|
SYSTEMATICS
Because there are millions of organisms on earth, we need some sort of
ordered system for keeping track of them and communicating about them. Without a
name the information about an organism is lost.
I. Systematics = the scientific study of
biological diversity
A. Taxonomy = the science of naming and classifying organisms
B. Polynomials - During medieval times Latin was the language of scholars
and most western scientific writing was done in Latin. Organisms were
identified by long descriptive Latin phrases called polynomials
C. Binomials - Linnaeus (mid 18th century) developed binomial
naming system:
1. Each species has a two part name, e.g. the scientific name for
humans is Homo sapiens
2. The first part of a binomial is the genus, e.g. Quercus
is the genus of oak trees
3. The second part of a binomial is the specific epithet, usually
a latinized descriptive. E.g. Quercus alba, Q. nigra
and Q. rubra are the white, black and red oaks, respectively
4. Note that a specific epithet is meaningless
without the genus name
5. Scientific names are always latinized and either italicized or
underlined, and the first letter of only the genus name is capitalized
6. Why use scientific names rather than common names?
a. the same organism may have more than one name in different languages or
different regions
b. the same common name is often applied to more than one organism,
e.g. daisy
c. many common names are confusing, e.g. a pineapple tree is neither a
pine nor apple
d. there are no rules for using common names, with millions of named
species there must be some rules
II. The taxonomic hierarchy - Species are grouped into an ordered
hierarchical system of classification, i.e. more inclusive groups:
A. Kingdom
Phylum
Class
Order
Family
Genus
Species = genus + specific epithet
B. The higher the category, the more inclusive it is
C. Members of a kingdom share general characters; members of a species
share quite specific characters
D. Characters are any structural, chromosomal, or molecular feature that
distinguishes groups
E. Additional levels of classification can be added by adding super-, sub-,
or infra- (e.g., suborder, subspecies).
III. Development of classification
A. Until relatively recently organisms were classified as either plants or
animals and two kingdoms were recognized.
1. Non-motile autotrophs were placed in the plant kingdom
2. Motile heterotrophs were placed in the animal kingdom
B. However, as more organisms were discovered and studied it became
apparent that many organisms did not fit into this system:
1. Euglena is a unicellular, motile autotroph
2. Fungi (mushrooms and molds), traditionally classified as plants, are
non-motile heterotrophs
C. Development of better microscopes led to the discovery that there are
two fundamental types of organisms, defined by their cell
types:
1. Prokaryotes (bacteria) - lack:
nuclei, organelles, 9+2 flagella, chromosomes, multicellularity and
sexuality
2. Eukaryotes (nearly all other organisms) - have:
nuclei, organelles, 9+2 flagella, DNA associated with histone proteins to
form chromatin/chromosomes, sexual reproduction and most are multicellular
D. Whitaker proposed a five kingdom system. All Prokaryotes were
placed into a single kingdom (Monera) and the Eukaryotes were placed into four
kingdoms: Plantae, Fungi, Animalia and Protista
IV. Three domains
A. Recent genetic and molecular investigations have demonstrated that there
are two major groups of prokaryotes. They differ radically in
the composition of their cell walls, membrane lipids, ribosomal RNA, and a
variety of other biochemical features
B. Therefore, our text recognizes three domains above
the rank of kingdom
1. Domain Bacteria - prokaryotes with muramic
acid in cell walls. Majority of bacteria plus cyanobacteria ("blue
green algae")
2. Domain Archaea (ancient bacteria) -
prokaryotes that lack muramic acid in cell walls. Many inhabit
"harsh" environments. Includes methane producers, extreme
halophiles, extreme thermophiles, acidophiles and one group which lacks cell
walls
3. Domain Eukarya - all
eukaryotes, four kingdoms:
a. Kingdom Animalia (Animals) - motile,
multicellular, lack plastids and cell walls, heterotrophic via
ingestion, sexual reproduction
b. Kingdom Plantae (Plants) - nonmotile,
multicellular, plastids and autotrophic via photosynthesis, cell
walls made of cellulose, adapted for life on land, mostly sexual
reproduction.
"Algae" are not included in this kingdom
c. Kingdom Fungi (Mushrooms and Molds) - nonmotile, filamentous,
lack plastids, cell walls are made of chitin, heterotrophic via
absorption of nutrients from dead (saprophytic) or living (parasitic)
matter. Virtually all are multicellular except yeast. Both sexual and
asexual reproduction
d. Kingdom Protista - lack multicellularity.
Heterogeneous assemblage of unicellular, colonial and multicellular
Eukaryotes that do not have the distinctive characters of plants, animals
or fungi.
They have various types of reproduction from simple cell division
through sexual, and various types of nutrition
Includes all groups previously called protozoa as well as all the
algae except blue greens. Also includes some organisms previously called
fungi
V. Classifying organisms
A. There are two ways to classify organisms:
1. Phenetic - based on overall similarity of form and structure. Must
be based on homologous, not analogous features
a.
Homologous structures - similarity in structure due to common
descent, irrespective of the diverse uses to which they may be put. E.g.
vertebrate forearms: human hand, bat wing, dolphin flipper.
b. Analogous structures - similarity in structure based on adaptation
for the same function, not common descent. E.g. wings have developed independently in
insects, reptiles, birds, and bats.
2. Phylogenetic - based on common evolutionary descent
a. Now the preferred method because it has predictive value
b. Phylogeny = the evolutionary history of a group of organisms
Modern phylogenetic investigations are based on molecular data, primarily nucleotide
sequences. Basically, the more closely related two organisms are, the more
nucleotide sequences (genes) they will have in common.
LIFE CYCLES
I. Organisms may exhibit one of three different life cycles:
A. Zygotic meiosis - zygote divides via
meiosis to form 4 haploid cells which divide by mitosis to produce more haploid cells or a
multicellular haploid individual that eventually gives rise to gametes by differentiation.
Found in some algae.
B. Gametic meiosis - haploid gametes are
formed via meiosis in a diploid individual. These fuse to form a diploid zygote that
divides via mitosis to produce a multicellular diploid organism. This is characteristic of
animals, some Protista and some brown algae.
C. Sporic meiosis - the diploid individual (sporophyte)
produces haploid spores as a result of meiosis. These spores do not function as gametes
but undergo mitotic division to produce multicellular haploid individuals (gametophytes),
which eventually produce gametes that fuse to form a diploid zygote. These zygotes
differentiate via mitosis into diploid individuals (sporophyte). This is characteristic of
true plants and some algae.
1. Spore - a cell that can grow directly into a new individual, e.g. ferns and
mosses. In most plants meiosis takes place within flowers
and it results in the production of spores.
2. Alternation of
generations - In some plants there are both multicellular haploid and diploid phases which
alternate during their life cycle
a. In plants the haploid, gamete-producing phase is called the gametophyte and
the diploid spore-producing phase is called the sporophyte
b. Isomorphic alternation of generations - the haploid and diploid forms are morphologically
indistinguishable, e.g. some algae
c. Heteromorphic alternation of generations - the haploid and diploid phases have different
phenotypes. This is characteristic of most true plants.
|