Phylogenies

TEXTBOOK NOTES 6 Pages: 113-116, 122-126, 130-133, 180-182, 217-220 Phylogenies and the History of Life Phylogenetic Trees  Phylogeny: the evolutionary history of a group of organisms  Phylogenies are usually depicted in a form of a phylogenetic tree which shows ancestor- descendant relationships among populations or species  In a phylogenetic tree: - Branch: represents a population through time - Node: the point where two branches diverge and represents the point in time when an ancestral species split into 2 or more descendant species - Tip: (or terminal node) is the endpoint of a branch and represents a group that is living today or is extinct How do Researchers Estimate Phylogenies  Phylogenetic trees are an extremely effective way of summarizing data on the evolutionary history of a group of organisms  Two general strategies used to estimate trees: - Phenetic Approach: computing a statistic that summarizes the overall similarity among populations based on the data - Ex. Researchers use gene sequences to compute an overall genetic distance between two populations into computers which then organizes the most similar genes onto one branch and the more divergent ones onto another branch - The Cladistic Approach: uses synapomorphies in order to estimate trees - ^ Synapomorphies are traits that exists in one organism and not in any other - ^ allows biologists to recognize monophyletic groups (clades or lineages) - In a phylogenetic tree, at the different nodes, two species split into different lineages  Ancestral trait: trait that existed in an ancestor  Derived Trait: modified form of the ancestral trait (found in the descendent) - When comparing animals to their ancestors: say they have derived traits - When comparing two different species (humans and whales): say they have ancestral traits How can Biologists Distinguish Homology from Homoplasy?  Traits can be similar in two species not because of common ancestor but because of the traits evolved independently in two distantly related groups What is Homoplasy?  Homology: occurs when traits are similar due to shared ancestry  Homoplasy: occurs when traits are similar for reasons other than common ancestry  Convergent Evolution: process where organisms that are not closely related (not monophyletic) evolve similar traits as a result of having to adapt to similar environments - EXAMPLE: flight in different species: insects and birds - Convergent is opposite of divergent evolution where related species evolve DIFFERENT traits Evidence for homology  Many animals on lineages that branches off between insects and mammals have similar genes  OBSERVATION: similar genes due to common ancestor means similar traits from ancestor Phylogenetic Trees  Polytomy: when a node consists of more than two descendants (more than two branches)  Taxonomy: is a method of naming and classifying organisms - Species are categorized depending on their features, genes, and behaviour  Taxon: is any group in which related organisms are classified - For example: a phylum, order, family, genus, or species  Classification: a method for arranging organisms into categories - Based on their structural and evolutionary properties - EXAMPLE: Binomial nomenclature  Specific Epithet: is the second part of a specific name - EXAMPLE: Homo sapiens: Homo is a generic name whereas sapiens is the specific epithet  Clade: a group of ALL its descendants along with its common ancestor The boxes on the left and right are clades but the middle one is a paraphyletic group. The second box represents a polyphyletic group (one species). The last box represents a paraphyletic group.  Monophyletic Group - ^ SAME AS CLADE  Paraphyletic Group – a group consisting of a common ancestor and SOME of its descendants  Parsimony – it is a principle that the simplest explanation that can explain the data is to be preferred - A hypothesis of relationships that requires the smallest number of character changes is most likely to be correct - Hypothesis 1 requires six evolutionary changes and Hypothesis 2 requires seven evolutionary changes, with a bony skeleton evolving independently, twice. Although both fit the available data, the parsimony principle says that Hypothesis 1 is better since it does not hypothesize unnecessarily complicated changes.  Hierarchy – is an arrangement of items in which the items are represented as being above, below or at the same level as one another  Fork: represent the divergence of two species - The closer a species is to a fork in the tree, the more closely it is related to the species from which it had diverged  Phenogram: a tree that is used in phenetic classification to show the similarities among taxa - Phenetic classification depends on the similarities between organisms - The more features (morphological or ecological) they share, they’re more likely in the same group - NOT related to similarities of common ancestor -