chapter 25

• Evolutionary biology is about both process and history.
  • The processes of evolution are natural selection and other mechanisms that change the genetic composition of populations and can lead to the evolution of new species.
  • A major goal of evolutionary biology is to reconstruct the history of life on earth.
• In this chapter, we will consider how scientists trace phylogeny, the evolutionary history of a group of organisms.
• To reconstruct phylogeny, scientists use systematics, an analytical approach to understanding the diversity and relationships of living and extinct organisms.
  • Evidence used to reconstruct phylogenies can be obtained from the fossil record and from morphological and biochemical similarities between organisms.
  • In recent decades, systematists have gained a powerful new tool in molecular systematics, which uses comparisons of nucleotide sequences in DNA and RNA to help identify evolutionary relationships between individual genes or even entire genomes.
• Scientists are working to construct a universal tree of life, which will be refined as the database of DNA and RNA sequences grows.

The figure represents a cross section of the sea floor through a mid-ocean rift valley, with alternating patches of black and white indicating sea floor with reversed magnetic polarities. At the arrow labeled "I" (the rift valley), the igneous rock of the sea floor is so young that it can be accurately dated using carbon-14 dating. At the arrow labeled "III," however, the igneous rock is about 1 million years old, and potassium-40 dating is typically used to date such rocks. Note: The horizontal arrows indicate the direction of sea-floor spreading, away from the rift valley.

Assuming that the rate of sea-floor spreading was constant during the 1-million-year period depicted above, Earth's magnetic field has undergone reversal at an average rate of once every

A) 10,000 years.
B) 25,000 years.
C) 100,000 years.
D) 250,000 years.
E) 1,000,000 years.

Which section of sea-floor crust should have the thickest layer of overlying sediment, assuming a continuous rate of sediment deposition?

A) A
B) B
C) C
D) D
E) E

If a particular marine organism is fossilized in the sediments immediately overlying the igneous rock at the arrow labeled "II," at which other location, labeled A—E, would a search be most likely to find more fossils of this organism?

A) B only
B) C only
C) D only
D) B and C
E) C and D