analogous traits A trait that is similar in two different species or taxa, not because of common descent, but rather as a result of natural selection operating in similar ways along separate evolutionary lineages. (p. 118)
chronograms A phylogenetic tree on which absolute time is denoted. (p. 114)
clade A taxonomic group including an ancestor and all of its descendants. (p. 110)
cladograms A phylogenetic tree in which cladistic (historical evolutionary) relationships are represented but in which branch lengths do not indicate the degree of evolutionary divergence. See also clade, phylograms. (p. 114)
convergent evolution The process in which natural selection acts in similar ways in different taxa, driving the independent evolution of similar traits in each taxon. See also analagous trait. (p. 119)
derived trait A trait that has changed form or state from the ancestral form over evolutionary time. (p. 120)
divergent evolution The process in which natural selection operates in different ways in each of two or more taxa that share a recent common ancestor, leading to different traits in these taxa. (p. 119)
homologous trait A trait shared by two or more species because those species have inherited the trait from a shared common ancestor. (p. 118)
homoplasy A trait that is similar in two species because of convergent evolution rather than common ancestry. (p. 121)
monophyletic group A group that consists of a unique common ancestor and each and every one of its descendant species, but no other species. (p. 110)
node A branch point on a phylogenetic tree, representing an ancestral population or species that subsequently divided into multiple descendant populations or species. (p. 108)
outgroup A distantly related group for which we already have information with respect to its evolutionary relationship to the taxon we are studying. Outgroups are used in rooting phylogenetic trees. (p. 110)
paraphyletic group A group that includes the common ancestor of all its members but does not contain every species that descended from that ancestor. (p. 111)
phylogenetic systematics An approach to classifying organisms based upon their evolutionary histories. (p. 103)
phylogeny The branching pattern of relatedness among populations (or occasionally, individuals) in a group or taxon. (p. 103)
phylograms A phylogenetic tree in which the length of each branch represents the amount of evolutionary change that has occurred along that branch. (p. 114)
polarity The order in which different variants of a trait evolved over evolutionary time. (p. 122)
polyphyletic group A group that does not contain the common ancestor of its members and/or all descendants of that common ancestor. (p. 111)
polytomy A node on a phylogenetic tree that has more than two branches arising from it. Polytomies are often used to represent our uncertainty about phylogenetic relationships on a phylogenetic tree. (p. 110)
root The basal (most ancestral) lineage on a phylogenetic tree. (p. 108)
rooted tree A phylogenetic tree in which the root is indicated and thus the direction of time is specified. (p. 111)
sister taxa Two taxa that derive from the same node on a phylogenetic tree. (p. 110)
symplesiomorphy A derived trait that has arisen so recently that it appears in only one of two sister taxa. Evolutionary biologists try to avoid using symplesiomorphies in phylogenetic reconstruction. (p. 122)
synapomorphy A derived trait that is shared in two populations because it was inherited from a recent common ancestor. Evolutionary biologists aim to use synapomorphies in phylogenetic reconstruction, as they provide useful information about the evolutionary relationships among populations. (p. 121)
taxon A group of related organisms. (p. 107)
traits Any observable characteristics of organisms, such as anatomical features, developmental or embryological processes, behavioral patterns, or genetic sequences. (p. 103)
unrooted tree A phylogenetic tree in which the root, and thus the direction of time, is unspecified. (p. 111)
vestigial traits Traits that have no known current function but that appear to have had a function in the evolutionary past. (p. 129)