"As buds give rise by growth to fresh buds ..." ~ Charles Darwin In 1837, Charles Darwin sketched a simple tree diagram in one of his notebooks, next to which he penned the tentative words: I think. From that point on, the image of a tree for Darwin persisted as a way to envision the sprouting of new species from existing forms. He later wrote in On the Origin of Species:
"As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever-branching and beautiful ramifications." ~ Charles Darwin, from Chapter IV. Natural Selection of On the Origin of Species
Today, trees diagrams have taken root as powerful tools for scientists to depict relationships among groups of organisms. As a result, an entire science with its own specialized vocabulary has developed around them. Here we'll look at the science surrounding evolutionary trees, also known as phylogenetics.
Phylogenetics is the science of constructing and evaluating hypotheses about evolutionary relationships and patterns of descent among organisms past and present. Phylogenetics enables scientists to apply the scientific method to guide their study of evolution and assist them in intepreting the evidence they collect. Scientists working to resolve the ancestory of several groups of organisms evaluate the various alternate ways in which the groups could be related to one another. Such evaluations look to evidence from a variety of sources such as the fossil record, DNA studies or morphology. Phylogenetics thus provides scientists with a method of classifying living organisms based on their evolutionary relationships.
A phylogeny is the evolutionary history of a group of organisms. A phylogeny is a 'family history' that describes the temporal sequence of evolutionary changes experienced by a group of organisms. A phylogeny reveals, and is based on, the evolutionary relationships among those organisms.
A phylogeny is often depected using a diagram called a cladogram. A cladogram is tree diagram that reveals how lineages of organims are interconnected, how they branched and rebranched throughout their history and evolved from ancestral forms to more modern forms. A cladogram depicts relationships between ancestors and descendents and illustrates the the sequence with which traits developed along a lineage.
Cladograms superficially resemble the family trees used in geneological research, but they differ from family trees in one fundamental way: cladograms do not represent individuals like family trees do, instead cladograms represent entire lineages—interbreeding populations or species—of organisms.