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Smithsonian National Museum of Natural History
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Artist’s reconstruction of a new fossil species Hemignathus vorpalis (bottom), based on its probable resemblance to adult males of its genus: H. wilsoni (Akiapola’au, above) and H. lucidus hanepepe (Kauai Nukupu’u, middle). Illustration © J. Hume


Artist’s reconstruction of a new fossil species Hemignathus vorpalis (bottom), based on its probable resemblance to adult males of its genus: H. wilsoni (Akiapola’au, above) and H. lucidus hanepepe (Kauai Nukupu’u, middle).

Illustration © J. Hume

 

“…natural selection is daily and hourly scrutinizing, throughout the world, the slightest variations; rejecting those that are bad,

preserving and adding up all that are good...”

-On the Origin of Species

 

Natural selection was Darwin’s great insight.


Natural selection explains why organisms are well suited—adapted—to their environment. Beneficial traits are preserved, while detrimental traits are “selected out.” Working on many traits over many generations, natural selection helps match populations to their environment.

Four conditions are necessary for natural selection:

Light- and dark-colored mice specimens from the Museum's collection. Photo (c) Smithsonian Institution

Sometimes individuals with particular trait variations survive and reproduce better than individuals with other variations. Owls hunting in an area with light soil, for example, are more likely to catch dark-colored mice than light-colored mice. Soon, mice with dark coats will disappear from the light soil.
Photo © Smithsonian Institution

Overproduction of individuals - Species produce many offspring, but only the tiniest fraction will survive.

Differing traits - Members of a species resemble one another, but a close look reveals that individuals differ in many ways.

Differential survival - Sometimes individuals with particular trait variations survive and reproduce better than individuals with other variations.

Heritability of traits - Many variations in traits are heritable—the traits are passed from parent to offspring through DNA.

Putting It Together


As long as:

  1. more individuals are produced than can survive, and
  2. differences in individuals’ traits influence their survival and are heritable,

then a population will evolve by natural selection.

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