Phylogenies

Defining and identifying species is one major area of research into the evolutionary process. Equally important is the study of how different species are related to each other. Because species originate by the branching process of speciation, each species living today has a history of descent that passes through many species that no longer exist. Every species that has ever existed shares a relationship with every other species through a common ancestor species that lived at some time in the past. For some pairs of animal species, like chimpanzees and humans, the ancestor is very recent, within the past few million years. For other pairs of species, such as humans and turkeys, the most recent common ancestor lived hundreds of millions of years ago. The account of all such ancestral relationships within a group of animals is called a phylogeny.

The concept of phylogeny was one of the most important insights of Charles Darwin as he developed his theory of the origin of species. As he hypothesized that species originate by a process of evolution from ancestral species, Darwin provided a new basis for explaining the patterns of similarities among species in nature (Gould 2002). Different species share common ancestors at different times in the past, and these phylogenetic relationships determine their similarities. While Darwin was the first to fully explain this pattern of relationships in an evolutionary framework, it was the German scientist Ernst Haeckel who most extensively illustrated hypotheses of phylogenetic relationships among different species in the wake of Darwins theories. Such illustrations made the analogy of a phylogeny with a tree explicit by incorporating imagery such as bark and leaves along with the names of species in nature.

Although our understanding of evolutionary processes and the actual pattern of relationships of different species have both changed substantially from that of the early evolutionists like Darwin and Haeckel, their conception of the phylogeny of life as a tree of relationships is essentially the same as todays view. At the same time, an understanding of phylogeny has become essential for understanding the evolutionary process, for several reasons:

  1. The evolutionary context in which morphological, behavioral, and genetic changes happen is determined in part by ancestry. Species with different ancestries may respond to environmental forces in different ways, because of their different genetic heritages.
  2. The selective pressures that cause most evolutionary changes are reflected in evolutionary changes that happen in closely related species. Different species that evolve in similar ways may provide evidence that a common process affects them.
  3. The history of life is in large part the history of relationships of different species. Humans place in nature, as part of that history, is determined by our relationships to both our nearest and our farthest relatives.

Scientists depict phylogenetic hypotheses in diagrams. These depictions may incorporate different amounts of information. Some diagrams include information about the timing of events such as speciations and extinctions, and the exact relationships of species, such as whether one species is ancestral to another or merely a close relative. The most extensive diagrams are called phylogenetic trees, or sometimes simply phylogenies, as they attempt to depict the full story of phylogenetic relationships.

Another type of diagram, called a cladogram, depicts only which species form related groups. Like a phylogenetic tree, a cladogram appears to show the branches that occurred during the evolution of a group of species, but it contains no information about either time or ancestor-descendant relationships. Each branch on a cladogram is a clade, a monophyletic group of species. Because all the members of a clade share a common ancestor, they often share similarities when compared with more distantly related species.

See also:

Speciation

Species concepts

References:

Gould SJ. 2002. The structure of evolutionary theory. Harvard University Press, Cambridge, MA. Amazon