Cladistics

Anoop Johny
Msc. Applied Zoology
University of Calicut

1. CLADISTICS ANOOP JOHNY 1 ST MSC. APPLIED ZOOLOGY UNIVERSITY OF CALICUT

2. TERMINOLOGY A  clade  ( monophyletic group  or  natural group ) is a group of organisms that are  monophyletic  – that is, composed of a common ancestor   and all its lineal descendants   on a  phylogenetic tree. Cladogenesis – Cladogenesis is an evolutionary  splitting of a parent species  into two distinct species, forming  a clade.

3. Monophyletic taxon is one that includes a group of organisms descended from a single ancestor Includes the most recent common ancestor of a group of organisms, and all of its descendents Such groups are sometimes called Holophyletic . Polyphyletic taxon is composed of unrelated organisms descended from more than one ancestor. Paraphyletic taxon is the one that includes the most recent common ancestor, but not all of its descendents

4. Plesiomorphy (“close form”) or ancestral state is a character state that a taxon has retained from its ancestors. Symplesiomorphy (from syn-, “together”) When two or more taxa that are not nested within each other share a plesiomorphy, it is a symplesiomorphy. Apomorphy (or derived trait) is a novel character or character state that has evolved from its ancestral form (or plesiomorphy).

5. Synapomorphy is an apomorphy shared by two or more taxa and is therefore hypothesized to have evolved in their most recent common ancestor
A Homology is a character shared between species that was also present in their common ancestor. Homoplasy , which is a convergent character shared between species but not present in their common ancestor.

6. INTRODUCTION ‘Klados ‘(Ancient Greek) – “Branch “. Cladistics is the classification of organisms based on their phylogenetic (evolutionary) relationship and the recency or antiquity of common ancestor, rather than on their observable similarities. The cladistic or phylogenic classification is based on phylogeny of the involved organisms and depends on the phylogenetic branching.

7. According to it organisms are placed in to taxonomic groups called Clades. Based on the possession of Synapomorphic characters. So organisms are classified based on the historical order of their evolutionary descent. It does not consider the phenotypic differences between the descendants of a common ancestor. Cladistic taxonomy upholds the monophyletic origin of different groups from a common ancestor through Cladogenesis.

8. HISTORY The original methods used in cladistic analysis and the school of taxonomy derived from the work of the German entomologist Willi Hennig, who referred to it as phylogenetic systematics Father of Cladistics – Willi Hennig What is now called the cladistic method appeared as early as 1901 with a work by Peter Chalmers Mitchell for birds and subsequently by Robert John Tillyard (for insects) in 1921 Willi Hennig 1972 Peter Chalmers Mitchell

9. The term “clade” was introduced in 1958 by Julian Huxley. Cladistic analysis by Willi Henning is to determine which character states are primitive and which are derived based on common ancestry. Robert John Tillyard

10. CLADOGRAM A cladogram (from Greek clados “branch” and gramma “character”) is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to descendants, nor does it show how much they have changed. A cladogram uses lines that branch off in different directions ending at a clade, a group of organisms with a last common ancestor. There are many shapes of cladograms but they all have lines that branch off from other lines. The lines can be traced back to where they branch off.

11. These branching off points represent a hypothetical ancestor (not an actual entity) which can be inferred to exhibit the traits shared among the terminal taxa above it.
This hypothetical ancestor might then provide clues about the order of evolution of various features, adaptation, and other evolutionary narratives about ancestors. Although traditionally such cladograms were generated largely on the basis of morphological characters, DNA and RNA sequencing data and computational phylogenetics are now very commonly used in the generation of cladograms, either on their own or in combination with morphology.

12. PARTS OF A CLADOGRAM ROOT The root is a common initial ancestor and is marked as the starting point for the diagram. An incoming line represents that the root comes from larger clades. NODES A node is a region that marks the point of divergence in cladograms and represents the hypothetical ancestor that further divides to bifurcate into two or more daughter taxa. CLADES A clade is a specific part of the cladogram that includes the recent ancestor and its descendants. It can be indicated by marking out a particular node and all of its associated branches.

13. BRANCHES The branches indicate the bifurcation of the root into nodes. Links between the organisms can be deduced via tracing out the branches. TAXON/OUTGROUP The taxon or the outgroup is the most distantly related organism in the entire chart. This group doesn’t usually form a clade and instead offers a point of comparison for the rest of the cladogram.

14. CLADISTIC CLASSIFICATION In the evolution of a taxon, a sequence of kinds can be seen at different periods of time and this sequence of kinds is called a ‘Lineage’ If we start from the recent taxon and trace it back through its lineage we can see its history and this is known as Phylogeny. cladistic or phylogenic classification is based on phylogeny of the involved organisms and depends on the phylogenetic branching.

15. Henning (1950) based his classification on genealogy (= history of the descent of taxa). Mayr and Ashlock (1991) stated that “A cladistic classification consists of a nested hierarchy of increasingly more inclusive holophyletic taxa; this hierarchy corresponds to a hierarchy of increasingly more inclusive synapomorphies.” In this type of classification the phylogeny is reconstructed (cladistic analysis) by analyzing the synapomorphic characters. The sequence of branching events in the evolutionary history of the group is determined and based on this, a cladogram is constructed.

16. The cladists who follow Hennigs believe that branching results when speciation takes place and only two new phyletic groups originate. Each of these groups can be recognized by its synapomorphic characters. Each species ceases to exist when it splits into two daughter species. The species form the base point of a cladistic analysis and each holophyletic taxon (holophyletic means pertaining to a group that consists of all the decendants of its most recent common ancestor) is derived from a particular stem species (Mayr and Ashlock, 1991).

17. Each character of a taxon must be evaluated to see whether it is apormorphic (derived) or plesiomorphic (primitive). Since the plesiomorphic characters do not help in locating branching points in a cladogram, they are usually ignored in cladistic analysis. A character can be considered as apomorphic when it is found only in a particular taxon.

18. Reference An Introduction to TAXONOMY - T. C. NARENDRAN. Basics of Cladistic Analysis - Diana Lipscomb George Washington University PRINCIPLES AND METHODS OF PHYLOGENETIC SYSTEMATICS: The University of Kansas - Museum of Natural History https://www.newworldencyclopedia.org/entry/Cladistics https://www.edrawsoft.com/article/what-is-cladogram.html https://en.m.wikipedia.org/wiki/Cladistics

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