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Phylogeny: Why Classify?

Lungfish and perch both live in water and look like fish, so pheneticists group them together (a), although the lineage leading to lungfish split from the other fish hundreds of millions of years ago, and the resemblance between them is due to convergent evolution. Cows and lungfish, although they look completely different, share a more recent common ancestor than either shares with a perch, so cladists group lungfish with cows (c). Humans have evolved a wide range of anatomical and behavioral specializations, while orangutans, gorillas, and chimps share many characteristics retained from early primates. Phenetic classification therefore groups the apes together in one family, Pongidae, while humans are separated out into the family Hominidae (b). Humans, chimps, and gorillas, however, share a quite recent common ancestor, while the orangutan lineage split off much earlier. Cladistic classification groups humans with chimps and gorillas (d).

Credits: Courtesy of Patricia J. Wynne

Phylogeny: Why Classify?

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Backgrounder

Phylogeny: Why Classify?:

One of the most basic human urges is to sort things into categories, look for patterns and apply labels. "Oh, blessed rage to order," the poet Wallace Stevens called it. Since ancient times, philosophers and scientists have sought a way to classify life itself, the innumerable different kinds of living things that populate the planet. It's estimated that as many as 30 million types of organisms exist on Earth today. But there's no one method of classification that everyone agrees is the best, and even today, new systems are being proposed and debated.

About 250 years ago the Swedish biologist Carl von Linne, known as Carolus Linnaeus, classified some 70,000 living things by grouping organisms with the most similar visible characteristics in groups. And these groups themselves were classified in categories of more inclusive rank -- species, genus, family, order, class, phylum, and kingdom. This kind of scheme is called taxonomy, and Linnaeus' system, first published in 1735, is still used today (with many changes). Along with his system, Linnaeus introduced the two-part names, genus and species, reflected in Homo sapiens, modern human, or Rosa canina, the briar rose, for example.

The overall form of Linnaeus' classification is groups nested within groups of increasingly general scope. So several species can belong to the same genus, several genera to the same family, and so on through order, class, phylum, and kingdom. Phenetic classification is an extension of the Linnaean approach.

The publication of Darwin's On the Origin of Species led to a new approach to classification that sought to decipher the evolutionary history of species. The closest species in this scheme are not necessarily those that look the most alike, but those which share the most recent common ancestry.

The development of cladistic techniques, coupled with the availability of computers to analyze large quantities of data, have produced new insights into the history of life. Cladistic classification determines the evolutionary relationships between organisms by analyzing certain kinds of characteristics, or traits. In the course of evolution, a novel, heritable trait will emerge in some organism. That trait will be passed on to its descendants. Two organisms that share such a new, or derived, trait or group of traits are therefore more closely related to each other than to organisms that lack those traits. By treating recently evolved characteristics differently from ancestral characteristics, this technique emphasizes evolutionary relationships over structural similarities.

In recent years, another line of evidence for determining evolutionary relationships has emerged, based on DNA sequencing methods like those used in the human genome project. The millions of chemical letters of DNA code that are stored in the cells of all living things provide a distinctive genetic profile of the species. By comparing the similarity of DNA between two species, scientists can determine how closely they are related. These molecular similarities reveal the relationships of the organisms themselves -- like, say, a chimpanzee and a human being.

By combining information from analysis of fossils with these DNA techniques, researchers draw evolutionary trees showing where and approximately when the species branched off from common ancestors. These diagrams, known as phylogenetic trees, or cladograms, are hypotheses -- they represent our best estimate of the true evolutionary relationships, based on existing evidence. They are constantly revised as new data becomes available, bringing our view of the tree of life into ever-clearer focus.

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