Biology: evolution and natural selection (grade 9) · lesson

How natural selection works

💡 When inherited traits differ, the ones that help survival and reproduction become more common over generations.

Natural selection needs three things: variation (individuals differ), inheritance (the differences are passed on in genes), and a struggle to survive. Individuals with helpful traits leave more offspring, so those traits spread.

Individuals do not change their genes by trying or by using a body part. A finch that cracks hard seeds does not grow a deeper beak for its chicks. Instead, finches that already had deeper beaks survive a drought and have more young.

This is how bacteria become resistant to a drug. A few already resist it. They survive each dose and reproduce, so later the drug works less well. The change in inherited traits over generations is called evolution.

🧠 Let's work it out

Use the order: variation first, then the environment favors some, then they reproduce more, then the population changes.
For data, find the share in each generation: 100 of 400 striped is 25%, and 75 of 500 is 15%.
That is a drop of 10 percentage points.

What do you think comes next?

variation: differences between individuals
evolution: change in a population's inherited traits over generations

🔗 PhET: Natural Selection · PhET Interactive Simulations (free) ↗

Adaptations

💡 An adaptation is an inherited trait that helps a living thing survive where it lives.

A cactus's water-storing stem is an adaptation: it is passed on in genes and helps in dry places. A suntan or a learned trick is not, because it is not passed on in genes.

Similar environments can shape unrelated animals in similar ways. Sharks (fish) and dolphins (mammals) both have streamlined bodies because that shape helps any fast swimmer, not because they are close relatives.

🧠 Let's work it out

Ask two questions.
Is the trait passed on in genes?
Does it help survival in that place?

What do you think comes next?

adaptation: an inherited trait that helps survival

🔗 Khan Academy: Mechanisms of evolution · Khan Academy (free) ↗

Evidence of common ancestry

💡 Shared body plans, shared DNA, and the fossil record show that species descend from common ancestors.

Homologous structures share a bone pattern even when used differently, like a human arm, a bat wing, and a whale flipper. That points to a shared ancestor. Bird and insect wings look useful in the same way but are built differently, so they don't show close kinship.

DNA differences build up over time after two groups split. Fewer differences means a more recent common ancestor.

In rock layers that have not been disturbed, lower layers formed first. So fossils in lower layers are older.

🧠 Let's work it out

A and B differ in 2% of a sequence; A and C differ in 15%.

Less difference means less time apart, so A is closer kin to B.
Living species are cousins, not ancestors of each other.

What do you think comes next?

homologous: built on the same basic pattern because of shared ancestry

How new species form

💡 When groups are cut off from each other, they change separately until they can no longer interbreed.

A river, mountain, or sea can split one population into two. Genes no longer mix between them.

Each group gets its own random mutations and faces its own selection. Over many generations the differences add up. When the groups can no longer interbreed, they are separate species.

🧠 Let's work it out

Ask: can genes still flow between the groups?
If not, every difference that builds up stays.

What do you think comes next?

species: a group that can interbreed and have fertile offspring