Theory of Natural Selection

Darwin Proposed the Theory of
Natural Selection to Explain
Evolution
 Evolution , or change over time, is the process by which modern organisms have descended from ancient organisms.
 A scientific theory is a well-supported testable explanation of phenomena that have occurred in the natural world.

Ideas that Shaped Darwin’s Thinking
James Hutton:
 Proposed a theory of geological change
 States that natural forces change earth’s surface shape
 States that these changes are slow and the Earth was much older than thousands of years

Ideas that Shaped Darwin’s Thinking
Charles Lyell
 Wrote the book
“
Principles of
Geography”
 States that geographical features can be built up or torn down
 Darwin thought if earth changed over time, what about life?

Ideas that Shaped Darwin’s Thinking
Jean Baptist Lamark
 Proposed that species change with a tendency toward perfection
(Giraffe necks)
 Traits that are used are passed down to offspring
 Traits that are not used are not passed down

Ideas that Shaped Darwin’s Thinking
Thomas Malthus
 19th century English economist who wrote
“On the Principle of
Population”
 Stated that if a population grew too large, then there would be insufficient living space and a lack of food so population would decline

• Populations tend to rise and fall overtime
• As the population decreases, food supply returns
• As food supply returns, population grows again

Ideas that Shaped Darwin’s Thinking
Selective Breeding-
 The process of humans selecting traits they want to appear in offspring to produce desired traits
 Used in farming to create sheep with fine wool
 Used to create different breeds of dogs and cats

Ideas that Shaped Darwin’s Thinking
Russell Wallace
 wrote an essay summarizing evolutionary change from his field work in
Malaysia
 Gave Darwin the drive to publish his findings

Darwin’s Conclusion:
Evolution happens through the process of natural selection!
Also called “Survival of the Fittest”
 Some individuals are better suited for their environment
 These individuals will survive and reproduce better than the others
 These favorable traits will be passed down in offspring and appear more in future generations

Natural Selection
 Over time, natural selection results in changes in inherited characteristics of a population.
 These changes increase a species fitness in its environment

Requirements for Natural Selection
Overproduction:
There must an overproduction of offspring so that some can live while others will die.

Requirements for Natural Selection
Genetic Variation:
There must be differences among the individuals so that some do better than others.
Ex. Color, aggression, intelligence, behaviors, speed

Requirements for Natural Selection
Competition:
There must be a struggle to survive due to limited resources.
Ex. Limited resources could include food, water, shelter, mates, territory

Requirements for Natural Selection
If none are able to, then the population will go extinct!
Successful
Reproduction:
There must be successful individuals who pass down their favorable traits to offspring.