User talk:Rdwhitetiger

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Please stop removing speedy deletion notices from articles. If you continue to remove them, you will be blocked from editing Wikipedia. --Brat32 05:14, 9 August 2006 (UTC)

This is your last warning. The next time you vandalize a page, you will be blocked from editing Wikipedia. -→ Buchanan-Hermit ™ / ?!  05:37, 9 August 2006 (UTC)

18-2 Modern Evolutionary Classification�

Taxonomic groups above the level of species are “ invented” by researchers who decide how to distinguish between one genus, family, or phylum, and another. Linnaeus grouped species into larger taxa, such as genus and family mainly according to visible similarities and differences.

Darwin’s ideas about descent with modification have given rise to the new study of Phylogeny, the evolutionary relationships among organisms. Biologists now group organisms into categories that represent lines of evolutionary descent, or phylogeny, not just physical similarities. Evolutionary Classification is the strategy of grouping organisms together based on their evolutionary history.

Species within a genus are more closely related to each other than to species in another genus. According to evolutionary classification, that is because all members of a genus share a recent common ancestor. Similarly, all genera in a family share a common ancestor. The higher the level of the taxon, the farther back in time is the common ancestor of all the organisms in the taxon.

Organisms that appear very similar may not share a recent common ancestor. Natural selection, operating on species in similar ecological environments, has often caused convergent evolution. To refine the process of evolutionary classification,many biologist now prefer a method called cladistic analysis. Cladistic analysis identifies and considers only those characteristics of organisms that are evolutionary innovations. New characteristics that appear in recent parts of a lineage but not in its older members are called derived characters.

Derived characters can be used to construct a cladogram, a diagram that shows the evolutionary relationships among a group of organisms. Cladograms are useful tools that help scientists understand how one lineage branched from another in the course of evolution. Just as family tree shows the shows the relationships among different lineages within a family, a cladogram represents a type of evolutionary tree, showing evolutionary relationships among a group of organisms.

All organisms use DNA and RNA to pass on information and to control growth and development. Hidden in the genetic code of all organisms are remarkably similar genes. 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. Similarities in DNA can be used to help determine classification and evolutionary relationships.

Comparisons of DNA can also be used to mark the passage of evolutionary time. A model know as a molecular clock uses DNA comparisons to estimate the length of time that two species have been evolving independently. Simple mutations occur all the time, causing slight changes in the structure of DNA. Some mutations have a major positive or negative effect on an organism’s phenotype.