Chapter 18 Classification (18-2)

18-2 Modern Evolutionary Classification

Problems with Traditional Classification

q Sometimes due to convergent evolution, organisms that are quite different from each other evolve similar body structures.

? Biologists now group organisms into categories that represent limes of evolutionary descent, not just physical similarities.

q This strategy of grouping organisms together based on their evolutionary history is called evolutionary classification.

Classification Using Cladograms

q Characteristics that appear in recent parts of a lineage but not in its older members are called derived characters.

q Derived characters are used to construct a cladogram, a diagram that shows the evolutionary relationship among a group of organisms.

Similarities in DNA and RNA

q Because DNA and RNA are so similar across all forms of life, these molecules provide an excellent way of comparing organisms at their most basic level—their genes.

? The genes of many organisms show important similarities at the molecular level. These similarities can be used as criteria to help determine classification.

Molecular Clocks

q A molecular clock uses DNA comparisons to estimate the length of time that two species have been evolving independently.

q Neutral mutations accumulate in the DNA of different species at about the same rate