Physical ChangePhysical Change

S7L5a Explain that physical characteristics of S7L5a Explain that physical characteristics of organisms have changed over successive organisms have changed over successive

generations (e.g., Darwin’s finches and generations (e.g., Darwin’s finches and peppered moths of Manchester).peppered moths of Manchester).

Changes Over TimeChanges Over Time

• In 1831, the HMS Beagle left England on a scientific journey. Charles Darwin was the ship naturalist and was responsible for collecting plant and animal samples from around the world.

• Darwin visited many places and studied plants, animals and fossils from all over the world, including the Galapagos Islands.

• The Galapagos Islands are located off the coast of South America. Some of the islands are dry and some get lots of rain.

Darwin’s FinchesDarwin’s Finches• Darwin noticed that the birds on the Galapagos

looked similar to the finches on the South American continent. A finch is a kind of seed-eating bird. Every bird Darwin saw on the Galapagos Islands was a type of modified finch. The finches were about the same size and all very similar in color.

• The only differences in the finches Darwin saw were in their beaks and what kind of food they ate. There were finches that ate insects, seeds, plant matter, egg yolks and blood. The finches on the Galapagos Islands looked very similar to one type of finch on the South American continent, but none of the Galapagos Island finches were found on the South American mainland.

• Darwin theorized that all the island finches were offspring of one type of mainland finch that might have been blown to the Galapagos Islands during a storm.

• The population of finches was changing over time because of their environment.

• The process through which organisms change over time is called adaptation.

• Thousands of adaptations lead to the process of evolution.

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More on the FinchesMore on the Finches

• The diet of the Galapagos finches changed in response to their new environment.

• There was a change of eating habits because there were limited food resources on the islands.

• Some physical traits may also be changed over time. As the finches began to eat different types of food, the way their beaks worked and looked began to change.

• Finches that began to eat insects needed longer beaks for digging beetles out of their burrows. Finches that ate seeds or nuts required thicker beaks to crack the seed shells.

How Did the Finches Change Their Beaks in How Did the Finches Change Their Beaks in Response to These Needs?Response to These Needs?

• Scientists now view the process of natural selection like this: – The finches’ beaks did not change overnight, but rather

over many, many generations.– Among the population of beetle-eating finches, those that

were born with longer, sharper beaks naturally had access to more beetles than those finches with shorter beaks.

– The sharp-beaked insect eating finches thrived, and had many offspring, while the short-beaked insect eating finches gradually died out.

– The sharp beak was a trait that was, in effect, selected by nature to thrive.

– The same thing happened in each finch population, until finches from a given population began to look similar to each other and different than other finches.

To Be ClearTo Be Clear

• The individual physical traits of a finch are NOT modified by the finch (his beak does NOT grow and change to suit his changing needs).

• RATHER, the finches who were born with the trait that is favored by the current environmental pressures survive and pass that trait on to their offspring.

• This ensures, that over time, the expression of the favored trait becomes more pronounced, and other traits disappear.

Peppered MothsPeppered Moths

• There are two basic types of peppered moths– a light-colored variety and a dark-colored variety.

• Birds are the main predator of the moths using mostly eyesight to capture their prey.

• In the beginning of the 19th century, birch trees in England had white or light-colored bark and the most common form of peppered moth was the light-colored one. The light-colored moth was easily hidden on the light-colored birch tree

• The industrial revolution soon caused soot to cover the birch tree’s bark, changing the bark color to black.

• Over the next few years, the most common form of peppered moth slowly changed to become the dark-colored one.

• The dark-colored moth was easily hidden on the dark-colored birch tree.

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