E VOLUTION

C HANGE OVER T IME If someone asked you to describe a frog, you might say that a frog has long hind legs, eyes that bulge, and a habit of croaking from time to time. Frogs have different adaptations that enable them to survive.

C HANGE OVER T IME An adaptation is a characteristic that helps an organism survive and reproduce in its environment. Adaptations can include structures and behaviors for finding food, for protection, and for moving from place to place.

C HANGE OVER T IME Living things that share the same characteristics and adaptations may be members of the same species. A species is a group of organisms that can mate with one another to produce fertile offspring.

C HANGE OVER T IME The species on Earth today range from bacteria that lack cell nuclei to multicellular fungi, plants, and animals.

C HANGE OVER T IME Earth is a very old planet. Scientists estimate that it is 4.6 billion years old. Since life first appeared on Earth, a great number of species have died out and have been replaced by newer species.

W HAT CAUSES SPECIES TO CHANGE ? Scientists think that newer species have descended from older species through the process of evolution. Evolution is the process by which populations accumulate inherited changes over time.

Because of evolution, scientists think that all living things, from daisies to crocodiles to humans, share a common ancestor.

In 1831, 21-year-old Charles Darwin had just graduated from college. Like many young people just out of college, Darwin didn’t know what he wanted to do with his life.

He was able to talk his father into letting him sign on for a 5- year voyage around the world. He served as a naturalist – a scientist who studies nature – on a British naval ship called the HMS Beagle.

During this voyage, Darwin collected thousands of plant and animal samples and kept detailed notes of his observations. During the journey, the ship visited the Galápagos Islands.

Darwin observed that the animals and plants on the islands were very similar, yet not identical, to the animals and plants on the nearby South American mainland. For example, he noted that the finches living on the Galápagos Islands differed slightly from the finches in Ecuador.

The finches on the islands were different not only from the mainland finches, but also from each other. The birds differed from each other mainly in the shape of their beaks and in the food they ate.

After Darwin returned to England, he spent many years working on his theory of how evolution happens. In 1858, about 20 years after he returned from his voyage on the HMS Beagle, Darwin theorized that evolution occurs through a process he called natural selection.

T HIS PROCESS IS DIVIDED INTO FOUR PARTS : 1. Overproduction 2. Genetic Variation 3. Struggle to Survive 4. Successful Reproduction

O VERPRODUCTION each species produces many more offspring than will survive to maturity

G ENETIC V ARIATION individuals in a population are slightly different from one another. Each individual has a unique combination of traits. Some traits increase the chances that the individual will survive and reproduce. Other traits decrease the chances of survival. These variations are genetic and can be inherited.

S TRUGGLE TO S URVIVE a natural environment does not have enough food, water, and other resources to support all the individuals born. In addition, many individuals are killed by other organisms. Only some of the individuals in a population survive to adulthood.

S UCCESSFUL R EPRODUCTION this is the key to natural selection. The individuals that are well adapted to their environment (have better traits for living), are more likely to survive and reproduce. The individuals that are not well adapted to their environment are more likely to die early or produce few offspring.

One of the observations on which Darwin based his theory of evolution by natural selection is that parents pass traits to their offspring. During the 1930s and 1940s, biologists combined the principles of genetic inheritance with Darwin’s theory. This combination of principles explained that the variations Darwin observed within a species are caused by mutation – changes in a gene.