U3L5–6: Evolution & Natural Selection

Fossils are typically found in layers of sedimentary rock. Understanding the order of these layers is crucial for determining the history of life on Earth.

Geologists use the position of rock layers to determine whether a fossil is older or younger than another. In undisturbed layers, the oldest fossils are found at the bottom, and the youngest are at the top.

Multiple lines of evidence suggest that different species share common ancestors.

Similar patterns of early development in different species (such as embryos having similar structures) provide evidence of a shared evolutionary history.

Comparing the genetic material of different species shows how closely related they are. Species with very similar DNA sequences are more likely to share a recent common ancestor.

These are physical features in different organisms that have a similar underlying structure but may serve different functions (e.g., the front limb of a cat vs. the wing of a bat).

Some organisms have body parts that no longer serve a function but are remnants of a structure that was useful to an ancestor (e.g., tiny leg bones in certain snakes).

Evolution is the process by which populations of organisms change over vast periods of geologic time.

During his travels to the Galapagos Islands, Charles Darwin observed that finches had different beak sizes and shapes. He realized these traits were specific adaptations to the types of food available on different islands.

This is the process by which individuals that are better adapted to their environment are more likely to survive and reproduce.

Populations produce more offspring than the environment can support.

Individuals in a population have different traits due to differences in their DNA.

Certain traits give individuals an advantage in surviving and finding resources.

The advantageous traits become more common in the population over generations.

Unlike natural selection, this is a human-driven process where people breed plants or animals for specific desired traits.

Genetic variation comes from mutations—random changes in DNA.

A mutation may result in an adaptive trait that makes an organism more likely to survive and reproduce in its environment.

In bacteria, mutations can allow individuals to survive exposure to antibiotics. These resistant bacteria then reproduce, causing the entire population to become resistant over time.

Physical traits, such as a mammoth’s thick furry coat, are adaptations that allow a species to survive in specific environments like cold climates.

Extinction occurs when a species completely dies out. This often happens because the environment changes more quickly than the species can adapt. Factors include loss of habitat, increased competition for food, or new predators.