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Chapter 4

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BIOL 2400
Cortland Griswold

Basics of Phylogenetic Estimation Cladistics, Chapter 4, p. 83-94 4.1 Tree Thinking 1830’s - Darwin pictures evolution as a tree growing new branches a population may become subdivided into 2 populations that no longer exchange genes, separating into 2 distinct species hard to draw entire tree of life; more than 1.9 million species known to date, millions left to discover difficult to know the diversity of life over history Phylogeny: a visual representation of the history of populations being compared Tips: terminal ends of an evolutionary tree, representing species, molecules, or populations being compared Branches: lineages evolving through time b/w successive speculation events Node: a point in a phylogeny where a lineage splits (a speciation event) (common ancestors) Internal Nodes: nodes w/n a phylogeny representing ancestral populations or species Clade: an organism and all of it’s descendants - smaller clades can be nested in larger ones trees can be rooted or unrooted - rooted includes the focal group, along w/ a closely related outgroup species - they contain info about the ancestral states of characters when a phylogeny only shows the relationship b/w a species it’s called a cladogram branches do not precisely measure period of time many different ways to draw and visualize phylogenetic trees - different representations can give different focus and importance 2 trees with their branches rearranged can give a diff. illusion but they mean the exact same things w/n a clade you can choose how many species you wish to represent more or less detail can give the tree a lot of significance 4.2 Phylogeny and Taxonomy evolutionary biologists will sometimes condense phylogenetic trees so entire clades become single tips it’s a convenient way to represent large clades the idea that mammals represent a taxonomic group emerged long before Darwin Carl Linnaeus established a system of classification based on shared traits species could be grouped into genera, then into families, orders and so on some believed this nested hierarchy reflected a preexisting structure in God’s mind that was represented in creation Darwin was able to explain nested hierarchies w/o recourse to the mind of the creator the traits that distinguished taxonomic groups evolved along the groups the hallmarks of all vertebrates, such as skulls, and spines, evolved in the common ancestor of gold- fish, frogs, lizards, birds and mammals we mark those traits w/ a line crossing the vertebrate clade this only tells that all these species are related and some evolved later in time than others w/n vertebrates the more recent traits define a smaller group: the tetrapods a taxonomic unit is only considered legitimate if it represents a clade Monophyletic: a term used to describe a group of organisms that form a clade different conclusions can result from diff. lines of evidence. Linnaeus’s system was built on an un- derstanding of structural similarities that was relatively basic compared w/ the tools of scientists to- day Definitions Monophyletic –Aterm used to describe a group of organisms that form a clade. Characters – Heritable aspects of organisms that can be compared across taxa. Taxon (plural, taxa) –Agroup of organisms that a taxonomist judges to
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