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15.2 Darwin's Theory of Evolution -- Part 3
Extreme differences among breeds have been caused by the process of breeding by humans.
Artificial selection was described by Darwin as a model for understanding natural selection.
The environment provides the selective force with natural selection.
Teosinte has a hard, thick outer shell that makes it difficult to use as a food source, and it looks very different from the corn we grow for food.
Evidence from archaeology and genetics supports the idea that prehistoric humans used softer shells and more kernels to produce modern corn.
Darwin believed that if humans could create a wide variety of organisms by artificial selection, then natural selection could also produce diversity, but with the environment, not humans.
The finches in the Galapagos have beaks that are adapted to the food they eat.
Peter and Rosemary Grant are two of the investigators who are documenting natural selection on the Galapagos Islands.
Each of the 13 species of finches has a beak.
The finches like to eat small, tender seeds.
They have to eat larger seeds, which are harder to crush, when the weather turns dry.
During the dry periods, the birds with a larger beak have an advantage.
The Grants' research shows that evolutionary change can sometimes be observed within a human lifespan.
Depending on the weather, the average beak depth of medium ground finches varies from generation to generation.
Different beak depths are better suited to eating different types of seeds because of the weather.
Over the course of a decade, average beak features changed many times.
This is a way in which evolution by natural selection can be observed.
New and revolutionary tools have been created to document the evolution of the genes.
Sean Carroll is a researcher at the University of Wisconsin, Madison, who studies the genes that determine variation in the color of fruit flies.
The male fly dances in this spot.
The spot on the wing of the male fly has evolved from a few simple mutations that have changed how a wing gene is switched on and off during development.
This study and others like it show how a few changes in the DNA code can lead to a new trait.
It was thought that only 10% of the population was dark.
With the advent of industry and an increase in pollution, the number of dark-colored moths exceeded 80%.
Legislation to reduce pollution caused a reversal in the ratio of light-colored and dark-colored moths.
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