Classification: Ordering the Natural World
As we learned in Unit 1, Carolus Linnaeus developed what is known as the Linnaean system of classification. This basic system is still used today, although it is focused on illustrating evolutionary relationships between organisms. This may sound familiar and it should---think phylogentic tree! Now we take a moment to look in more depth at classification.
Why do we bother with classification systems? Classification systems are important to help scientists organize the massive diversity of the natural world. It’s a method that helps scientists from different countries talk about the natural world without confusion. Modern classification is considered a natural system representing real relationships between organisms. One thing to keep in mind when looking at classification systems is that it is basically a hypothesis about evolutionary relationships between species, which is why you may see different schemes as researchers may interpret data differently.
The Linnaean System
This hierarchical system organizes organisms into ever increasingly similar categories. Linnaeus’ system has been modified from when he originally published Systema Natura in 1735. He published ten editions of the book, adding new levels to the hierarchy.
According to the Linnaean system, human beings are classified as follows (Larsen 2008: 36):
Category Level Common Characteristics
Kingdom Animalia mobile multicellular organisms that consume other organisms for food and develop during an embryo stage
Subkingdom Eumetazoa all major animals except sponges that contain true tissue layers, organized as germ layers, which develop into organs in humans
Phylum Vertebrata group of vertebrate and invertebrate animals that have a notochord, which becomes the vertebral column in humans and other primates
Superclass Tetrapoda vertebrate animals with four feet or legs, including fish, amphibians, birds, dinosaurs, and mammals
Class Mammalia group of warm-blooded vertebrate animals that produce milk for their young in mammary glands; they have hair or fur and specialized teeth
Subclass Theria group of mammals that produce live young without a shelled egg
Order Primates group of mammals specialized for life in the trees, with large brains, stereoscopic vision, opposable thumbs, and grasping hands and feet
Suborder Anthropoidea group of primates, including monkeys, apes, and humans, but not prosimians; they have long life cycles and are relatively large-bodied
Family Hominidae group of anthropoids, including humans and great apes, and human ancestors; they have the largest bodies and brain sizes of all the primates
Genus Homo group of hominids including modern humans, their direct ancestors, and extinct relatives; they are bipedal and have large brains
Species sapiens modern and ancestral modern humans; they have culture, use language, and inhabit every continent except Antarctica
Subspecies sapiens modern humans alone
Rules of Classification
Taxonomy or systematics is the field that establishes the rules of classification. As established by Linnaeus, organisms are first classified by their physical similarities. Remember, today classification is interested in illustrating evolutionary relationships, so it is not enough simply to establish that there are physical similarities; it is important to try and understand how the similarities arose…was it because of a close common ancestor or some other reason.
To begin classification, researchers identify anatomical features that have the same function as that of similar structures in other species. Then they determine whether the similar features are due to a homology or analogy. A homology is a trait found in two or more different organisms that was inherited from a common ancestor. The more homologies shared by two organisms, the closer the genetic relationship. The structure of your arm, the wing of a bird, the fin of a whale, and the forelimb of lizard are examples of homologies. Confusing? Think of this as cousins who inherit the same trait from their grandfather (Understanding Evolution). These types of traits are called homologous structures or simply, homologies. Click here for another image.
However, not all similarities are homologies. An octopus has limbs, but the structure is not the same as the tetrapods referred to above. Octopus limbs evolved independently after they split from their shared ancestor with tetrapods. This type of similarity is called an analogy or analogous structure. How can analogous structures evolve in separate species? Analogies are a response to similar pressures. The Understanding Evolution website has a short animation that helps to explain this: http://evolution.berkeley.edu/evolibrary/article/0_0_0/similarity_hs_08.
In order to determine whether a trait is a homology or analogy, researchers have some criteria to help them make the determination:
- Do the bones have the same basic structure?
- Do the bones have the same relationship to other features?
- Is the development of the structure the same?
