Darwin’s theory of evolution entails the following fundamental ideas. The first ideas were already under discussion among earlier and contemporaneous naturalists working on the “species problem” as Darwin began his research. Darwin’s original contributions were the mechanism of natural selection and copious amounts of evidence for evolutionary change from many sources. He also provided thoughtful explanations of the consequences of evolution for our understanding of the history of life and modern biological diversity.

The Process of Natural Selection Darwin’s process of natural selection has four components.

  1. Variation. Organisms (within populations) exhibit individual variation in appearance and behavior. These variations may involve body size, hair color, facial markings, voice properties, or number of offspring. On the other hand, some traits show little to no variation among individuals—for example, number of eyes in vertebrates.
  2. Inheritance. Some traits are consistently passed on from parent to offspring. Such traits are heritable, whereas other traits are strongly influenced by environmental conditions and show weak heritability.
  3. High rate of population growth. Most populations have more offspring each year than local resources can support leading to a struggle for resources. Each generation experiences substantial mortality.
  4. Differential survival and reproduction. Individuals possessing traits well suited for the struggle for local resources will contribute more offspring to the next generation.






Homologous Structures. Homologous structures are body parts that are alike because the species in question share a common ancestor. These structures may serve the same or different functions. For example, the wing of a bird and the arm of a human serve very different functions, but are considered homologous structures because of the evolutionary relationship between birds and humans. Body parts that are alike in that they serve similar functions (regardless of evolutionary relationship) are analogous structures.




Whale "missing links."The recent discoveries of new species have filled in the gaps between the hypothetical ancestral link between the whales and hoofed mammals, and present-day whales.

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Evolution of shell shape in oysters.Over 12 million years of the Early Jurassic Period, the shells of this group of coiled oysters became larger, thinner, and flatter. These animals rested on the ocean floor in a special position and it may be that the larger, flatter shells were more stable in disruptive water movements.
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Evolutionary change in body size of horses. Lines show the broad outline of evolutionary relationships. Although most change involved increases in size, some decreases also occurred







Body Size (kg)
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Evolutionary change in the length of a girrafe's neck: Time has shown that the original giraffe neck was short, and as time passed, it adapted to it's environment by evolving and growing a longer neck in order to reach leaves from tree's to consume.


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Links:
http://bioweb.cs.earlham.edu/9-12/evolution/HTML/live.html