Bio B Tau - Glossary: Phylogeny (Less.5)

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adaptation

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Change in a organism resulting from natural selection; a structure which is the result of such selection.

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ancestor

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Any organism, population, or species from which some other organism, population, or species is descended by reproduction.

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Earliest diverging group

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The first group to diverge within a clade; for instance, to hypothesize that sponges are the earliest diverging animals group is to suggest that the lineage leading to sponges diverged from the lineage that gave rise to all other animals.

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character

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Heritable trait possessed by an organism; characters are usually described in terms of their states, for example: "hair present" vs. "hair absent," where "hair" is the character, and "present" and "absent" are its states.

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clade

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A monophyletic taxon; a group of organisms which includes the most recent common ancestor of all of its members and all of the descendants of that most recent common ancestor. From the Greek word "klados", meaning branch or twig.

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convergence

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Similarities which have arisen independently in two or more organisms that are not closely related. Contrast with homology.

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derived

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Describes a character state that is present in one or more subclades, but not all, of a clade under consideration. A derived character state is inferred to be a modified version of the primitive condition of that character, and to have arisen later in the evolution of the clade. For example, "presence of hair" is a primitive character state for all mammals, whereas the "hairlessness" of whales is a derived state for one subclade within the Mammalia.

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diversity

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Term used to describe numbers of taxa, or variation in morphology.

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extinction

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When all the members of a clade or taxon die, the group is said to be extinct.

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homology

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Two structures are considered homologous when they are inherited from a common ancestor which possessed the structure. This may be difficult to determine when the structure has been modified through descent.

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ingroup

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In a cladistic analysis, the set of taxa which are hypothesized to be more closely related to each other than any are to the outgroup.

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lineage

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Any continuous line of descent; any series of organisms connected by reproduction by parent of offspring.

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monophyletic

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Term applied to a group of organisms which includes the most recent common ancestor of all of its members and all of the descendants of that most recent common ancestor. A monophyletic group is also called a clade.

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outgroup

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In phylogeny a taxon which is hypothesized to be less closely related to each of the taxa under consideration than any are to each other. In a cladistic analysis, any taxon used to help resolve the polarity of characters,

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paraphyletic

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Term applied to a group of organisms which includes the most recent common ancestor of all of its members, but not all of the descendants of that most recent common ancestor.

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parsimony

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Refers to a rule used to choose among possible trees. Parsimony states that the tree implying the least number of changes in character states is the best.

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phylogenetics

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Field of biology that deals with the relationships between organisms.

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phylogeny

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The evolutionary relationships among organisms; the patterns of lineage branching produced by the true evolutionary history of the organisms being considered.

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plesiomorphy

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A primitive character state for the taxa under consideration.

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polarity of characters

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The states of characters used in a cladistic analysis, either ancestral or derived. Ancestral characters are those acquired by an ancestor deeper in the phylogeny than the most recent common ancestor of the taxa under consideration. Derived characters are those acquired by the most recent common ancestor of the taxa under consideration.

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polyphyletic

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Term applied to a group of organisms which does not include the most recent common ancestor of those organisms; the ancestor does not possess the character shared by members of the group.

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Ancestral

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Describes a character state that is present in the common ancestor of a clade. An ancestral character state is inferred to be the original condition of that character within the clade under consideration. For example, "presence of hair" is an ancestral character state for all mammals, whereas the "hairlessness" of whales is a derived state for one subclade within the Mammalia.

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rank

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In traditional taxonomy, taxa are ranked according to their level of inclusiveness. Thus a genus contains one or more species, a family includes one or more genera, and so on.

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relatedness

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Two clades are more closely related when they share a more recent common ancestor between them than they do with any other clade.

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sister group

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The two clades resulting from the splitting of a single lineage.

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synapomorphy

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A character which is derived, and because it is shared by the taxa under consideration, is used to infer common ancestry.

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taxon

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Any named group of organisms, not necessarily a clade; a taxon may be designated by a Latin name or by a letter, number, or any other symbol; plural = taxa.

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taxonomy

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The science of naming and classifying organisms.