Evolution How do organisms change over time?

Key Terms to Understand Species a group of similar organisms that can only mate with each other Evolution gradual changes in a species over time Variation individual differences among organisms of the same species Adaptation any trait that an organism inherits (is born with) that allows the organism to survive in its environment Charles Darwin the man who gave us working idea for HOW species evolve

Voyage to the Galapagos Islands In 1831, Darwin set sail on the HMS Beagle to study the natural world. One of the most important places he visited on his voyage was the Galapagos Islands located off the west coast of South America. On these islands, Darwin observed many different varieties of plants and animals. His observations allowed him to develop his theory for how species change over time.

Darwin’s Observations While traveling, Darwin saw giant land tortoises. Depending on which island he was visiting, each tortoise had a different shell shape. THINK! – Why might the tortoises on different islands have different shaped shells? The shape of the shell was an adaptation that helped the tortoise survive in its environment. Darwin concluded that all the land tortoises probably came from one common ancestor. Over time, the tortoises evolved to better survive in their environment.

Darwin’s Observations Darwin also observed 13 different species of finches. Each species had a different beak shape. THINK! – Why would each finch have a different beak? The different beaks were adaptations to obtain different kinds of food. Darwin concluded that the different finches probably evolved from a common ancestor. Over time, the finches evolved to better survive in their environment .

Darwin’s Evolution of Finches

Darwin’s Observations He also saw giant iguanas, blue-footed booby birds, and Sally light-foot crabs

Darwin’s Idea of Natural Selection After his trip, Darwin came up with the idea for how species change over time. This idea is known as the theory of natural selection.

Darwin’s Idea of Natural Selection Natural selection works as follows: The organisms in a population have small differences or VARIATIONS. These variations make some organisms better ADAPTED to their environment. The best adapted organisms end up SURVIVING. They will now REPRODUCE and pass on the helpful trait. Over time, many of the organisms in the population have the trait. We say the population has EVOLVED or changed.

Example of Natural Selection Suppose a new-fast swimming shark moves into a turtle’s habitat. The turtle population has variation. Some are fast swimmers and some are slow swimmers. Turtles that are able to swim fast are better adapted because they can swim away from the shark. The fast swimming turtles will survive. They will reproduce and pass on their “fast swimming” genes. Over time, the turtle population will evolve and more turtles will be able to swim fast.

Example of Natural Selection THINK! – Use this image to explain how natural selection is occurring?

Example of Natural Selection In the 1700s most peppered moths were light gray in color. They had an advantage over the black moths since birds could not see them against the trees. However, when the Industrial Revolution began in the late 1700s people built factories and blackened the tree trunks. Now, the black moths had the advantage. Natural selection favored the black moths and by about 1850, the population changed and most moths were black.