Convergent evolution the independent evolution of similarity in 2 or more distantly related lineages Since some synapomorphies are indicative of evolutionary relationships and others are not how do cladists estimate phylogeny o By interpreting as many synapomorphies as possible as homologies and interpreting the remaining synapomorphies as coincidence 2 Types of Coincidental Evolutionary Phenomena leftovers a Parallelism a trait independently acquired in two or more closely related species 2 Evolutionary reversals Fig 3 7 Example salamanders 1 Maximizing congruence among derived characters 1 Convergence parallelism Fig 3 5 3 6 Illustrations of 2 Congruence parallelism 3 Evolutionary reversals rather than A B C Punch line 1 Explain as many synapomorphies as possible homologs 4 The two congruent synapomorphies vs 1 incongruent are evidence for A B C a Example start by explaining the distribution of evidence by common ancestry then treat leftovers as independent events find maximum congruence among the evidence 2 All homologies are synapomorphies but not all synapomorphies are homologies Species A Species B Species C Matrix X 1 1 0 Key 1 Has trait 0 Does not have Y 1 1 0 Assume the presence of a character is the derived characteristic Z 0 1 1 X A B Y A B x y Step 1 Step 2 X Y Step 3 A y B C x A B B Z B C z Z C z Step 4 A B x y C z z Example of convergence B C 4 steps z derived trait x y homologous traits z independent trait A B z y x C z Example of evolutionary reversal z then z z ancestral trait Neither can preferred without further information 4 steps A B C B x y z 5 Steps C A y x Parsimony dictates the topology of the tree C The Role of Parsimony in Phylogenetics Systematics What is the general law of Parsimony Occam s Razor Parsimony is at the root of much science How is parsimony defined in systematics That the simplest sufficient hypothesis is to be preferred even though others are possible The principle of invoking the minimal number of evolutionary changes to infer phylogenetic relationships or Choosing the hypothesis of species genealogy that best fits the available evidence Some critics argue that parsimony assumes that nature is simple of that evolution is parsimonious o Not True A most parsimonious hypothesis is evolutionary relationships based on minimal assumptions and is at very least a starting point for genealogical studies Any author who prefers a less parsimonious tree should be required to justify this choice based on additional biological assumptions and evidence D Polarity direction of change Polarity the direction of evolutionary change within a transformation series o Transformation series evolutionary sequence of changes in a homologous character Why do we need to estimate polarity in phylogenetic systematics So we can tell derived from ancestral character and character series o Cladists only use synapomorphies for phylogenetic estimation How do we estimate polarity There are at least 3 criteria that have been used to judge polarity 1 Outgroup Polarity 2 Use fossils if available paleontology 3 Ingroup comparisons Outgroup Comparison neonatologists researchers Currently the most commonly used method for determining polarity at least among Ingroup in cladistics any group of theoretically closely related species of interest to the Outgroup in cladistics taxa at least two that are closely related to the monophyletic group of taxa whose relationships are under study Ingroup 1 Outgroup criterion for Polarization 2 a The distribution of characteristics of the outgroups summarized at the outgroup node is used to infer the characteristics of the ingroup node ancestor of the ingroup b This procedure finds the best fit to the evidence c Best fit simplest explanation parsimony