A clade (from Ancient Greek: κλάδος, klados, "branch") is a group of organisms that consists of a common ancestor and all its lineal descendants, and represents a single "branch" on the "tree of life".
The common ancestor may be an individual, a population, a species (extinct or extant), and so on right up to a kingdom. Clades are nested, one in another, as each branch in turn splits into smaller branches. These splits reflect evolutionary history as populations diverged and evolved independently. Clades are termed monophyletic (Greek: "one clan").
Over the last few decades, the cladistic approach has revolutionized biological classification and revealed surprising evolutionary relationships among organisms. Increasingly, taxonomists try to avoid naming taxa that are not clades; that is, taxa that are not monophyletic.
In order to organize and categorize things in science, we have to create graphs, charts, tables, etc. A clade is just a type of organizational method found within these graphs and charts. We will discuss a clade here and then test your knowledge with a quiz.
What Is A Clade?
Have you ever seen a family tree laid out on paper or a screen? Try to picture it in your mind -- one section of the tree might start out with two people before branching out to show their children, grandchildren, and so on. Well, the individual lines that form this particular branch of the family tree are referred to as a clade.
In science, a clade is used to chart connections between organisms. It does this by connecting an organism to its particular ancestors or descendants. It is monophyletic, so it is only about one particular type of organism and its own descendants. Scientists use these to better organize and understand whatever particular part of taxonomy they are looking at.
The idea of a clade and a cladogram are essential to the approach of classification known as cladistics. In cladistics, we classify organisms based on whether or not they have common unique features that were also found in a more recent common ancestor and not necessarily a distant one. The closer the common ancestor is to the groups, the more related we find them to be.
Originally, cladistics was based on morphological (physical features and appendages) data, but now we take into account DNA sequencing. The cladograms that are created are similar in scope to a family tree with many different branches but are not quite the same as those. We see many different forms of cladograms but they are essentially the same, just oriented in slightly different ways.
In biology and biological taxonomy, a clade is a group consisting of a single common ancestor, all the descendants of that ancestor, and nothing else. Over centuries of work, biological taxonomy has endeavored to split groups into clades, rejecting non-clade classifications, which are referred to as "paraphyletic." True clades are "monophyletic."
An example of a true clade would be birds. Birds are believed to all descend from a common ancestor that lived about 150 million years ago. However, reptiles and apes are not clades. Reptiles aren't a clade because birds descended from dinosaurs, considered reptiles, and birds aren't considered reptiles. A group that excludes descendants of a common ancestor isn't a clade. Apes aren't a clade because humans descended from apes and humans generally aren't considered apes. If you include humans and the extinct relatives of humans, like Neanderthals, to be apes, then apes are a clade, but this generally isn't done.
Simpler organisms, such as arthropods (crustaceans, insects, millipedes, etc.) are more difficult to organize into clades, because there are fewer genetic and morphological features that can be used to determine common ancestry and ancestral lines. For instance, for decades in the late 20th century, scientists thought that arthropods (animals with external exoskeletons and jointed appendages) evolved on several separate occasions from soft-bodied ancestors such as annelid worms. Subsequent morphological and genetic analysis has found this to be false -- arthropods are indeed a clade, descending from a common ancestor that split from soft-bodied ancestors just once.
Determining clades at levels more specific than phyla can be challenging, especially for relatively simple animals. After decades of study, we still don't know how different arthropod groups are related to one another. Did land arthropods evolve from fairy shrimp, or some other group? We don't know for sure, and scientists are busy publishing papers and conducting analyses to find out.
Determining clades is difficult partially because much of the morphological and genetic data is ambiguous. Sometimes, a certain morphological feature -- like spines -- evolve via parallel evolution rather than manifesting in a single species and most (or all) of its descendants. Genetic data can be ambiguous because evolution occurs in different species at different rates, throwing off calculations that attempt to date time-of-divergence between species by comparing genetic commonality. To make matters worse, morphological specialists and genetics specialists tend to quarrel about the relative significance of their respective approaches. Correct determinations about animal clades only emerge after years or decades of in-depth research representing hundreds or thousands of papers and studies.
A clade is a grouping that includes a common ancestor and all the descendants (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.