BIODIVERSITY CHAPTER 23 SYSTEMATICS AND THE PHYLOGENETIC REVOLUTION text first four sections pg 456 470 23 1 Systematics systematics phylogeny difference between species reconstruction and study of evolutionary relationships systematic construction of an evolutionary tree based on the similarities and o represents a hypothesis about patterns of relationship among species Branching diagrams depict evolutionary relationships in Darwin s view all species were descended from a single common ancestor and that the history of life could be depicted as a branching tree o twigs of the tree represent existing species o the joining of twigs and branches reflects the pattern of common ancestry back in time to the single common ancestor of all life descent with modification from common ancestry shows that all species are related in a hierarchical way o evolutionary history is depicted using branches or phylogenic trees key to depicting a tree is to look at the species that are closely related Similarity may not accurately predict evolutionary relationships similarity is not a great predictor of how long it has been since two species shared a common ancestor o evolution occurs rapidly and slowly and is not unidirectional o evolution is not always divergent convergent evolution can cause two species that are not closely related to eventually become more related evolve convergently because similar habitats are used where similar adaptions are favored 23 2 Cladistics cladistics derived characteristics that came from a common ancestor approach to systematics that distinguishes the similarities between species by the o shared derived characters determining evolutionary relationships only things found to be useful and informative when The cladistic method requires that character variation be identified as ancestral or derived o characters can be any aspect of the phenotype morphology physiology behavior and DNA Examples of ancestral versus derived characters o presence of hair is a shared derived feature of mammals and to contrast presence of lungs is ancestral b c it is also present in amphibians and reptiles this does NOT tell us that mammal species are more closely related to one another than to reptiles or amphibians DOES tell us that the derived feature of hair suggests that all mammal speices have a common ancestor that existed more recently than common ancestor of mammals reptiles and amphibians Determination of ancestral versus derived o polarize determine whether particular character states are ancestral or derived after the data is assembled first step of manual cladistic analysis ex to polarize teeth sysematists determine which state presence or absence was shown by the most recent common ancestor of this group o outgroup comparison outgroup group under study method used to assign character polarity species or group of species that is related to but not a member of it when group under study exhibits multiple character states and one of those states is exhibited by the outgroup that state is considered ancestral and the other states are conserved to be derived keep in mind outgroup species also evolve from ancestors so outgroup species will not always show the ancestral condition Construction of a cladogram o cladogram o clade shared derived characters construction that depicts a hypothesis of evolutionary relationships created after all characters have been polarized group of species that share a common ancestor indicated by the possession of evolutionary units that refer to a common ancestor and all of its descendants derived character shared by called members synapomorphy ancestral states plesiomorphies symplesiomorphies shared ancestral states not informative about phylogenic relationships reflect character states inherited from a distant ancestor do not imply that species exhibiting that state are closely related Homoplasy complicates cladistic analysis some characters have evolved independently in several species makes phylogenics difficult o these characters would be shared derived characters but would be false signals of close evolutionary relationships homoplasy exhibiting that character state a shared character state that has not been in inherited from a common ancestor o results from convergent evolution or evolutionary reversal ex absence of a tail is synapomorphy that unites gorillas humans and frogs however frogs have neither an amniotic membrane nor hair synapomorphies for clades with humans and gorillas for when there are conflicts between characters favors hypothesis that principle of parsimony requires fewest assumptions o phylogeny that needs the fewest evolutionary happenings is considered the best hypothesis of phylogenetic relationships DNA sequencing also used to make phylogenies b c of the large amount of characters that can be obtained through sequencing character states are polarized by reference to the outgroup cladogram is constructed that minimizes the amount of character evolution required Other phylogenetic methods work better than cladistics in some situations principle of parsimony works well in reconstructing evolutionary relationships when and if characters evolve from one state to another at a slow rate in comparison to the frequency of speciation events o underlying assumption that shared derived similarity is suggestive of recent common ancestry is generally correct except when characters evolve rapidly cause principle to be misleading Rapid rates of evolutionary change and homoplasy o some stretches of DNA appear to not yet have a function and as a result mutations that occur here are not removed through natural selection rate of evolution of new character states can be high as a result of genetic drift o because there are only four character states for a nucleotide base A T G C that creates a high probability that two species with independently evolve he same derived character state at any base if homoplasy dominates the character data set assumptions of principle of parsimony are violated this method is deemed inaccurate o more effective when dealing with homoplasy when evolutionary change rates are high Statistical approaches than using parsimony The molecular clock o molecular clock states that rate of evolution of a molecule is constant through time divergence in DNA is used to calculate the times at which branching events have occurred amount of DNA divergence separating two clades isd ivided by the length of time separating the two clades
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