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L20 & 21 - Phylogeny

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Western University
Biology 1001A
Tom Haffie

L20 - Phylogeny Speciation w/out allopatry Adaptations to a new food type can cause sympatric speciation (ranges overlap) 1. Hawthorn maggot introduced to North America 2. Subset of maggots began feeding only on apples Competition, disruptive selection & assortative mating  sympatric speciation * stickleback species closely related but do not interbreed All life is connected 1. Reading Phylogenies - two ways to represent relatedness - two types of phylogenies give same types of information - different branching points represent how closely related particular species are (how long ago the branching point occurred) - time moves bottom to top, left to right (most commonly) - gives information about relative order of branching events - no scale b/c we don’t actually have any indication of time - splits shown at same time did not necessarily occur at the same time - some phylogenies give different information ( distance from branching point represents number of changes that have occurred) - rotating nodes changes nothing - be skeptical of people talking about species which are more highly evolved (this is meaningless) Phylogenies are like pedigrees - like family trees 2. Building phylogenies - how do we know which groups are more closely related - this is identified by similarity - the more traits a group has in common the more likely it is that they share a recent common ancestor - homology: similarity that reflects recent common ancestry -homoplasy – misleading similarity, OR misleading dissimilarity (ex. Convergent evolution, similar due to similar selection pressures) - similar trait that does not reflect evolutionary relationship Homologous traits lead us towards the correct phylogeny Homoplasious traits mislead - w/o knowing the phylogeny in advance how do you know if its convergence or homology? * sometimes can be inferred from structure/development - solution? Cladistic analysis * only some similarities are worth attention * synapomorphy – shared traits that are derived from a common ancestor Synamorphic traits are informative ones - shared by two or more groups - derived (form common ancestor) Symplesiomorphy - shared by two or more groups - ancestral (trait was already present before the groups diverged) Autapomorphy - unique to a single group, derived within the group L21 - Phylogeny II Derived vs Ancestral Traits - outgroup analysis (compare butterflies to a fly, closely related but not a butterfly) - evolution does not always proceed towards greater complexity * Butterfly example: * (4 legs does not always evolve towards 6 legs) * Derived state is a lower number of legs (64) Building Phylogenies - “related to” ≠ “descended from” - Ex. You are related to your cousins but NOT descended from your cousins - more complex ≠ “more evolved” - relatedness ≠ similarity * not all similarities are homologies * not all homologies are synapomorphies * cladistics ONLY pays attention to synapomorphies * symplesiomorphy example: shared genetic code (NOT derived) - ingroup – more closely related to each other
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