Transcript Natural Selection

Natural Selection
What is natural selection?
• In 1831, naturalist Charles Darwin sailed to the
Galápagos Islands. There, he observed that
many living things had slight differences from
those he had seen in South America. For
example, iguanas on the islands had larger,
sharper claws.
• Darwin’s observations convinced him that
organisms change over time. He proposed that
this is the result of natural selection.
• Natural selection is the process by which individuals that are better
suited to their environment are more likely to survive and reproduce than
other members of the same species.
• Evolution is the change in a species over time (at least several
generations).
• Charles Darwin is known for development of the theory of natural
selection that is the foundation for the theory of evolution.
Natural selection is a process that involves these factors:
• Overproduction: Most species produce far more offspring than
can possibly survive to adulthood.
• Variation: Members of a species differ from one another in
many of their traits. Any difference between individuals of the same
species is called a variation.
• Competition: The members of a species must compete with
one another to survive because food, space, and other resources
are limited. A variation that helps an organism to survive is called
an adaptation.
The “survivors” are more likely to live to adulthood and reproduce,
passing the adaptations (helpful traits) on to the next generation.
What are some changes in genetic traits due to
natural selection?
• Today it is known that the traits of members of a species
pass to offspring through the genes of the organisms. A
trait that is inherited through genes is a genetic trait.
• Darwin observed the beaks of finches on the Galápagos
Islands. Beak shape is a genetic trait. The beaks of
finches found in any given area of the islands were
adapted for eating the food available in that area.
Scientists are still observing changes in the beaks of medium
ground finches on Daphne Major, a tiny Galápagos Island. Over
several generations, scientists have observed the beak shape of
this bird change from a medium size, to larger and heavier, and
back to medium. These changes occurred as food sources
changed in response to changing amounts of rainfall.
Selective Breeding (aka Artificial Selection)
• In selective breeding, people select organisms with
desired traits to be parents of the next generation.
• The plants we use for many of our major crops have
been selectively bred. For example, over thousands of
years people saved the seeds a type of wild grass that
produced the best food. Today we know that crop as
corn.
• Hens that lay more eggs; race horses that run faster; all
modern breeds of dogs for various reasons.