How Does Evolution Happen? Section 6.2

Discussion Describe a dinosaur. Why are there no dinosaurs alive today? Why do you think dinosaurs became extinct? RealPlayer Videos\HowStuffWorks Videos When Dinosaurs Roamed America The Great Extinction.flv

Discussion Dinosaurs were well adapted to their environment. Dinosaur species existed for over 150 million years on Earth. A catastrophic event occurred which changed their environment faster than they could adapt. Adaptation takes generations. Their surroundings changed in a matter of days, weeks or months. people.dbq.edu

So how does evolution happen? This is the question that naturalists (scientists who study nature) were trying to answer in the 1800’s. Most geologists up to that time hadn’t believed that Earth was that old. Geologic evidence was found which showed that gradual processes had been shaping Earth’s surface over millions of years.

Charles Darwin Served as the naturalist on the HMS Beagle, which sailed around the world. It was supposed to be a 2 year journey, but took nearly 5 years. Darwin spent most of this time exploring on land: three years and three months on land, 18 months at sea. en.wikipedia.org mulibraries.missouri.edu

Charles Darwin Along the journey they visited the Galápagos Islands, which were separated from the mainland of South America by around 600 miles (965 km), near Ecuador. These islands are volcanic; they were recently formed (geologically speaking), so they were never joined to the mainland. upload.wikimedia.org

Charles Darwin As he studied the local plant and animal life, Darwin noted that although they were similar to that found in Ecuador, there were distinct differences too.

Charles Darwin For example, the finches on the island were slightly different from those on the mainland, and they also differed between the islands as well. Their beaks differed depending upon their diet.

Charles Darwin Darwin hypothesized that the island finches were descended from the mainland finches. However, over generations they had adapted to their particular island’s environment.

Charles Darwin The birds on the islands were faced with a water barrier, which made it difficult to travel far from their own island. Birds in North America are usually classified into two groups, birds east of the Rocky Mountains and birds that are west of the Rocky Mountains. Why do you think ornithologists (scientists who study birds) use this system?

Other Scientists’ Influence Darwin waited about 20 years before he proposed his theory of natural selection. In 1858 he received a letter from Alfred Russel Wallace, another naturalist, who had come to the same conclusions. Wallace had spent 10 years researching along the Amazon and in the islands of southeast Asia. His letter included an essay explaining Wallace’s theory that natural selection pressures species to change. probaway.files.wordpress.com

Other Scientists’ Influence Charles Lyell’s research in geology led him to the belief that natural processes occurring over millions of years have shaped Earth’s features (known as uniformitarianism – the present is the key to the past). This knowledge that the Earth was vastly older than previously believed also influenced Darwin’s formulation of the theory of natural selection. theora.com kentsimmons.uwinnipeg.ca

Other Scientists’ Influence Thomas Malthus noted that humans have a tendency to reproduce rapidly, and that food supplies could not support unlimited population growth. web65.rollins.edu probaway.files.wordpress.com Darwin knew that species could produce many offspring, but populations were limited by the food supply, disease, competition and predation.

Other Scientists’ Influence Darwin realized that only those with traits that were best suited for survival would live long enough to pass on these traits to their offspring. Thus, organisms well suited to their environment survive, and they reproduce more successfully than less well adapted organisms do – natural selection. Go to this site and view “Evolving Ideas: Who Was Charles Darwin?”

The 4 Steps of Natural Selection

Genetics and Evolution The study of genetics was just beginning in Darwin’s time, so he didn’t know how offspring inherited traits, he just observed that they did. We now know that traits are passed on through our genes, and that some genes make it more likely that an organism will survive and reproduce. Selection occurs when only those with certain genes are able to survive to reproduce. RealPlayer Videos\Evolution Primer 4 How Does Evolution Really Work.flv

Science Blooper Earlier in the 19th century, around 1809, a naturalist named Jean Baptiste Lamarck had theorized that if an animal changed a body part through use or nonuse, that change would be inherited by its offspring (a theory called acquired selection). For example, larger leg muscles caused by extensive running would get passed on to offspring. This theory was proven to be incorrect by genetic studies in the 1930’s and 1940’s.

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