Ecosystems and change over time (grade 8) · lesson

The fossil record

💡 Fossils in rock layers show that life has changed over time, from simpler life in old layers to more complex life in younger ones.

A fossil is the remains or traces of a living thing kept in rock. Bones, shells, and footprints can all be fossils.

In flat rock layers, deeper layers are older. Older layers mostly hold simpler life. More complex life shows up in younger layers.

Fossils also show in-between forms. Early whale fossils had small back legs.

🧠 Let's work it out

Line up two clues.
Old whale fossils had back legs.
Modern whales have tiny hip bones but no legs.

What do you think comes next?

fossil: the remains or traces of a living thing kept in rock

🔗 Khan Academy: Natural and artificial selection · Khan Academy ↗

Similar body plans

💡 Similar bone patterns in different animals suggest they share a common ancestor.

A human arm, a bat wing, and a whale flipper do very different jobs.

Yet they have the same set of bones in a similar pattern. Scientists see this as a sign of a shared ancestor.

🧠 Let's work it out

Separate what the part does from how it is built.
The jobs are different: grabbing, flying, swimming.
The build is the same.

What do you think comes next?

common ancestor: an earlier species that two or more later species came from

Natural selection

💡 Individuals with helpful traits survive and reproduce more, so those traits become more common over generations.

Individuals in a population differ. Some differences help them survive where they live.

Those individuals live longer and have more offspring. The offspring inherit the helpful trait. Over many generations, the trait becomes more common. Darwin called this natural selection.

Individuals do not change on purpose. A beak does not grow deeper from use. The population changes because of who survives and reproduces.

🧠 Let's work it out

Read the data as survival.
On brown soil, brown beetles went from 50% to 70% to 90%.
Birds spotted green beetles more easily.
So brown beetles survived and passed on their color.
It is the same for germs that survive a medicine.

What do you think comes next?

natural selection: the process where helpful traits become more common because they help survival
generation: one step in a family line, like parents to children

🔗 PhET simulation: Natural Selection · PhET Interactive Simulations ↗

When people choose traits

💡 In artificial selection and genetic engineering, people, not nature, choose which traits get passed on.

In artificial selection, people choose which plants or animals reproduce. Farmers breed the cows that give the most milk.

In genetic engineering, scientists change an organism's DNA directly. They might add a gene so corn makes a protein that kills pests.

In natural selection, the environment does the choosing. No one picks the parents.

🧠 Let's work it out

Ask who did the choosing.
People picked the parents? Artificial selection.
People changed the DNA directly? Genetic engineering.
Birds or weather did it? Natural selection.

What do you think comes next?

artificial selection: people breeding living things for traits they want
genetic engineering: changing an organism's DNA directly