What Is Evolution? Big Ideas and Basic Terms

1. The Big Idea: What Biologists Mean by Evolution

When scientists talk about biological evolution, they are not talking about one animal suddenly turning into another kind of animal.

Working definition (modern biology):

> Evolution is a change in the heritable traits of a population over many generations.

Break this down:

• Change – something is different now than it was before.
• Heritable traits – features that can be passed from parents to offspring through DNA (for example, eye color, beak shape, leaf shape).
• Population – a group of individuals of the same species living in the same area.
• Over many generations – not just from Monday to Friday, but over the lifetimes of many sets of parents and offspring.

Key point for this course:

• Individuals do not evolve. Populations evolve.

You can think of evolution as a long-term shift in what is common or rare in a population. For example, if dark-colored beetles become more common in a beetle population over hundreds of generations, that is evolution.

2. Individuals vs. Populations: Who Actually Changes?

To understand evolution, we must clearly separate individual change from population change.

Individual change (NOT evolution)

• A person gets stronger by exercising.
• A dog learns a new trick.
• A plant bends toward the light during the day.

These are changes in one individual’s body or behavior during its lifetime. They are not automatically passed on to offspring.

Population change (evolution)

A population is:

> A group of individuals of the same species living in the same place at the same time.

Population change means:

• The percentages of different heritable traits in the population shift over generations.

Example idea:

• In 1900, 5% of a bird population had a long beak and 95% had a short beak.
• By 2000, 70% had a long beak and 30% had a short beak.

The population has evolved, even though each bird still only lives one lifetime and does not personally transform from short-beaked to long-beaked.

3. A Simple Visual: Beetles on a Log

Imagine you are looking down at a log covered with beetles.

Generation 1 (starting point)

• There are 100 beetles.
• 80 green, 20 brown.
• Green and brown are heritable traits (controlled by genes).

If you drew this, you might sketch:

• A log with many small green dots (beetles) and a few brown dots.

Over many generations

• Birds can see green beetles more easily on the brown log and eat more of them.
• Brown beetles survive a bit better and have more offspring.

Generation 50 (many generations later)

• There are still about 100 beetles.
• Now 20 green, 80 brown.

What changed?

• No single beetle changed color during its life.
• But the population’s overall color pattern changed.

This shift in the frequency (how common a trait is) of green vs. brown beetles in the population over generations is evolution.

4. Thought Exercise: Individual Change or Evolution?

Decide whether each situation describes individual change or evolution of a population. Reflect before you read the answers.

1. A person gets a suntan after spending the summer at the beach.
2. Over many generations, more people in a high-altitude village are born with larger lungs, helping them use oxygen more efficiently.
3. A cat loses part of its tail in an accident.
4. Over hundreds of years, a type of fish in a polluted river becomes mostly resistant to a chemical in the water.

Think first, then check yourself:

• 1. Suntan → Individual change. Skin cells change, but this tan is not a new heritable trait.
• 2. Larger lungs at high altitude → Evolution of a population. The proportion of people with genes for larger lungs has increased over generations.
• 3. Injured tail → Individual change. The injury is not coded in the cat’s DNA and will not be passed on genetically.
• 4. Chemical-resistant fish → Evolution of a population. A higher fraction of the population now carries genes that give resistance.

Key takeaway: If it’s not a heritable genetic change in a population over generations, it’s not evolution.

5. What Is a Species?

We often talk about evolution causing new species to appear and others to disappear.

A species is commonly defined (for sexually reproducing organisms) as:

> A group of organisms that can interbreed in nature and produce fertile offspring.

In practice, biologists now use several species concepts (including genetic and evolutionary definitions), especially for organisms that do not reproduce sexually (like many bacteria). But for this course, use this working idea:

• Members of the same species:
• Usually look broadly similar.
• Share many of the same heritable traits.
• Can (under natural conditions) have offspring together that can also have offspring.

Examples:

• All humans alive today belong to one species, Homo sapiens.
• Domestic dogs (Chihuahuas, Great Danes, etc.) are all one species, Canis familiaris, because they can interbreed and produce fertile puppies.

Why this matters for evolution:

• Evolution can change what is typical within a species.
• Over very long times, populations can diverge so much that they become different species.

6. Traits and Variation: The Raw Material of Evolution

To understand evolution, you need three linked ideas:

1. Trait
2. Heritable trait
3. Variation

Trait

A trait is any observable feature of an organism.

• Examples: eye color, height, blood type, beak length, leaf shape, fur pattern.

Heritable trait

A heritable trait is a trait that can be passed from parents to offspring through genes (DNA).

• Examples: natural hair color, blood type, many aspects of body shape.
• Non-example: a scar from an injury is generally not heritable.

Variation

Variation means there are differences in traits among individuals in a population.

• Example: In a class of 30 students, some are tall, some medium, some short. That is variation in height.
• Example: In a population of flowers, some are red, some white, some pink.

Why variation matters for evolution:

• Without variation in heritable traits, every individual would be genetically almost the same.
• Evolution needs heritable variation so that some traits can become more common and others less common over generations.

7. Putting It Together: Human Height as an Example

Let’s use human height as a simplified example to connect the terms.

In a population of 1,000 adults:

• Some are shorter than 160 cm.
• Most are between 160–180 cm.
• Some are taller than 180 cm.

Here’s how our key terms apply:

• Species: All are Homo sapiens (humans).
• Population: These 1,000 adults living in the same city.
• Trait: Height.
• Variation: Different people have different heights.
• Heritable trait: Height is influenced by genes and environment (like nutrition). The genetic part is heritable.

If, over many generations:

• Better nutrition and certain genetic variants become more common,
• And the average height in this population increases,

then we can say:

• There has been evolution in this population regarding the heritable components of height.

Important nuance:

• An individual growing from a child to an adult is development, not evolution.
• Evolution is about changes in the population’s genetic makeup over generations, not about one person getting taller as they grow.

8. Quick Check: What Counts as Evolution?

Choose the best answer. Then check the explanation to see the reasoning.

9. Apply the Definition to a New Case

Read the scenario and answer the guiding questions in your own words (mentally or in writing).

Scenario:

In a certain region, there is a population of wild rabbits. Some are born with thicker fur, and some with thinner fur. The fur thickness is a heritable trait. Over several decades, the climate becomes colder. Rabbits with thicker fur survive winters better and have more offspring. After many generations, most rabbits in this region have thick fur.

Questions to guide your thinking:

1. What is the population in this scenario?
2. What is the main trait being discussed?
3. Is this trait heritable? How do you know from the description?
4. What is the variation in this trait?
5. Has evolution occurred? If yes, explain in one sentence using the phrase “change in the heritable traits of a population over generations.”

Sample answers to compare with your own (read after you think):

1. The population is the group of wild rabbits living in that region.
2. The main trait is fur thickness.
3. Yes, it is heritable, because the scenario clearly states that fur thickness is a heritable trait.
4. Some rabbits have thicker fur and some have thinner fur — that’s variation.
5. Yes, evolution has occurred: there has been a change in the heritable trait of fur thickness in the rabbit population over many generations, with thick fur becoming more common.

10. Review Key Terms

Flip through these cards (mentally) and see if you can give the definition before revealing the back.

11. Final Check: Populations and Traits

One more question to solidify the difference between individuals and populations.