3 Phylogenetic systematics cladistics Willi Henning a German entomologist published his famous book in 1950 in German and it was later 1966 translated into English Title Phylogenetic Systematics This book has revolutionized Evolutionary biology and cladistics which has become the dominant school of systematics What are the principle distinctions between cladistics and competing schools Cladistics emphasizes 1 Special similarities synapomorphies a Synapomorphy the common possession of a derived trait amorphic character or character state b Example single digit in zebras horses etc in order Persissodactyla 2 Character Congruence agreement maximized by parsimony Fig 2 10 a Parsimony the principle of invoking the minimal of evolutionary changes to infer phylogenetic relationships 3 Classifications should be consistent with phylogeny and base solely on monophyletic groups a Monophyletic group a group of taxa derived from a single ancestral taxon containing all of the descendants of the most recent common ancestor b Paraphyletic group a group of taxa derived from a single ancestral taxon but one which does not contain all the descendants of the most recent common ancestor based on common possession of plesiomorphic traits Shark Lungfish Cow Shark Lungfish Cow Paraphyletic Group Choanoates monophyletic Fish paraphyletic group o Based on overall similarity Lungfish cows monophyletic group o Based on synapomorphies special similarities What would the classifications look like Evolution systematics Fishes Shark Lungfish Mammals Cows Phylogenetics Systematics Chondrichthyes Shark Choanates Lungfish Cow So phenetics and evolution systematics can produce artificial classifications while phylogenetic systematics produces natural classifications based on genealogy rather than overall similarity Details of Cladistic Methodology A Assumptions of Cladistics 1 Evolution has occurred descent without modification 2 Species evolve from ancestral species 3 Contemporary species are reproductively isolated from each other 4 2 3 There is a species genealogy These three assumptions force us to believe that 1 The biological world is non random a Example species are hierarchically arranged in nature 2 The attributes of species characters are hierarchically arranged as well Example A B C A B C D Ancestral Species C Axyz Bxyz Cz A B x y z These hierarchically distributed traits serve as the evidence for evolutionary relationships Not all traits have the same generality but 2 or more can have some generality x y have the same generality Z has a different greater level of generality than x y does How do these character transitions actually occur All characters first appear in a single or a few individual ontogeny growth and development of an individual to maturity o Therefore due to interbreeding A novel character spreads to other individuals of the population Then spreads to other populations within the species The spread of the same novelties to other contemporary species is impossible due to reproductive isolation This process leads to nested levels of novelties characters B C A x Time Trait x can be passed down to Species A B but not to C B Special Similarity and concept of Homology 1 Only use synapomorphies for establishing phylogenies 2 There are classes of observations that are non evidence for phylogeny i Example 1 Symplesiomorphy 2 Autapomorphy a derived character that is not shared 3 Here x is an autapomorphy x provides no evidence on the evolution relationships among A C A x B C 4 Overall Similarity Synapomorphy Symplesiomorphy Autapomorphy A B C x A B C x Symplesiomorphy AxBxCx shared ancestral character Monophyletic group Synapomorphy AxBxC The type of evidence portrayed in the first cladogram is common in the evolutionary biology literature but o X as a symplesiomorphy says nothing about how species A B and C are related to each other says only that they form a monophyletic group B A C x Tricotomy Polytomy A2b A1a A1b A2a B C A1 A2 B C A B C T3 T2 T1 x T1 x autapomorphy T2 x synapomorphy T3 x symplesiomorphy How does homology relate to these ideas Homology that similarity is inherited from a similar condition in the common ancestor of organisms being studied Fig 3 3 support of the A B clade o Example x is a homologous characteristic and is the only relevant evidence in A B C x Not all shared derived traits are homologous o Example vertebrate eyes v cephalopod eyes The presence of image forming eyes is a derived characteristic and is shared by both vertebrates and cephalopods synapomorphy but these eyes are not homologous They were not present in the common ancestral lineage which eventually gave rise to vertebrates and cephalopods those eyes represent convergent evolution Fig 3 4