Systematics PDF

Classifications and Systematics Chapter 18 M. Beals  Introduction Taxono my Specie s Classificati Systemat on ics  Background  The scientifi...

Classifications and Systematics Chapter 18 M. Beals  Introduction Taxono my Specie s Classificati Systemat on ics  Background  The scientific study of biological diversity and evolutionary history as we know it is known as systematics. It is a comparative science  Prior to the use of binomial names plant species had excessively long names.  All living organisms were given a single, two-word Latin scientific name  Only one correct scientific name applies to all individuals of a species, NO MATTER WHAT!  Many common names are given to the same species, but only one common name can be applied to a # of different species  Concepts (1 of 2)  Phylogeny is the hereditary relationships of any group of organisms. − The evolutionary history of the group  At the end of the 19th century, taxonomists adopted these goals. − Developing a natural system of classification, in which closely related organisms are grouped − Assigning plant names on the basis of phylogenetic relationships  Concepts (2 of 2)  Goals of modern plant systematics − To understand each of these evolutionary lines − To have a system of names (nomenclature) that reflects their relationships accurately  Phylogeny is a reality; our classification systems are hypotheses.  Because our knowledge is incomplete and imperfect, the current classification systems are only approximations.  Development of Concepts of Evolution and Classification  Our modern classifications have their origins in ancient Greece. The first attempt to organize and classify plants was by a Greek philosopher and biologist, Theophrastus.  The most important book on plant classification from the ancient world was written by Dioscorides, a Roman military surgeon. Materia Medica.  Our system of nomenclature, of scientific names, can be traced directly to Carolus Linnaeus.  Every species had both a genus name and a species epithet, the basis of our present binomial system of nomenclature.  Taxonomy  method by which scientists, conservationists, and naturalists classify and organize the vast diversity of living things on this planet in an effort to understand the evolutionary relationships between them.  study of plant classification  the science of documenting biodiversity  The term was originally coined by Augustin Pyramus de Candolle in 1813 (Stuessy; 1990, 1994)  Levels Of Taxonomic Categories (1 of 3)  Species are a set of individuals closely related by descent from a common ancestor. − The most fundamental level of classification. − Members of a species can interbreed successfully, but cannot interbreed with individuals of any other species. − Some species may be capable of interbreeding with closely related species and producing a viable hybrid.  Closely related species are grouped together into genera (singular, genus).  Levels Of Taxonomic Categories (2 of 3)  A group of species is monophyletic if: −All of the species included in the genus are related to each other by a common ancestor. −All descendants of that common ancestor are in the same genus.  The species are polyphyletic if: − Members have evolved from different ancestors.  Polyphyletic species may resemble each other only as a result of convergent evolution.  Levels Of Taxonomic Categories (3 of 3)  The level above genus is family, with each family composed of one, several, or often many genera. − The levels above family are order, class, division, and kingdom. − Except for kingdom, genus, and species, the names must have a certain ending to indicate the classification level. Taxon Ending Example Division -phyta Chlorophyta Class -opsida Magnoliopsid a Order -ales Cornales Family -aceae Cornaceae  Cladistics (1 of 5)  Distantly related plants have been on separate lines of evolution for millions of years. − Many mutations can accumulate so that the plants resemble each other only slightly.  Phylogenetic studies are complicated by the fact that plants can resemble each other for two distinct reasons.  Cladistics (2 of 5) 1. They have descended from a common ancestor. − Similar features are synapomorphies (homologous features). 2. They have undergone convergent evolution. − Features like this are homoplasies (analogous (c) AnnSteer/iStock features).  Cladistics (3 of 5)  Determining whether a similarity is due to homology (common ancestry) or analogy (convergent evolution) can be difficult.  Cladistics (4 of 5)  Studying lack of similarity can be difficult too. −In some cases a small genetic change results in dramatic phenotypic changes.  Cladistics (5 of 5)  Systematists attempt to look at as many features as possible. − They assume that misleading evidence will be outweighed by valid characters.  Genealogy Versus Clades  A genealogy maps the genetic convergence of characters from many ancestors into one descendant over a few generations.  A phylogeny maps the evolutionary divergence of characters and progeny over thousands or millions of generations.  Cladograms (1 of 3)  A cladogram is a diagram that shows evolutionary patterns by means of a series of branches. − Each point (node) at which a cladogram branches represents the divergence of one taxon into two. − All of the branches that extend from any particular point represent the descendants of the original group (their common ancestor).  Cladograms (2 of 3)  A paraphyletic group does not contain all the descendants of the most recent common ancestor. − A derived condition is an apomorphy compared to the ancestral condition. − In constructing a cladogram, we search for shared derived characters, synapomorphies.  To infer a phylogenetic tree from information we have, we use a principle called parsimony. − We prefer the simplest possible explanation and we don’t make a hypothesis any more complicated than it needs to be. (Occam’s razor)  Cladograms (3 of 3)  All of the species depicted, plus their common ancestor at node 1, form a clade.  Groups A through D plus E do not form a clade but a paraphyletic group. − Doesn’t include all the descendants of the most recent common ancestor.  Cladograms and Taxonomic Categories  The only taxonomic unit with an objective definition is a species: individuals that can interbreed.  Many systematists have decided to continue using the old names for genera, families, and orders unless they are shown to be definitely polyphyletic or paraphyletic.  Cacti and Evolutionary Diversification  The cactus family (Cactaceae) is a large group that has undergone extensive evolutionary diversification.  Cacti and Evolutionary Diversification  One branch leads to Blossfeldia, a monotypic genus (there is only one species, B. liliputana) of plants that have many derived characters.  Until recently, B. liliputana was placed up by node10.  DNA sequencing indicates that it is a sister to all of the cacti from node 5.  Other Types Of Classification Systems Artificial Systems of Classification  Several key characters, often very easy to observe, are chosen as the basis of artificial classification (such as roadside floras and common field guides).  These systems are artificial because the plants often do not share a close common ancestor.  Taxonomic Studies (2 of 2)  The International Code of Botanical Nomenclature describes precisely the steps necessary for naming a new species. − The species must be uniquely named. − Its description must include the designation of a type specimen—a single preserved plant that truly carries the name.  Major Lines of Evolution  All organisms are grouped into three domains: Bacteria (with cyanobacteria), Archaea, and Eukarya.  Major Lines of Evolution (1 of 3)  The most significant event in evolution was the origin of life itself, probably about 3.5 billion years ago.  The next major evolutionary event was the conversion of a prokaryote into a eukaryote.  After eukaryotes evolved to have mitochondria, endoplasmic reticula, and true nuclei, numerous evolutionary lines emerged.  Major Lines of Evolution (2 of 3)  One clade contained organisms that would later diverge into animals and fungi, and the other gave rise to algae. − The early algae continued to diversify, and approximately 400 million years ago, some became adapted to living on land. − They established the clade of true plants, Kingdom Plantae.  Major Lines of Evolution (3 of 3)  The major evolutionary lines diverged from early algae. − Simple plants with neither seeds nor vascular tissues (mosses, liverworts, and hornworts) − Plants that do not produce seeds but do have xylem and phloem (ferns and similar plants) − Seed-bearing vascular plants (gymnosperms and angiosperms)