Chapter 25- Phylogenies and the History of Life

25.1 Tools for Studying Life’s History: Phylogenetic Trees

• The branching evolutionary history of species or other groups of organisms is called a phylogeny. 
• A phylogenetic tree is a simplified diagram of this history. 
• Systematics is the discipline of biology that characterizes and classifies the relationships among all organisms on Earth.
• The tree of life is the most universal of all phylogenetic trees. 
• Sister groups are two descendants that split from the same node and therefore are each other’ s closest relatives.
• A useful character, or trait, is any heritable genetic, morphological, physiological, developmental, or behavioral characteristic that varies among the taxa to be studied.
• Outgroup is a taxon that is not part of the taxonomic group being studied but is closely related.
• An ancestral trait is a character that existed in an ancestor. 
• A derived trait is one that is a modified form of the ancestral trait, found in a descendant.
• A synapomorphy is a trait found in two or more taxa that is present in their most recent common ancestor but is missing in more distant ancestors. 
• Synapomorphies are important because they allow biologists to recognize monophyletic groups-also called clades or lineages.
• A monophyletic group is an evolutionary unit that includes an ancestral species and all of its descendants, but no others.
• Convergent evolution is the independent evolution of similar traits in distantly related organisms due to adaptation to similar environments and lifestyles. 

25.2 Tools for Studying Life’s History: The Fossil Record

• Paleontologists are scientists who study fossils, pieces of physical evidence from an organism that lived in the past. 
• The fossil record is the total collection of fossils that paleontologists and amateur collectors have found throughout the world and archived in thousands of private and public collections.
• The Paleozoic (“ancient life”) era begins with the appearance of most major animal lineages and ends with the obliteration of almost all multicellular life-forms at the end of the Permian period.
• The Mesozoic (“middle life”) era begins with the end-Permian extinction events and ends with the extinction of the dinosaurs ( except birds) and other groups at the boundary between the Cretaceous period and Paleogene period.
• The Cenozoic (“recent life”) era is divided into the Paleogene, Neogene, and Quaternary periods. 

25.3 Large-Scale Pattern in Life’s History: Adaptive Radiation 

• When a single lineage rapidly produces mαny descendant species with a wide range of adaptive forms, biologists say that an adaptive radiation has occurred.

25.4 Large-Scale Pattern in Life’s History: Mass Extinction

• A mass extinction refers to the rapid extinction of a large number of diverse species around the world.
• Background extinction refers to the lower, average rate of extinction observed when a mass extinction is not occurring
• ==Some researchers estimate that the current extinction rate is 100 or even 1000 times higher than the background extinction rate==

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