Topic 5.4 - Cladistics

Cladograms are tree diagrams that show the most probable sequence of divergence in clades. Cladograms are often generated using DNA or mDNA evidence. The point at which organisms diverge from one another on a cladogram is a node. Nodes correspond with a divergence between organisms. Time is represented vertically. Tips for identifying a clade: The clade must end at a single ancestor. The clade must highlight organisms that share a single characteristic. Can you identify a clade on the cladogram above? How many nodes are present in this cladogram?

A clade is a group of organisms that have evolved from a common ancestor. Clades can be of different sizes and can include different numbers of organisms Clades include all organisms and their common ancestor Evidence for which species are part of a clade can be obtained from the base sequences of a gene or the corresponding amino acid sequence of a protein. These organisms at the nodes are not present anymore, they have either: Evolved into a modern organism or become extinct. Click the dragons to the right to practice building a cladogram for these mythical dragons. http://ccl.northwestern.edu/dtttl2009/LS_426_-_The_Design_of_Technological_Tools_for_Thinking_and_Learning/Past_Projects/Entries/2008/3/11_Here_is_a_project_title_files/TreeInteractive-02.swf

Mutations that occur in DNA do so at a consistent rate. Application: Be able to analyze cladograms including humans and other primates. Each node is associated with a trait that is developed. Mutations that occur in DNA do so at a consistent rate. As such, we can use this rate as a pseudo-clock to calculate how long ago species diverged from one another – Taking DNA from two modern species, you analyze sequences that produce common proteins, and analyze the numbers of differences between the sequences. The number of differences implies the amount of time it has been since they diverged – i.e. there is a positive correlation between the number of differences and the times since divergence.

Application: Be able to analyze cladograms including humans and other primates to deduce evolutionary relationships. Notice, these species can be placed on a cladogram by morphology fairly easily, up until the great apes, when we require more advanced technologies to distinguish evolutionary relationships. The combination of genetics and morphological technologies has led us to understand that Chimpanzees and Bonobos are the closest relatives to humans out of the great apes. Bonobos – 98.7% similarity to humans Chimpanzees – 99% similarity to humans Bonobos to chimpanzees – 99.6% to each other

Application: Be able to analyze cladograms including humans and other primates to deduce evolutionary relationships. What animals does the sperm whale share a common ancestor with what animals? How closely related are the modern dolphin and hippopotamus? Which animal diverged first out of the living animals?

Analogous Structures – same evolved function, evolved independently Traits can be analogous or homologous. Analogous Structures – same evolved function, evolved independently Homologous Structures – same structure, evolved from a common ancestor Nature of Science – Falsification of theories with one theory being superseded by another—plant families have been reclassified as a result of evidence from cladistics. For many years, taxonomists relied on morphology to derive evolutionary relationships, however, once genetics technologies advanced far enough, they began to realize that they had mis-classified many organisms!!!

Evidence from cladistics has shown that classifications of some groups based on structure did not correspond with the evolutionary origins of a group or species. The figworts were a family of angiosperms that had almost 5000 species based on morphology alone. Genetic evidence allowed for: Merging of two small families 50 genera moved to plantain family 12 genera moved to boomrape family 13 genera moved to new family This is one of the most drastic cases of reclassification that have been undertaken, reducing this family from the 8th largest family to the 39th Weird Fact: Archaea did not exist as a separate domain until 1977, due to a lack of technology.

“Class”-ify your Classmates Using one of the methods we have described for how scientists classify organisms, you must classify your fellow classmates. Three methods you may use: Visual Dichotomous Key Simplified Dichotomous Key Cladogram You may not use any offensive or derogatory characteristics, but be creative with how you classify them! Make sure the key is easily readable and professional looking.