Phylogenetics Chapter 16 18 So how do we go from here to here Student learning objectives Interpret relationships among taxa as visualized on a phylogenetic tree start at individual level and work up to hierarchical clusters Identify characters as homoplasious or homologous Identify clades as monophyletic If given a character matrix draw the corresponding phylogenetic tree Use correct terminology to discuss different parts of the trees or tree inference Over that time those mechanisms have led to divergence all organisms are related through descent Over that time those mechanisms have led to divergence all organisms are related through descent Over that time those mechanisms have led to divergence all organisms are related through descent Consider a beetle named Mimi This species reproduces sexually So a parent passes on copies of half of his or her genes to each offspring This species lives 1 year Here are the last 3 generations There is occasional migration that brings in genes from other populations gene flow Each species descends through time as a bundle of genetic connections isolated from other species lineage Often barriers develop that prevent gene flow between populations of the same species Divergence genetic drift or natural selection leads to populations being different enough they no longer exchange genes Divergence genetic drift or natural selection leads to populations being different enough they no longer exchange genes Two different species Each is its own isolated bundle of ancestor descendant connections About 1 5 mya speciation event occurred Another way to look at the process Mutation Natural selection Genetic drift Migration Non random mating Another way to look at the process e m ti y r a n o ti u o v E l Mutation Natural selection Genetic drift Gene flow Non random mating A species is composed of a group of interbreeding populations Interbreeding populations River Semi isolated population When are these populations different enough that we should call them different species How might we decide What data could we gather If we go further back in the Mimi s history we see a sequence of beetle lineages splitting Sister taxa Species or Taxon Tip Node Branch If we go further back in the Mimi s history we see a sequence of beetle lineages splitting Sister taxa Species or Taxon Tip Node Branch Spots Biological Species Concept A species is group of populations reproductively isolated from others that occupies a specific niche in nature The ability to successfully interbreed is central to the concept Figure The Western meadowlark left and the Eastern meadowlark right appear to be identical and their ranges overlap but their distinct songs prevent interbreeding Two morphological species Lion Tiger Liger What about organisms that don t have sex Evolutionary Species Concept An evolutionary species is a single lineage of ancestor descendant populations that maintains its identity from other such lineages and that has its own evolutionary trajectory This definition accommodates both sexual and asexual forms Phylogenies Evolutionary relationships among organisms Similarities and differences between species are products of their evolutionary history Phylogenetic trees are a graphical representation of genealogical relationships between taxa Tree of Life all organisms related through descent Closely related species share more characteristics because of their more recent common ancestors Remember Mimi We started with a simple family tree Divergence genetic drift or natural selection leads to populations being different enough they no longer exchange genes Another way to look at the process Mutation Natural selection Genetic drift Migration Non random mating If we go further back in the Mimi s history we see a sequence of beetle lineages splitting Sister taxa Sister taxa Sister taxa Sister taxa Spots The sister species of F is A B C F H I G D E A Extant B Extinct C not derived from the same common ancestor as species F D not yet evolved E None of the above Based on this phylogeny crocodiles are A More closely related to lizards than to birds B Are equally closely related to lizards and dinosaurs C Are more closely related to birds than to lizards D None of the above Changing the angle at which you draw the lines does not change relationships All these figures show the same relationships Rotation of internal nodes does not change the relationships All these figures show the same relationships These two trees depict the same relationship A True B False These two trees depict the same relationship A True B False Rotation of internal nodes does not change the relationship Groups of close relatives form clades or monophyletic groups We use these relationships to form a classification scheme A phylogeny is built from the study of characters that vary among species Character similarity that results from common ancestry is called homology The four limbs of tetrapods homologous characteristic Reconstructing phylogeny requires determining ancestral and derived character states The character state that was present in the common ancestor is ancestral Characters that arose later are derived Spots All the taxa pictured are extant None is an ancestor of another The oldest split in the tree is when the ancestors of lungfish split from the ancestors of the tetrapods Different lineages may evolve similar features independently these characters are homoplasious Characters that are similar but do not share common descent are nonhomologous homoplasy convergent evolution Homology vs Homoplasy X a characteristic e g fur wing X X X X Homology similar characteristic derived from common ancestor Homoplasy similar characteristic not caused by common ancestor convergent evolution Parsimony the simplest explanation is our initial hypothesis How do we estimate the timing of species divergence Fossils Where are they stratigraphically Radiometric dating of isotopes Problems with fossils Why most organisms are not fossilized Only the hard parts of organisms are fossilized Habitat bias only organisms near water and sediments can be fossilized Recent fossils are more common than more ancient ones Highly non random Fossilization is unlikely so it is mostly the common species that are fossilized How do we estimate the timing of species divergence Molecular data Basic assumption of molecular clocks changes in sequence accumulate at equal rates over time generations http www blackwellpublishing com ridley a z