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Natural Selection



Natural selection is the process by which biological organisms with favorable traits survive and reproduce more successfully than organisms that do not possess such traits.


Types of Natural Selection

1.  Sexual Selection:

Intrasexual: competition among individuals of the same sex to reproduce.  Most commonly occurs between males of a species.  Genes that help individuals to compete for mates are passed on.

Intersexual: one sex controls access to reproduction and chooses from a population of available mates.  Most common where females choose from a selection of males.  Genes that help males to be chosen are passed on to future generations.  Often leads to sexual dimorphism (phenotypic difference between males and females)

2.  Environmental Selection:

Individuals who can survive in their ecosystem to reproduce pass on their genes to future generations.  Genes that help individuals reach reproductive age and reproduce are passed on:

    • Survive (eat, drink, & don’t die)
    • Reproduce


In the 1850’s, Charles Darwin observed and studied 13 different species of finches living on the Galapagos Islands.  He proposed that all these different finches had evolved through natural selection.  Here are the main points of Darwin’s ideas about natural selection:

1.  Organisms produce more offspring than can survive.

2.  Variations exist between members of a population.

3.  Variations are passed down to offspring.

4.  Individuals with helpful variations (adaptations) are more likely to survive and reproduce.

5.  Over time, offspring of individuals with helpful variations make up more of a population.


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1.  Why do organisms produce more offspring than can survive?

2.  Write a definition for variation.

3.  Give an example of variations in a population of humans.

4.  Write a definition for adaptation.

5.  Give an example of an adaptation that a wolf has.

6.  Explain why albinism is NOT an adaptation.


Evolution:  change in a kind of organism over time; process by which modern organisms have descended from ancient organisms

human evolution

Natural selection:  process by which individuals that are better suited to their environment survive and reproduce most successfully; also called survival of the fittest

natural selection

 differences among members of the same species


Artificial Selection:  selection by humans for breeding of useful traits from the natural variation among different organisms

Competition:  The struggle between organisms to survive as they attempt to use the same limited resource(s)

 inherited characteristic that increases an organism’s chance of survival; (helpful variation)


segments of DNA and RNA that perform a function in an organism.  Different species share many genes in common.  Genes that aid survival are passed on to future generations.


All of the observable characteristics of an organism; (physical appearance).  Phenotype results from the expression of an organism’s genes.

  a characteristic

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