Chapter 25 Phylogeny Systematics Phylogeny p p the

Chapter 25 Phylogeny & Systematics

Phylogeny p p the evolutionary history of a species or a group of related species the construction of phylogenies is based on: n n the fossil record (the sequence in which fossils appear in the layers of sedimentary rock) morphological & molecular homologies Systematics p an analytical approach to understanding the diversity & relationships of organisms n n traditional systematics – uses morphological homologies to infer evolutionary relationships molecular systematics – uses molecular homologies (similarities in DNA, RNA, proteins, & other molecules) to infer evolutionary relationships

What are homologies? p p p similarities between two species due to shared ancestry result of divergent evolution can be: n shared derived characters – features that are unique to a particular taxon (classification group) n shared primitive characters – features that a group of organisms may share with other organisms outside their taxon (ie: with organisms that they are not as closely related to)

Analogies p p p similarities between two unrelated species that evolved independently of each other as 2 lineages adapted to similar lifestyles result of convergent evolution n (ex) 4 -chambered heart of birds & mammals NOTE: the more points of resemblance 2 characteristics (ex: skulls, DNA sequences) have the less likely they are analogies ( the more likely they are homologies)

Taxonomy p the science of classification n p ordered division of organisms into categories based on a set of characteristics used to assess similarities (shared derived characters) & differences features of taxonomy useful in phylogenetic systematics: n binomial nomenclature p n Genus species names used to describe species hierarchical classification p includes 8 taxa: DKPOCFGS

Phylogenetic Trees p p p diagrams that depict hypotheses about evolutionary relationships the branches of phylogenetic trees reflect the hierarchical classifications of groups nested within more inclusive groups the sequence of the branching is NOT related to the age of the species

Cladograms p p diagrams that depict patterns of shared characteristics among taxa do not imply evolutionary history but if the shared characteristics are due to common ancestry, can form the basis of a phylogenetic tree

Clades p p groupings within a cladogram clades can be: n monophyletic – made up of an ancestral species and all of its descendents n paraphyletic – made up of an ancestral species and only some of its descendents n polyphyletic – a grouping that leaves out the common ancestor of the species included

Cladistics p the analysis of how species may be grouped into clades n n monophyletic clades (valid clades) are defined by characteristics that are unique to the group (shared derived characteristics) these characteristics are identified by comparing ingroup species with an outgroup species that does not have the shared derived characteristic

Phylogenetic Trees & Rate of Evolution p p by varying the lengths of the branches in a phylogenetic tree, we can show the rate at which a homologous gene (a shared gene due to common ancestry) has evolved in different lineages this type of diagram is called a phylogram

Phylogenetic Trees & Geologic Time p p p another way phylogenetic trees can be altered is to place evolutionary branch points in the context of geologic time this type of diagram is called an ultrametric tree the ultrametric tree to the right shows us that invertebrates & chordates diverged during the Neoproterozoic Era and the two groups have had an equal amount of type to evolve since then (even if the rates of evolution within the lineages are different)

Which phylogenetic tree is most accurate? p p as we increase the number of species included in a phylogenetic tree, the number of ways to arrange them also increases leading to many different phylogenetic hypotheses the 1 st step in evaluating phylogenetic hypotheses is to follow the principle of maximum parsimony: n n p the simplest explanation that is consistent with the facts should be investigated first in other words, investigate the phylogenetic tree that requires the fewest evolutionary changes first the 2 nd step is to apply the principle of maximum likelihood n investigate the phylogenetic tree that reflects the most likely sequence of evolutionary events (based on certain rules about how DNA changes over time)

Types of Homologous Genes p orthologous genes n n n p genes that are passed in a straight line from one generation to the next but have ended up in different gene pools because of speciation often shared by distantly related species (ex) 99% of the genes of humans & mice and 50% of the genes of humans & yeast are orthologous paralogous genes n n n genes found in multiple copies within the same genome results from gene duplication may lead to gene families that have related functions p n p (ex) the -globin & -globin gene families (which both produce subunits of hemoglobin) diverged from an ancestral globin gene that likely duplicated and, then, mutated often shared by closely related organisms studying both types can give us clues to an organism’s evolutionary history

Molecular Clock method used to measure the absolute time (ie: the numerical age in mya) of evolutionary change based on the observation that some genes & other regions of the genome appear to evolve at constant rates p based on the assumption that the number of nucleotides substituted in genes is proportional to the amount of time since… p n n a species branched from its common ancestor (orthologous) the genes duplicated (paralogous)

Preview: Universal Tree of Life p consists of 3 domains: n n n p Bacteria Archaea Eukarya early history unclear