Systematics

Biological systematics is the study of the diversification of life on the planet Earth, both past and present, and the relationships among living things through time.

Classification

Method by which biologists group and categorize organisms by biological type, such as genus or species. Biological classification is a form of scientific taxonomy.

Phylogeny

The study of evolutionary relatedness among groups of organisms (e.g. species, populations), which is discovered through molecular sequencing data and morphological data.

Taxonomy (Taxon, Taxa)

Taxonomy is the classification, identification, and naming of organisms.

Evolutionary Systematics

A branch of biological classification that seeks to classify organisms using a combination ofphylogenetic relationship and overall similarity. This type of taxonomy considers taxa rather than single species, so that groups of species give rise to new groups.

Paraphyly is accepted!

Cladistics

 A method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants (and nothing else).

Paraphyly is not accepted!

Monophyletic

 A monophyletic group is a taxon (group of organisms) which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group.

Holophyletic

A term posited as a semantically correct replacement for the term monophyletic.

Polyphyletic

A group whose members have different ancestors.

Paraphyletic

Consists of an ancestral species and some (but not all) of its descendants.

Pleisomorphic

 In cladistics, this term describes primitive or generalized characteristics that arose early in the evolutionary history of a taxonomic group. These will be very widespread and will therefore not help in dividing the group into lower-level taxa.

Apomorphic

Sympleisiomorphic

Synapomorphic

A trait that is shared by two or more taxa and their most recent common ancestor, whose ancestor in turn does not possess the trait.

Autapomorphic

A distinctive anatomical feature, known as a derived trait, that is unique to a given terminal group.

Homologous

Any characteristic of organisms that is derived from a common ancestor. 

Analogous

 Similarities between organisms that were not in the last common ancestor of the taxa being considered but rather evolved separately.

Homoplasy

Similarity due to convergent evolution. These similarities are not due to common heredity but rather to similar environments.

Convergent Evolution

The emergence of biological structures or species that exhibit similar function and appearance but that evolved through widely divergent evolutionary pathways.

Parallel Evolution

The independent evolution of similar traits, starting from a similar ancestral condition.

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Dendrogram

A tree diagram frequently used to illustrate the arrangement of the clusters produced by hierarchical clustering. 

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Cladogram

Horotely

A normal or average rate of evolution per million years, of genera within a given taxonomic group.

Tachytely

A rate of evolution within a group that is much faster than the average horotelic rate. Such accelerated evolution typically occurs when an organism enters a new adaptive zone and initiates an adaptive radiation to fill the available niches.

Bradytely

An exceedingly slow rate of evolution, manifest by slowly evolving lineages which survive much longer than would normally be expected. Living fossils (e.g. the coelacanth) and groups that have remained relatively stable with time (e.g. opossums and crocodiles) represent the low end of the bradytelic range. 

Maximum Parsimony

Seeking the simplest explanation that is consistent with the facts. 

(In trees based on morphology, the most parsimonious tree requires the fewest evolutionary events.)

Maximum Likelihood

A tree that reflects the most likely sequence of evolutionary events.

Typological Species Concept

A group of organisms in which individuals are members of the species if they sufficiently conform to certain fixed properties or "rights of passage".

Biological Species Concept

A set of actually or potentially interbreeding populations.

Evolutionary Species Concept

A group of organisms that shares an ancestor; a lineage that maintains its integrity with respect to other lineages through both time and space. At some point in the progress of such a group, some members may diverge from the main population and evolve into a subspecies, a process that eventually will lead to the formation of a new full species if isolation (geographical or ecological) is maintained.