Phylogenetic trees -Tools in evolution biology

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1 Phylogenetic trees -Tools in evolution biology
Fylogenetiske træer er værktøjer, som man bruger i evolutionsbiologi

2 Intended Learning Outcomes
Participants will comprehend the biological basis for phylogenetic trees Participants will be able to apply the principle of parsimony to analyse simple phylogenetic trees Participants will be able to modify simple phylogenetic trees to account for new data Læringsmål for øvelsen: forstå biologiske basis; kunne bruge parsimoni-princippet; kunne modificere simple fylogenetiske træer for at inkludere nye data

3 What is a phylogenetic tree?
phylo- means race or tribe genesis means origin or coming into being  A phylogenetic tree is a diagram which illustrates a hypothesis about the origin of certain species A phylogenetic tree is a hypothesis!

4 An early phylogenetic tree
The point about phylogenetic trees is that for each two species, if you go far enough back in their evolutionary history, you will find a common ancestor. Mention time axis. Charles Darwin (1859)

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6 Theropod Icarosaurus Archosaur Bird Crocodile
(Ancestor) Bird Stamformen til de jordiske dyr vi skal kigge på er denne her archosaur, som var et primitivt krybdyr der uddøde for 300 mill år siden. Efterkommerne af denne stamform er fugle, theropod-dinosaurer, en linje af flyvende eller svævende firbens-agtige dyr, og krokodiller og deres slægtninge. Jeres opgave er nu at danne en hypotese om, hvordan disse dyr er beslægtet med hinanden (næste slide). There is some debate about when archosaurs first appeared, but some time between 260 mya and 249 mya is likely. Archosaurs were the dominant land creatures during the mesozoic period ( mya). Quadruped, oviparous, teeth. Crocodile

7 Theropod Icarosaurus Archosaur Bird Crocodile
(Ancestor) Bird Stamformen til de jordiske dyr vi skal kigge på er denne her archosaur, som var et primitivt krybdyr der uddøde for 300 mill år siden. Efterkommerne af denne stamform er fugle, theropod-dinosaurer, en linje af flyvende eller svævende firbens-agtige dyr, og krokodiller og deres slægtninge. Jeres opgave er nu at danne en hypotese om, hvordan disse dyr er beslægtet med hinanden (næste slide). Crocodile

8 Species with most derived traits
Species with most ancestral traits Evolutionary time Exercise 1: Arrange the four animal species according to the tree on your handout (this is your hypothesis!) Handout 1: one hypothesis (tree); four animals for each. Give three minutes. Take hypotheses on transparency/overhead (construct from dictation from group). The points to make here are: Does anyone have comments for this? Do you agree? What criteria did you use? (resolve sequence using morphology) Convention is that the species with the most ancestral traits is placed on the first branch, and the species with the most derived traits is placed on the farthest branch. ERGO: there is no way for participants to resolve the tree further on the basis of the information they have. Ancestral archosaur

9 Traits of bird relatives
Archosaur Crocodile Icaro-saurus Theropod Bird Lays eggs x Has teeth X Walks on four legs Is able to fly or glide Has feathers

10 Which hypothesis best explains the data?

11 The principle of parsimony
(Occam’s Razor) Under maximum parsimony, the preferred phylogenetic tree (or hypothesis) is the tree that requires the smallest number of evolutionary changes. So let’s examine the number of evolutionary changes in each hypothesis!

12 No teeth Feathers Bipedal stance Bipedal stance Ability to fly or glide

13 No teeth Feathers Ability to fly or glide Ability to fly or glide Bipedal stance

14 The hypotheses explain the data equally well!

15 Scientific controversy…
The scientific controversy regarding the origin of birds. At one point in this controversy, the two main positions were that birds were the descendants of ground-living theropod dinosaurs; and that birds were descendants of arboreal, non-dinosaur archosaurs such as Longisquama or Icarosaurus which had wing-like structures in order to glide from tree to tree.

16 ”From the trees down” ”From the ground up”
Feathers evolved among species that already had a tree-living habit and the ability to glide ”From the ground up” Feathers evolved as balance aids or insect traps among bipedal running species

17 In 1863, Richard Owen found this fossil!
It had teeth and was bipedal It was classified as a theropod, but... It had feathers! So, now revise your phylogenetic tree to take this ancestor into account! Archaeopteryx

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20 Exercise 2: Revise your hypothesis to reflect the new data.

21 Traits of bird relatives
Archosaur Crocodile Icaro-saurus Theropod Bird Lays eggs x Has teeth X Walks on four legs Is able to fly or glide Has feathers

22 No teeth Ability to fly or glide Bipedal stance Ability to fly or glide Feathers

23 No teeth Feathers Feathers Bipedal stance Bipedal stance Ability to fly or glide

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25 Characteristics of phylogenetic trees
Phylogenetic trees may be constructed on the basis of physical traits The principle of parsimony: the simplest possible explanation is preferred Phylogenetic trees must often be modified or revised to account for new data

26 Physical characteristics

27 The Tree of Life