2 Fossil Criterion for polarization Oldest character state is ancestral state is ancestral state Ax A x Rock Layer If A x appears earlier in the fossil record then x is the ancestral character state If oldest character state ancestral this assumes the oldest fossil represents the common ancestor of the newer fossil Ax A x Ax Punch line process The oldest fossil only represents the minimum age of a species older specimens possessing character x may not have been preserved in the fossil record due to vagaries of the fossilization 3 Ingroup Criterion for Polarization Most common character state is ancestral state Problem Assumes a constant rate of speciation Therefore a more speciose group s characteristics become ancestral A1a A1b A2a A2b B1a B1b B2a B2b C x A1 A2 B1 B2 A B C D x x E x F Time Outgroup polarization of characters is the method currently preferred by most evolutionary biologists E Why hypotheses of evolutionary relationships phylogenies are like a time machine or What good are phylogenies anyway 1 Estimating the History of Character Evolution In order to infer casual evolutionary processes i e the how questions one must first estimate what evolutionary transitions have actually taken place How is this done By reference of a phylogenetic tree Once a robust hypothesis of evolutionary relationships has been constructed for a group of organisms parsimony can be used to estimate by minimizing the number of evolutionary events the number and sequence of evolutionary transitions in all sorts of characteristics coincidence Freshwater Mussels a Example A basic dichotomy single v multiple origins example of serendipity How many times did the bivalve shell evolve in the Bivalvia 1 Are a very old group of bivalves dating back to at least the early Triassic i e at least 200 mil Years old derived from marine ancestor 2 Are a geologically widespread taxon living in rivers and freshwater lakes on all continents except for Antarctica but there are freshwater bivalve fossils on that continent 3 In relatively undisturbed habitats are often dominant macro invertebrate of the benthos 4 Are one of the most endangered groups of animals in the world 5 Are of the order Unionoida which contains two superfamilies and five families Order Unionoida Superfamily Unionoidea Family Unionoidae Family Hydriidae Family Margaritiferidae Superfamily Etheroidea Family Iridinidea Family Mycetopodidae Why is 6 important 6 Have an obscured evolutionary history in that the closest marine relatives were unknown i If my goal is to estimate Unionoid phylogeny and character evolution using parsimony I ll need to root my trees using the outgroup method to determine character polarities I Hypothesis 1 Veneroid bivalves are sister taxon to Unionoids II Hypothesis 2 Trigonoid bivalves are the sister taxon to Unionoids The resulting DNA nucleotide sequences were analyzed using parsimony and other methods to 1 Estimate a phylogeny for bivalved mollusks 2 Assess the monophyly of the Unionoidea a Example is this a natural group 3 Determine the closest marine relative for Unionoidea Punch lines for Bivalvia Tree 1 The Unionoidea is monophyletic 2 Neotrigonia a Trigonoid bivalve genus is the closest marine lineage sister taxon to Unionoid bivalves 3 The Molluscan class Bivalvia is not a monophyletic group a Example of serendipity What are the evolutionary implications of this tree Use character optimization via parsimony to assess the history of character change How does this work 1 Ancient habitat shift a How many Unionoid invasions of freshwater are suggested by the Bivalvia tree 2 Morphological change a What does the Bivalvia shell say about evolution So how many times did the bivalved shell evolve within Bivalvia More than once according to data set b Direction of Evolution o Phylogenetic trees can used to estimate the direction of evolutionary change in a multistate character Example the evolution of brooding location in freshwater mussel higher taxa Unionoid Brooding Background Info 1 Unionoid bivalves have four ctendia gills two on each side of the animal 2 Unionoid bivalves are primarily dioecious with males releasing spermatozoa into the water and females taking them up with their ctendia 3 Females release eggs that are fertilized by the acquired spermatozoa and then the early embryos are deposited into modified ctendia called marsupia where they develop larvae 4 The marsupia can be made up of a All four ctendia Tetrageny b The inner two Endobranchy c The outer two Ectobranchy A classification for the group with brooding location indicated Order Unionoida Superfamily Unionoidea Family Unionoidae Ectobranchy Tetrageny Family Hydriidae Endobranchy Family Margaritiferidae Tetrageny Superfamily Etheroidea Family Iridinidea Endobranchy Family Mycetopodidae Endobranchy So if Unionoids are monophyletic as suggested by the previous analysis the character brooding location with three character states must have changed its state a minimum of two times during Unionoid phylogensis But what was the order of character state transitions Traditional Hypotheses Endobranchy Tetrageny Ectobranchy Derived Ancestral Derived Based on the story that Margaritiferidae which has Tetrageny possessed the ancestral character states for the Unionoida due to Margaritiferids having simpler ctendial structures than other Unionoids To evaluate this hypothesis Sequenced a 710 nucleotide fragment of the cytochrome c oxidase subunit I COI gene encoded in the circular mitochondrial DNA in various Unionoid genera representing the five families ingroup with their different brooding locations and Neotrigonia Unionoid sister taxon outgroup