Biological diversity is expressed by arranging organisms into kingdoms,
phyla, classes, orders, families, genera, and species. These groups reflect
evolutionary history and common ancestry as much as possible. Evolutionary
relationships responsible for these arrangements are often depicted in
family trees. The aim of phylogenetics is to reconstruct family trees and base
classifications on them.
Binomial nomenclature: Each species has a two-word name. The first
word (capitalized) is the name of the genus; the second (lower case) is
the name of the species. Example: Homo sapiens.
The Linnaean system: Uses binomial nomenclature throughout.
Species are grouped into genera and genera into higher groups. Any one
of these groups, at any level, is called a taxon (plural, taxa).
The complete Linnaean hierarchy (ranking) of groups is as follows:
Kingdom (the most inclusive group)
Phylum (plural, phyla, sometimes called a "division" in plants)
Class
Order
Family
Genus (plural, genera)
Species (same spelling in both singular and plural)
(Mnemonic: "King Philip Came Over For Good Soup")
Extra ranks are added to this hierarchy as needed, such as subphylum (just
below phylum) or superfamily (just above family).
Example: Humans belong to the Kingdom Animalia, Phylum Chordata,
subphylum Vertebrata, class Mammalia, order Primates, family Hominidae,
genus Homo, species Homo sapiens.
Phylogeny: A family tree of species.
Phylogenetics: The study of family trees.
- Phylogenetic methods use both the fossil record and resemblances among
living species as evidence to reconstruct phylogenies. Species sharing
many similarities are considered to be descendents of a common ancestor
that also shared these similarities. When conflicting evidence arises
from different characters, further study is undertaken to see whether
some of the similarities could have evolved by convergence.
- An important task in phylogenetics is therefore recognizing
homology (resemblance due to common ancestry) and distinguishing
it from analogy or convergence.
- The aim of classification based on phylogenetics is to group together
those species that derive their similarities from a common ancestor.
That means that, insofar as possible, each taxon should be made
monophyletic by including the common ancestor within the taxon.
Taxonomy is the theory behing the making of classifications.
- Phenetic taxonomy: Classifications based on resemblance alone
have long been in disfavor because they do not distinguish convergence
from other causes of resemblance.
- Phylogenetic taxonomy: Modern classifications are based on
phylogenetics, meaning that species that share a common ancestry are
grouped together as much as possible. Strict adherence to this principle
is the basis of cladistics. Cladistic taxonomists construct family
trees first, then base their classifications strictly on the geometry of
branching, ignoring such matters as the diversity or degree of change within
each branch.
Drawing cladograms
Cladistics example
- Monophyletic group (clade): A group which includes a single ancestor
and all of its descendants (a natural group).
- Paraphyletic group: A group which includes a common ancestor
and some but not all of its descendants (an incomplete group).
- Polyphyletic group: An unnatural group whose common ancestor is not part
of the group.
- Plesiomorphy: A primitive trait, possessed by an ancestor.
- Symplesiomorphy: A shared plesiomorphy, often defining a paraphyletic group.
- Apomorphy: A derived trait, possessed by a descendant but not by an ancestor.
- Synapomorphy or homology: A shared trait, derived from a common ancestor, thus defining a monophyletic group.
- Homoplasy: A misleading resemblance (such as a convergence), defining a polyphyletic group.
Evolutionary (phylogenetic) classification is based on branching descent:
Biological classification
reflects the results of a branching evolutionary process. Insofar as
possible, classifications should be genealogical. Each taxon should
ideally represent one branch of the evolutionary tree, with the smaller
included taxa representing its sub-branches.
Three domains: Most biologists now arrange organisms
into three great domains:
- Domain Archaea contains only the Kingdom Archaebacteria,
a group of oxygen-intolerant procaryotes with RNA sequences different
from those of all other organisms.
- Domain Bacteria (or Eubacteria) contains only a single kingdom
of the same name, including the majority of procaryotes.
- Domain Eucarya contains several major kingdoms of eucaryotes:
- An assortment of simple eucaryotes, called "protists": generally one-celled,
containing neither tissues nor embryos
- Kingdom Mycota: Fungi, with cell walls but no plastids.
- Kingdom Plantae: Plants, containing plastids and chlorophyll.
- Kingdom Animalia: Multicellular animals, developing from blastulas.
REVIEW:
Study guide and vocabulary
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