polytomy
A branch point from which more than two taxa emerge. Indicates that the evolutionary relationships between the descendant taxa are not yet clear.
phylogenetic tree
A branching diagram that represents a hypothesis about the evolutionary history of a group of organisms.
shared derived character
An evolutionary novelty that is unique to a particular clade.
taxon
A named taxonomic unit at any given level of classification.
systematics
A scientific discipline focused on classifying organisms and determining their evolutionary relationships.
paralogous genes
Homologous genes within a species that result from a gene duplication event.
orthologous genes
Homologous genes found in different taxa as a result of speciation event.
resolution
In microscopy: A measure of the clarity of an image; it is the minimum distance two points can be separated an still be distinguished as tow points.
sister taxa
Groups of organisms that share an immediate common ancestor and hence are each other's closest relatives.
shared ancestral character
A character, shared by members of a clade, that originated in an ancestor that is not a member of that clade.
domain
A taxonomic level consisting of Bacteria, Archaea, and Eukarya.
polyphyletic
A group of taxa that includes distantly related organisms but does not include their most recent common ancestor.
phylogeny
The evolutionary history of a species or group of related species.
taxonomy
A scientific discipline concerned with naming and classifying the diverse forms of life.
horizontal gene transfer
The transfer of genes from one genome to another through mechanisms such as transposable elements, plasmid exchange, viral activity, and perhaps fusion of different organisms.
binomial nomenclature
Two part, latinized format for naming a species, consisting of the genus and specific epithet.
basal taxon
In a specified group of organisms, a taxon whose evolutionary lineage diverged early in the history of the group.
analogous structures
Structures that are similar because of convergent evolution, not homology.
magnification
In microscopy: the ratio of an object's image size to its actual size.
field of view
In microscopy: the area that is visible when looking through a microscope.
branch point
The representation on a phylogenetic tree of the divergence of two or more taxa from a common ancestor.
paraphyletic
A group of taxa that consists of a common ancestor and some, but not all, of its descendants.
contrast
In microscopy: the difference in the light intensity between different parts of a sample and the background.
homologous structures
Structures in different species that are similar because of common ancestry.
molecular clock
A method for estimating the time required for a given amount of evolutionary change, based on the observation that some regions of genomes evolve at constant rates.
monophyletic
A group of taxa that consists of a common ancestor and all of its descendants.
maximum likelihood
A principle that states when considering multiple phylogenetic hypotheses, one should take into account the hypothesis that reflects the most likely sequence of evolutionary events.
rooted
A phylogenetic tree containing a branch point representing the most recent common ancestor of all taxa in the tree.
maximum parsimony
A principle that states that when considering multiple explanations for an observation, one should first investigate the simplest explanation that is consistent with the facts.
cladistics
An approach to systematics in which organisms are placed into groups called clades based primarily on common descent.