Researchers also try to distinguish whether or not structures are due to parallel evolution or convergent evolution. Parallel evolution refers to situations where organisms that are unrelated or distantly related have similar adaptations because of similar environmental pressures. When parallel evolution occurs “in distantly related [organisms] that are morphologically similar in overall appearance” (Armstrong 2002-2008) it is called convergent evolution. An example that will hopefully make this concept more clear comes from the plant world. Cactaceae (North American cactuses) and Euphorbiaceae (South American euphorbias are distant relatives that both have characteristics we attribute to succulents—thick stems, spines, and adapted to deserts, but they belong to different plant families).
Some scientists use the terms interchangeably, making them even more confusing then they already are; this group of scientists proposes that what we are seeing are two different ends of a continuum. They suggest that we simplify terminology and call it convergence. To help make the distinction between 1) similarity due to a common ancestor and 2) similarities due to distantly related ancestors, some researchers use the term homology to refer to the former condition (#1 above) and homoplasy to refer to the latter (#2 above) (Armstrong 2002-2008).
There are two basic schools of thought in constructing classifications or evolutionary relationships: Evolutionary systematics and cladistics. Evolutionary systematics is the traditional approach that relies on analysis of homologous characters (or traits), phylogeny, divergence, and adaptational level to establish evolutionary relationships. It arose out of the Modern Synthesis, in particular the work of Ernst Mayr, G. G. Simpson, and Thomas Cavalier-Smith, and has a time component to it. In more recent times, DNA sequences have been incorporated into the construction of evolutionary relationships.
Cladistics, or phylogenetic systematics, also reconstructs evolutionary relationships but only uses derived homologous characters. Derived traits are those characters that are different from the ancestor in form or function. An example of a derived trait for humans is the chin. Ancestral traits, which are sometimes referred to as primitive traits, are those characters inherited from the ancestor. For humans, the opposable thumb is an ancestral primate trait. Cladistic evolutionary relationships are presented in a cladogram and do not indicate time. Groups of organisms that share a common ancestor are part of clade.
Learn more about Reading Phylogenies and Cladograms from the Understanding Evolution web site.
Armstrong WP. c.2002-2008. Parallel and convergent evolution [Internet] [cited 2015 Jun 8]. Available from: http://waynesword.palomar.edu/convevol.htm
Carnegia J, Bruno LC, editors. 2001. Classification. Detroit (MI): UXL. p. 132-136. (UXL Complete Life Science Resource; vol. 1).
Jurmain R, Kilgore L, Trevathan W. Essentials of physical anthropology, 4th edition. Belmont (CA): Wadsworth, Cengage Learning; 2013. 437 p.
Larsen CS. Our origins: discovering physical anthropology. New York (NY): W.W Norton & Company, Inc.; 2008. 430 p.
Lerner JK, Lerner KL. Cladistics. In: Lerner KL , Lerner BW, editors. The Gale Encyclopedia of Science, Vol. 2. Farmington Hills (MI): Gale; 2014. p. 964-965
Lerner KL, BW Lerner, editors. 2014. Evolution, Convergent. Farmington Hills (MI): Gale. p. 1675-1676. (The Gale Encyclopedia of Science; vol. 3).
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O’Neill D. c.1998-2012. Classification of living things: An introduction to the principles of taxonomy with a focus on human classification categories [Internet] [cited 2015 Jun 8]. Available from: http://anthro.palomar.edu/animal/default.htm
Similarities and differences: understanding homology and analogy [Internet]. Berkeley (CA): University of California Museum of Paleontology; c2015 [cited 2015 Jun 8]. Available from: http://evolution.berkeley.edu/evolibrary/article/similarity_hs_01
Understanding Phylogenies [Internet]. Berkeley (CA): University of California Museum of Paleontology; c2015 [cited 2015 Jun 9]. Available from: http://evolution.berkeley.edu/evolibrary/article/0_0_0/evo_05