 Read Chapter 4.  All living organisms are related to each other having descended from common ancestors.  Understanding the evolutionary relationships.

 Read Chapter 4

 All living organisms are related to each other having descended from common ancestors.  Understanding the evolutionary relationships between groups enables us to reconstruct the tree of life and gain insight into history of evolutionary change.

 Phylogeny is the study of the branching relationships between populations over evolutionary time.  A phylogenetic tree is built up by analyzing the distribution of traits across populations.

 A trait (or character) is any observable characteristic of an organism.  Could be anatomical features, behaviors, gene sequences, etc.

 Traits are used to infer patterns of ancestry and descent among populations. These patterns are then depicted in phylogenetic trees.  By mapping other traits onto trees it is possible to study the sequence and timing (history) of evolutionary events.

 It’s important to bear in mind that phylogenetic trees are hypotheses about the evolutionary relationships between groups.  When additional evidence is acquired it can be used to test a tree.

 Each branch tips represents a taxon (a group of related organisms).  Interior nodes (where branches meet) represent ancestral populations that are the common ancestors of the taxa at the ends of the branches.

 Phylogenetic trees are generally drawn in either a Tree format or a Ladder format.  They convey the same information about the relatedness of taxa

 It is important to remember that a particular set of evolutionary relationships can be depicted in multiple different ways in a phylogenetic tree.  Any node in a phylogenetic tree can be rotated without altering the relationships between taxa.

 The purpose of building phylogenetic trees is to use them to figure out the evolutionary relationships between taxa and to identify “natural” groupings among taxa, those that reflect their true evolutionary relationships.  A key idea is that natural groupings called clades are monophyletic groups.

 Clade : a group of taxa that share a common ancestor.  Monophyletic group: consists of an ancestor and all of the taxa that are descendants of that ancestor.

 In the next slides elephants, manatees and hyraxes plus their common ancestor form a monophyletic group.  Similarly tapirs, rhinoceroses and horses plus their common ancestor form another monophyletic group.

 A taxon is polyphyletic if it does not contain the most recent common ancestor of all members of the group.

 A polyphyletic group requires the group members to have each had an independent evolutionary origin of some diagnostic feature.  E.g. Referring to Elephants, rhinoceroses and hippopotamuses as “pachyderms.” Pachyderms are a polyphyletic group because each group evolved thick skin separately.

 A taxon is paraphyletic if it includes the most recent common ancestor of a group and some but not all of its descendents.

 An example of a paraphyletic group among vertebrates would be “fish.”  All of the tetrapods (four-legged animals) are descended from lobe- finned fish ancestors, but are not considered “fish” hence “fish” is a paraphyletic group because the tetrapods are excluded.

 Trees we’ve seen so far have been rooted and these trees give a clear indication of the direction of time.  However, computer programs that produce phylogenetic trees often produce unrooted trees.

 In an unrooted tree branch tips are more recent than interior nodes, but you cannot tell which of multiple interior nodes is more recent than others.

 An unrooted tree can be rooted at any point and depending where it is rooted very different rooted trees will be produced.

 Obviously, there is only one true tree of evolutionary relationships and we would like to identify that tree.  To do that we need to root the tree correctly. One of the easiest ways to root a tree is to use an outgroup to root it.

 An outgroup is a close relative of the members of the ingroup (the various species being studied) that provides a basis for comparison with the others.

 The outgroup lets us know if a character state within the ingroup is ancestral or not.  If the outgroup and some of the ingroup possess a character state then that character state is considered ancestral.

 Consider an unrooted tree of four magpie species.

 To root the tree we need a group that split off earlier from the lineage that led to these four species of magpies.  Azure-winged magpie is a suitable unrooted outgroup. One this added to the unrooted tree we can root the tree.

 In some phylogenetic trees branches are drawn with different lengths.  In these trees the branch lengths represent the amount of evolutionary change that has occurred in that lineage.

 Phylogenetic trees can be used to generate hypotheses about the evolution of traits.  This is done by mapping the trait states on the tree and trying to reconstruct the simplest (most parsimonious) explanation that accounts for the observed distribution of traits.

 Light sensitive pigments called opsins are responsible for color vision. Humans have three different cone opsins.  Other vertebrates have as many as four or as few as two opsins.

 By mapping the presence or absence of different opsins onto a phylogenetic tree of vertebrates we can attempt to reconstruct the evolutionary history of color vision in these vertebrates.

 It is clear that the ancestral trait is to possess four opsins (as both birds and reptiles do).  The mammal lineage appears to have lost two opsins (probably because the animals were nocturnal) and one opsin was later reevolved on the lineage leading to old world primates and humans.

 Homologous traits are those derived from a common ancestor.  E.g. all mammals possess hair. This is a homologous trait all mammals share because they inherited it from a common ancestor.  Analagous traits are shared by different species not because they were inherited from a common ancestor but because they evolved independently.

 Divergent evolution occurs when closely related populations diverge from each other because selection operates differently on them.  Such new species will possess many homologous traits in common.

 Analagous traits are the result of a process of convergent evolution whereby the same or similar solution to an evolutionary problem is converged upon by different organisms independently of each other.

 We want to avoid including analagous traits when constructing phylogenetic trees because they can mislead us.  An analagous trait in trees is referred to as a homoplasy.

 When building a phylogenetic tree we want to use characters inherited from ancestors. Such a character found in two or more taxa is referred to as a shared derived character or synapomorphy.

 Read Chapter 4.  All living organisms are related to each other having descended from common ancestors.  Understanding the evolutionary relationships.

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 Read Chapter 4.  All living organisms are related to each other having descended from common ancestors.  Understanding the evolutionary relationships.

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Presentation on theme: " Read Chapter 4.  All living organisms are related to each other having descended from common ancestors.  Understanding the evolutionary relationships."— Presentation transcript:

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