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NIS - BIOLOGY
Lecture 63 – Lecture 64
Darwin’s Theory of Evolution by Natural
Selection
Ozgur Unal
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Developing the Theory of Evolution
About 200 years ago, the average person believed that the
world was about 6000 years old.
Almost everyone thought that animals and plants were
unchanging.
In 1831, young Charles Darwin boarded the HMS Beagle, which
set sail from England.
The primary mission of Beagle was to survey the
coast of South America.
Darwin’s role on the ship was as naturalist and
companion to the captain.
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Developing the Theory of Evolution
Over the course of the ship’s 5-year voyage, Darwin made
extensive collections of rocks, fossils, plants and animals.
He also read a copy of Charles Lyell’s Principles of Geology – a
book proposing that Earth was millions of years old.
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Developing the Theory of Evolution
In 1835, the Beagle arrived in the Galapagos Islands off the
coast of South America.
These volcanic islands contain their own, slightly different
varieties of animals.
After he returned to England, he found
out that almost every specimen he
collected on the islands was new to
European scientists.
Darwin began to suspect that the
populations from the mainland (South
America) changed after reaching the
Galapagos.
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Artificial Selection
Darwin hypothesized that new species could appear gradually
through small changes in ancestral species. How?
The process of directed breeding to produce offspring with
desired traits, referred to as selective breeding in Chapter 13 was
called artificial breeding by Darwin.
Artificial breeding also occurs when developing new breeds of
dogs or new strains of crop plants.
Darwin inferred that if humans could change species by artificial
selection, then perhaps the same process could work in nature.
Darwin thought that given enough time, perhaps this process
could produce new species.
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Natural Selection
After reading Thomas Malthus’ ideas on the effects of
increasing human population, Darwin thought that these ideas
could be applied to the natural world.
Darwin reasoned that some competitors in the struggle for
existence would be better equipped for survival than others.
Those less equipped would die  process of natural selection
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Natural Selection
Darwin’s theory of evolution by natural selection has 4 basic
principles:
1- Individuals in a population show differences or variations.
2- Variations can be inherited, meaning that they are passed
down from parent to offspring.
3- Organisms have more offspring than can survive on available
resources.
4- Variations that increase reproductive success will have a
greater chance of being passed on than those that do not
increase reproductive success.
http://www.edparadigm.com/sessions/science/selectionSim.html
http://bcs.whfreeman.com/thelifewire/content/chp23/2302001.html
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Natural Selection
Darwin’s finches:
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Natural Selection
Today, biologists use the term evolution to define cumulative
changes in groups of organisms through time.
Natural selection is not synonymous with evolution; it is a
mechanism by which evolution occurs.
Is an individual organism able to evolve over the course of its
life?
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