Cladistics and Phylogeny PDF

Constructing a Cladogram Introducing Cladistics and Phylogeny How Evolution Works and How we Figured it Out  Taxonomy ⚫ When a new species is discovered, taxonomists: Accurately describe the characteristic features of the species Determine its relationship to other kn...

Constructing a Cladogram Introducing Cladistics and Phylogeny How Evolution Works and How we Figured it Out  Taxonomy ⚫ When a new species is discovered, taxonomists: Accurately describe the characteristic features of the species Determine its relationship to other known species that share similar traits Give the new species a unique binomial name While it is possible to classify organisms in many different ways depending on our objectives, the most widely accepted approach to modern classification is to develop classification systems that consider evolutionary relationships – that is, they reflect the phylogeny or root of the organisms.  What is a Phylogeny? ⚫ studying inherited species’ characteristics and other historical evidence, we can reconstruct evolutionary relationships and represent them on a “family tree,” called a phylogeny ⚫ Phylogeny –reconstruct evolutionary relationships in “family tree” Based on careful evaluation of a wide range of evidence: fossil record morphology (physical characteristics) genetics (DNA) ⚫ This family tree is called a Cladogram  Cladistics & Phylogeny Cladograms - Illustrate/hypothesis of evolutionary relationships (phylogeny) of different groups of species of organisms - scientists can infer which groups are more closely related & the general sequence of events that gave rise to each group. 🡪 focuses on shared, derived (new) traits 🡪 uses branching approach to show relationships among species 🡪 assumes that all species are descended from a common ancestor Different ways of drawing cladograms We will learn how to draw this type of cladogram - Letters A-H at “branching points” represent the shared derived traits of groups that arise beyond that point. Which organism is most closely related to the butterfly? What letter represents the most recent shared derived trait of the housefly, dragonfly, and butterfly?  Terminology Cladistics: a method of determining evolutionary relationships based on the presence (+) or absence (-) of recently evolved traits (derived traits). Derived Traits: trait that has evolved relatively recently with respect to the species or groups being discussed Synapomorphy: a derived trait shared by two or more species or groups (ex. All birds have feathers & are more closely related to each other than reptiles, which do not have feathers). ⚫ Understanding a phylogeny is a lot like reading a family tree ⚫ Root = ancestor ⚫ Tips of branches = descendents ⚫ Move root to tip = moving forward in time ⚫ When a lineage splits = speciation 🡪represented as branching on a phylogeny. ⚫ gives rise to two or more daughter lineages ⚫ Phylogenies trace patterns of shared ancestry between lineages ⚫ each lineage has ancestors that are unique to that lineage and ancestors that are shared with other lineages—common ancestors. Using the Tree for Classification ⚫ Testing hypotheses about evolution ⚫ Learning about the characteristics of extinct species and ancestral lineages ⚫ Classifying organisms  Practice! ⚫ Which organisms branched off before evolving a complete digestive tube? ⚫ Which three organisms have a brain?  How to Construct a Cladogram? Step 1: Analyze and summarize the characteristics of the organisms being compared Step 2: Select an outgroup= the organism that is used as the starting point when constructing a cladogram. It is chosen because it is believed to be less closely related to the other organisms being studied (ancestral). Note: Organisms in the study group have the trait of the outgroup and the outgroup does not have the traits the study group have Step 3: Construct the cladogram by organizing study groups into clades showing derived traits in sequence Step 4: Label the derived traits, showing the point of speciation Example Study Group: tiger giraffe lancelet salamander gray squirrel brook trout How to Construct a Cladogram? ⚫ Step 1: draw a chart organizing the presence or absence of traits Brook trout Salamander Giraffe Gray squirrel Vertebrate Vertebrate Vertebrate Vertebrate Jaws Jaws Jaws Jaws Four Legs Four Legs Four Legs Lungs Lungs Lungs Amniotic Egg Amniotic Egg Endothermic Endothermic  ⚫ Step 1: Chart showing presence (+) or absence (-) of trait Organism Vertebrate Jaws Four Legs Lungs Amniotic Egg Endothermic Lancelet - - - - - - Brook trout + + - - - - Salamander + + + + - - Giraffe + + + + + + Gray + + + + + + squirrel How to Construct a Cladogram? Step 2: Outgroup (does not share ANY characteristic/trait) = lancelet Salamander Squirrel Giraffe Step 3: Trout amniotic egg, endothermic (warm-blooded) 4 legs, lungs Lancelet vertebrae, jaw How do we trace ancestry back to ancient humans? Cladogram of Phylogeny of Primates  Cladistics Classification Hypothetical cladogram: TIME 5 4 3 2 1 F E D C Sister group (most closely related B group in the cladogram) Out group (ancestral) A #1 – 5 are different species populations (taxa) that come from an ancestral taxa 🡪 when a taxa acquired a derived trait, the original species cease to exist A – F are favourable characters or traits acquired from e.g. random mutations 🡪 character/trait A is: shared, ancestral characteristic because it is shared by/found in ALL of the resulting 5 taxa 🡪 character/trait B is: derived, ancestral characteristic because it is shared by/found only in taxa 4 and 5 Now, practice time!!! Try the practice worksheets