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Natural Selection
What is Natural Selection?
• Charles Darwin
• Spent five years on HMS Beagle studying variation and adaptation
• Natural selection – the process by which organisms with favorable variations survive and
produce more offspring than less well-adapted organisms
• Proposed by Charles Darwin in 1859, based on his observations
• Natural selection affects populations, not individuals
• Natural selection produces change in populations due to
genetic variations present
• Ex) Leopard frogs
• Populations in genetic equilibrium do not change or evolve
• Natural selection upsets genetic equilibrium and
causes change in populations
Variation
• Process of Variation
1. Variation occurs in every population
2. Individuals with more favorable variations
produce more offspring
3. The next generation has a larger amount of the
favorable variation
4. The variation is shared by the entire population
• Adaptation – a trait that helps organisms of a particular
species to survive and reproduce in their unique environment
• Ex) Flamingo vs. kiwi bird
Heredity
• Acquired trait – a trait that is obtained due to environmental conditions during a life
• Genetic trait – a trait that is passed on to offspring; a result of genes
encoded in the DNA of organisms
• Ex) Family members share similar genetic traits
Biology 9.2 – Natural Selection
Natural Selection
Competition
1. Every habitat has finite resources
• Ex) Food, water, shelter
2. Individuals in a population compete for these resources
3. Individuals with favorable variations survive and pass these
variations on to next generation
4. Favorable variations become adaptations
• Competition leads to adaptations via natural selection
• Ex) Faster cheetahs survive and pass on genes
Differential Reproductive Success
• Natural selection causes differential reproductive success
• Differential reproductive success – the production of
more offspring by individuals with a certain variation
than by other individuals in a population
• Occurs as a result of inherited variation,
population growth, and finite resources
• Ex) Giraffe evolution is a result of differential
reproductive success
• Giraffes with long necks more likely to have more
offspring
• Longer necks becomes an adaptation of the population
Biology 9.2 – Natural Selection