Ch.19- Classification and Systematics

Introduction

  • phylogeny:

    • hereditary relationships of any group of organisms

    • assigning plant names on the basis of phylogenetic relationships

  • goals of modern plant systematics

    • to understand evolutionary lines

    • to have a system of names that reflects their relationships accurately

Levels of Taxonomic Categories

  • species: set of individuals closely related by descent from a common ancestor

    • most fundamental level of classification

    • members of a species can interbreed, but cannot interbreed with individuals of any other species

    • some may be able to interbreed with other species and produce a viable hybrid

  • closely related species are grouped together into genera (singular, genus)

  • 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

  • species is polyphyletic if:

    • members have evolved from different ancestors

    • may resemble each other only as a result of convergent evolution

  • level above genus is family

    • each family is composed of one, several, or often many genera

    • levels above family are order, class, division, and kingdom

    • except from kingdom, genus, and species, the names must have a certain ending to indicate the classification level

Cladistics

  • distant related plants have been on separate lines of evolution for millions of years

    • many mutations can accumulate so that plants resemble each other slightly

  • cladistics: method of analyzing phylogenetic evolutionary relationships

  • plants can resemble each other for two reasons:

    • they have descended from a common ancestor

      • similar features are synapomorphies (homologous)

    • they have undergone convergent evolution

      • features are homoplasies (analogous)

DNA Sequencing

  • DNA sequencing is a new tool for evolutionary relationship

  • DNA of each species must differ from that of all other species

  • objective studying phenotypes is to determine differences in genotypes

  • DNA can be examined directly and mutations can be identified

Understanding Cladograms

  • a diagram that shows evolutionary patterns by means of series of branches

    • each point (node) represents the divergence of one taxon into two

    • all of the branches that extend from any particular point represent the descendants of the common ancestor

  • paraphyletic group:

    • does not contain all the descendants of the most recent common ancestor

      • derived condition is an apomorphy compared to the ancestral condition

      • shared derived characters is known as synapomorphies

  • to infer a phylogenetic tree from information we have, we use parsimony

  • clade: all species plus their common ancestor

  • all angiosperms were once considered either monocots or dicots

  • basal angiosperms: early angiosperm that diverged before the split of monocots and eudicots

  • eudicots: angiosperms excluding monocots and basal angiosperms

Taxonomic Studies

  • declaring a plant to a new species is easy; proving it and describing it properly is difficult

  • type specimen: a single preserved plant that truly carries the name

  • loss of type specimen can cause problems

  • isotypes: specimens as similar as possible to the type specimen

Major Lines of Evolution

  • all organisms are grouped into three domains

    • bacteria (with cyanobacteria)

    • archaea

    • eukarya

  • major evolutionary lines diverged from early algae

Identifying Unknown Plants

  • key: helps to identify plants

    • constructed of pairs of choices called couplets

    • start at the first choice and determine which description matches your plant

    • proceed to next couplet

    • read couplets and eventually arrive at the name of the plant

  • keys are used for identification not classification


Review of Plant Parts

  • simple leaf: having one blade per leaf

  • compound leaf: having more than one leaflet per leaf

  • leaf margins

    • dentate: triangular tooth-like teeth

    • serrated/toothed: sharp saw-like teeth