CHAPTER 16 Summary

• The theory of evolution—what Darwin called “descent with modification”—draws two main conclusions about life: that all living things are related, sharing a common ancestor in the distant past; and that the species we see today are the result of natural selection operating over millions of years.
• The theory of evolution is supported by a wealth of evidence, including fossil, anatomical, and DNA evidence.
• Fossils are preserved remains or impressions of once-living organisms that provide a record of past life on Earth. Not all organisms are equally likely to form fossils.
• Fossils can be dated directly or indirectly on the basis of the age of the rocks they are found in, or their position relative to rocks or fossils of known ages.
• When fossils are dated and placed in sequence, they show how life on Earth has changed over time.
• As predicted by descent with modification, the fossil record shows the same overall pattern for all lines of descent: younger fossils are more similar to modern organisms than are older fossils.
• Descent with modification also predicts the existence of “intermediate” organisms, such as Tiktaalik, that possess mixtures of “old” and “new” traits. Tiktaalik has features of both fish and tetrapods (four-limbed vertebrates).
• An organism’s anatomy reflects adaptation to its ecological environment. Changed ecological circumstances provide opportunities for new adaptations to evolve by natural selection.
• Homology—the anatomical, developmental, or genetic similarities shared among groups of related organisms—is strong evidence that those groups descend from a common ancestor.
• Homology can be seen in the common bone structure of the forelimbs of tetrapods, the similar embryonic development of all vertebrate animals, and the universal genetic code.
• Many genes, including those controlling limb development, are shared among distantly related species, an example of molecular homology owing to common ancestry.
• DNA can be used as a molecular clock: more-closely related species show greater DNA sequence homology than do more-distantly related species.

• MORE TO EXPLORE

  • Tiktaalik roseae http://tiktaalik.uchicago.edu
  • Shubin, N. (2008) Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body. new York: Random House.
  • Shubin, n. H., et al. (2006) The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb. Nature 440:764–771.
  • Daeschler, E. B., et al. (2006) A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature 440:757–763.
  • Shubin, N., et al. (2009) Deep homology and the origins of evolutionary novelty. Nature 457:818–823.
  • Thomas, J. W., et al. (2003) Comparative analyses of multi-species sequences from targeted genomic regions. Nature 424:788–793.