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SIU BIOL 304 - 2016-Lecture (8)

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PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Estimating Evolutionary TreeAn evolutionary tree, also known as a phylogenetic tree or a phylogeny,is a diagram showing the history of divergence and evolutionary change leading from a single ancestral lineage to a suite of descendants.In other words, it depicts a group of organisms’ genealogical relationships as they are understood according to Darwin’s theory of descent with modification from common ancestors.How to Read Time on an EvolutionaryTree:In this diagram, from root to branches.In most phylogenies, the root is on the leftand time proceeds from left to right.How to Read Relationships on an Evolutionary Tree:Phylogenies are hypotheses about the history of descent with modification from a common ancestor. Time flows along a phylogeny from the root toward the branch tips.The evolutionary relationships among species on a phylogeny are defined by the relative time elapsed since they last shared common ancestors. Lineages that share more recent common ancestors are considered more closely related.Is the snow leopard more closely related to the Canada lynx or to the jaguar?Evolutionary Trees Do Not Show Everything.Evolutionary Trees Can Be Drawn in Various Styles.Always, Evolutionary relationships are depicted solely by the order of branching in a phylogeny.Evolutionary Trees Are Hypotheses:The Logic of Inferring Evolutionary Trees:In an ideal case, we can infer the evolutionary history of a set of species from their nested sets of shared evolutionary innovations.An ideal example:•The four species are descended with modification from the common ancestor on the left side, an undecorated bird with a short beak. Knowledge of the common ancestor’s characteristics allows us to identify the long beak and various decorations adorning the four species on the right as evolutionary novelties particular to this group of birds.• Each of the evolutionary novelties evolved exactly once. • Once each of the novelties evolved in a lineage, it was never lost.Identifying evolutionary novelties (also called derived characters).(1) Which ones are unique to one species?(2) Which ones are shared by more than one species.The characters unique to one species must have evolved after the lineages leading to these species diverged from the lineages leading to the other species.Shared:Tail barMaskWing tipsUnique:Long billDark tailThe characters unique to one species must have evolved after the lineages leading to these species diverged from the lineages leading to the other species.Recall:- Descent with modification from common ancestors automatically produces species displaying nested sets of shared evolutionary novelties (what were the examples we used?),- The order of nesting allows us to predict the order in which the novelties evolved,- We used this to test Darwin’s theory of descent with modification by checking such predictions against the fossil record.- The same logic is being used to hypothesize an evolutionary history by reconstructing a phylogeny.Key Concepts in inferring phylogenies:Apomorphy (separate form): an evolutionary novelty, or derived character. Pleisomorphy (near form): a preexisting, or ancestral character.Note: The proper application of these concepts depends on the context. For our imaginary birds:How would you categorize the mask(a)within the set of all four living species?(a)within the set of masked species? Monophyletic group also known as a clade: an ancestor and all of its descendants.Paraphyletic group consists of an ancestor and some, but not all, of its descendants.Polyphyletic group contains some, but not all, of an ancestor’s descendants, and also excludesthe ancestor.Synapomorphies identify monophyletic groups.Uncertainty about the order of branching in a phylogeny is indicated by polytomies - nodes where a lineage splits into more than two descendant lineages simultaneously.Phylogeny Inference in Non-ideal Cases:-Characteristics of the common ancestor not known.-Synapomorphies not certain; similar evolutionary novelties sometimes evolve independently in different lineages.-Evolutionary novelties are sometimes lost.Hypothetical example:Outgroup Analysis:Involves including one or more additional species related to the ingroup (the species whose relationships we wish to infer), but less closely related to them than the members of the ingroup are to each other.Assume there has been no evolutionary change in the outgroup’s lineage since it diverged from the lineage that gave rise to the ingroup.Parsimony AnalysisThe hypothesis that requires the fewest evolutionary changes in the characters of interest is preferred. We evaluate each character on each possible tree, looking for the simplest evolutionary scenario that can explain the distribution of the character states among the species at the tips. We add up the total number of evolutionary changes required by each hypothesis, and identify the hypothesis for which the total is lowest.Uninformative.Why?Parsimony analysis therefore leads us to favor Hypothesis 2 as our estimate of the evolutionary relationships among our ingroup antelopes.Can we say anything about the evolutionary history of dark tails?The Number of Possible Trees:An ingroup had only three species = three possible trees to consider.The number of possible trees increases rapidly with the number of species. Four species in the ingroup = 15 possible bifurcating trees. With 10 species, there are 34,459,425.The independent appearance in different lineages of similar derived characters is called convergent evolution.The loss of derived traits in a lineage, resulting in a return to the ancestral condition, is called reversal. Similarity in character states due to convergence and/or reversal is called


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