Chapter 26 Phylogeny and the Tree of Life 26 1 PHYLOGENIES SHOW EVOLUTIONARY RELATIONSHIPS Taxonomy names and classifies organisms BINOMIAL NOMENCLATURE Carolus Linnaeus came up with the binomial nomenclature for naming organism The first part of the name is the genus to which each species belongs too and the second part is called the specific epithet HIERACHIAL CLASSIFICATION Linnaeus also grouped organisms into a level of hierarchy that consisted of increasingly inclusive categories Each level is called a taxon Taxa broader than the genus are not italicized LINKING CLASSIFICATION AND PHYLOGENY An evolutionary history of a group can be represented by a branching diagram called a phylogenetic tree There can also be difficulties by aligning Linnaean classification with phylogeny and some scientists have proposed classification be based entirely on evolutionary relationships Phylogenetic tree represents a hypothesis about evolutionary relationships These relationships are depicted as a series of dichotomies or two way branch points Each branch point represents divergence of 2 evolutionary lineages from a common ancestor To understand how to read a phylogeny tree reference page 550 of the textbook WHAT WE CAN AND CANNOT LEARN FROM PHYLOGENETIC TREES Phylogeny trees could best be understood using these three points 1 They are intended to show patterns of descent not phenotypic similarity 2 The sequence of branching in a tree does not necessarily indicate the actual absolute ages of the particular species No assumptions should be made about when particular species evolved or how much change occurred in each lineage It should not be assumed that a taxon on a phylogenetic tree evolved from the taxon next t it 3 APPLYING PHYLOGENIES Understanding phylogeny can have practical applications For example Corn descended from Wild Grasses that may be its closest living relatives Thus wild grasses could provide useful alleles to corn because of their closeness to each other 26 2 PHYLOGENIES ARE INFERRED FROM MORPHOLOGICAL AND MOLECULAR DATA To infer phylogeny of organisms information is gathered about morphology genes and biochemistry of relevant organisms MORPHOLOGICAL AND MOLECULAR HOMOLOGIES Genes or other DNA sequences are homologous if they are descended from sequences carried by a common ancestor just like structures In general organisms that share very similar morphologies or similar DNA sequences are likely to be more closely related to organism than ones with different structures and sequences Furthermore morphological divergence could be great but genetic divergence not e g Hawaiian silversword plants SORTING HOMOLGOY FROM ANALOGY A potential source of confusion in constructing phylogeny is similarity due to convergent evolution which produces analogous structures Distinguishing between homology and analogy is crucial to determining phylogeny Consider bats and birds Bats and birds look more related to each than birds are to cats However a closer look shows that cats and bats have similar forelimbs Bats and birds descended from a common tetrapod 320 MYA Thus the underlying skeletal systems of bats and birds are homologous their wings are not Flight is enabled by membranes in bats versus feathers in birds The more elements in common between 2 organisms the higher the chances of having a common ancestor EVALUATING MOLECULAR HOMOLGIES Comparing DNA molecules often poses technical challenges for researchers The first step after sequencing the molecules is to align comparable sequences from the species being studied Closely related species differ in DNA sequencing at fewer sights than distantly related species The reason is insertions and deletions could accumulate over long periods of time For example certain noncoding DNA sequences near a particular gene are very similar in two species except that the first base of the sequence has been deleted in one of the species This causes the remaining sequence to shift back one not Without technology we would miss what would seem like a perfect match Two sequences that resemble each other at many points are most likely to be homologies In organisms that do not appear close the sequences that seem to be shared are called molecular homoplasies For organisms to share 25 of DNA sequences would result in a molecular homology 26 3 SHARED CHARCTERS ARE USED TO CONSTURCT PHYLOGENETIC TREES A key step in reconstructing phylogenies is to distinguish homologous features from analogous ones A method must also be chosen to infer phylogeny from these homologous characters A widely used set of methods is known as cladistics CLADISTICS Common ancestry is the primary criteria to classify organisms when using cladistics Biologists attempt to place species into groups called clades each of which includes an ancestral species and all of is descendants Clades are nested within larger clades A taxon is equivalent to a clade only if it is MONOPHYLETIC signifying that it consists of an ancestral species and all of its descendants PARAPHYLETIC groups consist of an ancestral speices and some but not all of its descendants POLYPHYLETIC groups consist of distantly related species but not the most recent common ancestor SHARED ANCESTRAL AND SHARED DERIVED CHARCTERS As a result of descent with modification organisms may share and not share characteristics with their ancestors For example all mammals have backbones but a backbone does not distinguish mammals from vertebrates because they all have backbones Thus this is SHARED ANCESTERAL CHARACTER a characteristic that originated in an ancestor of the taxon In contrast hair is a character shared by all mammals but not found in their ancestors thus it is a SHARED DERIVED CHARACTER an evolutionary novelty unique to a clade INFERRING PHYLOGENIES USING DERIVED CHARACTERS Because shared derived characters are unique to particular clades they are very useful for inferring phylogenies An out group is a species or group of species from an evolutionary lineage that is know to have diverged before the lineage that includes the species we are studying the in group By comparing members of the in group with each other and with the out group one can determine which characters were derived at the various branch points PHYLOGENETIC TREES WITH PROPORTIONAL BRANCH LENGTHS Phylogenetic trees can be proportional to the amount of evolutionary change or to the times at which particular events occurred The equal spans of chronological time can be