields of science.

1. The Fossil Record

Fossils are the preserved remains or traces of dead organisms. By examining the layers of rock (strata), scientists can see how organisms have changed over millions of years.

• Older rocks: Contain simpler organisms.
• Younger rocks: Contain more complex organisms, showing a gradual transition.
• Key Point: Fossils show transitional forms—organisms that have features of two different groups, demonstrating how one group evolved into another.

2. Antibiotic and Insecticide Resistance

This is evolution happening right now, quickly!

How Resistance Evolves:

1. Within a huge population of bacteria (or insects), random mutation causes a few individuals to be resistant to the drug (antibiotic) or poison (insecticide).
2. When the drug is applied, all non-resistant individuals die (this is the selection pressure).
3. The few resistant individuals survive and have no competition. They rapidly reproduce, passing the resistance gene to their offspring.
4. Soon, the entire population is resistant, and the treatment no longer works.

Did you know? This rapid evolution is why doctors warn against overusing antibiotics—it speeds up the selection for resistant "superbugs."

3. Comparative Anatomy and Biochemistry

Scientists look at similarities in the structure and chemistry of living organisms:

• Comparative Anatomy: The skeletons of humans, cats, whales, and bats look very different externally, but the basic structure of their forelimbs (arms/wings/flippers) is remarkably similar. This suggests they share a common ancestor.
• Biochemistry (DNA): All living things use DNA and RNA in fundamentally the same way. The closer the match between the DNA sequences or proteins of two species, the more recently they shared a common ancestor. For example, human DNA is extremely similar to chimpanzee DNA.