A clade is a grouping that includes a common ancestor and all the descendents (living and extinct) of that ancestor. Using a phylogeny, it is easy to tell if a group of lineages forms a clade. Imagine clipping a single branch off the phylogeny—all of the organisms on that pruned branch make up a clade.
Clades are nested within one another—they form a nested hierarchy. A clade may include many thousands of species or just a few. Some examples of clades at different levels are marked on the phylogenies below. Notice how clades are nested within larger clades.
Shared derived characters can be used to group organisms into clades. For example, amphibians, turtles, lizards, snakes, crocodiles, birds and mammals all have, or historically had, four limbs.
However, the presence of four limbs is not useful for determining relationships within the clade in green above, since all lineages in the clade have that character. To determine the relationships in that clade, we would need to examine other characters that vary across the lineages in the clade.
We use homologous characters—characters in different organisms that are similar because they were inherited from a common ancestor that also had that character. An example of homologous characters is the four limbs of tetrapods . Birds, bats, mice, and crocodiles all have four limbs. Sharks and bony fish do not.
The ancestor of tetrapods evolved four limbs, and its descendents have inherited that feature—so the presence of four limbs is a homology.
For example, birds and bats both have wings, while mice and crocodiles do not. Does that mean that birds and bats are more closely related to one another than to mice and crocodiles? No. When we examine bird wings and bat wings closely, we see that there are some major differences.
Bat wings consist of flaps of skin stretched between the bones of the fingers and arm. Bird wings consist of feathers extending all along the arm. These structural dissimilarities suggest that bird wings and bat wings were not inherited from a common ancestor with wings.
Bird and bat wings are analogous—that is, they have separate evolutionary origins, but are superficially similar because they evolved to serve the same function. Analogies are the result of convergent evolution.
Interestingly, though bird and bat wings are analogous as wings, as forelimbs they are homologous. Birds and bats did not inherit wings from a common ancestor with wings, but they did inherit forelimbs from a common ancestor with forelimbs.
Paraphyletic taxon : A group composed of a collection of organisms, including the most recent common ancestor of all those organisms. Unlike a monophyletic group, a paraphyletic taxon does not include all the descendants of the most recent common ancestor.
Examples : Traditionally defined Dinosauria, fish, gymnosperms, invertebrates, protists, etc.
A group consisting of an ancestor but not all of its descendants. It is defined by what it does not have.
Polyphyletic taxon : A group composed of a collection of organisms in which the most recent common ancestor of all the included organisms is not included, usually because the common ancestor lacks the characteristics of the group.
Copyright 2006 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California. All materials appearing on the UCMP Web Servers (WWW.UCMP.BERKELEY.EDU, EVOLUTION.BERKELEY.EDU)