Species and the Heirarchy of Life

Main Points:

  1. Hierarchies
  2. Individuals
  3. Emergent Properties
  4. Structural Hierachy
  5. Species
  6. Linnean Hierarchy
  7. Phylogentic Hierarchy
  8. Ecological Hierarchy

1. What are the entities of life?

What are the operational units, the interactors, the individuals, the organisms we need to discuss?

These are not trivial questions because the answers define the kinds of questions we ask about life and its role as well as define our place in the natural world. One aspect of The Life System course that is different than other Earth Science courses, is that it tends to deal with what at first might seem philosophical issues, but which in reality sets the tone for the real science that gets done.

One aspect of life that was used as a criterion for life in the previous lecture was that "Species make societies of kin and parasitic to symbiotic relationships with other species."

This suggests that life has a hierarchical nature, and indeed this seems to be a fundamental aspect of Life. However, the exact nature of this hierarchy is debated and forms the subject of this lecture.

It is very important to realize that we cannot pretend that this lecture does more than touch on these topics, since whole courses, even careers have been built on their exegesis. Similarly, the definitions below are not offered as "definitive" but rather as operational for this course and a philosophical jumping off point for our discussions. It is critical to realize however that these "philosophical issues" have immense practical implications. Our text is for the most part blissfully without such worries and it is important to be aware of this.

What is a hierarchy?

A hierarchy is a structural or classificatory system characterized by individuals or classes that are ordered by rank.

Hierarchies can be inclusive (i.e. nested) or pyramidal (i.e. non-nested). Tyler Volk makes a nice distinction here by noting that: "A body physically includes its organs, but the manager of a music shop does not." (Volk, 1995).

Inclusive hierarchies can be depicted as pyramidal, but many pyramidal heirarchies are not inclusive.

Non-inclusive pyramidal hierarchies can be purely functional as in control information systems (e. g. a military force), or ecosystems. Inclusive hierarchies can be nominal, as in a classificatory system that might or might not reflect something about reality, or they can be structural as in the parts of a cell.

Hierarchies have levels. The thing at a specific level in a hierarchy is called a holon. Within the Linnean hierarchy below, all of the holons are taxa, once of which is a taxon.

Holon (taxon) Holon (taxon) Name
           Homo sapiens

In this kind of hierarchical list, the uppermost levels are the most inclusive.

Biological examples of hierarchies include:

Inherent to the notion of hierarchies is the concept of the individual...

What is an individual?

An individual is a spaciotemporally localized material body that either remain relatively unchanged or undergoes relatively continuous change (paraphrased from Hull, 1992).

Individuals are the objects that can have properties that allow them to interact with the environment in a coherent way. For the concept to be meaningful we have to distinguish classes from individuals.

A class is an assemblage of entities. A class might or might not be an individual. A single chair can be considered an individual. But, a room full of chairs is not.

Within Life, you or I am an individual. The classroom full of students and professors is probably not. Our species, Homo sapiens might be, however. A very helpful discussion of what an individual is supplied by Hull (1992).

One of the most profound aspects of Life is the tendency for the nature of individuals to be very different across kinds of organisms and change dramatically through time. To understand what is meant by this we have to consider a properties that individuals have, which is emergent properties.

What is an emergent property?

An emergent property is one that is a consequence of the synergy of the parts within a system that makes the whole greater than the sum of its parts.

Often, it is quite impossible to predict the emergent properties of individuals.

For example, a book consists of papers with words comprised of letters, bound together. A single book is an individual. However, the meaning of the words, comprising the story that is told by the book, is an emergent property, that cannot be deduced from an analysis of the components alone.

In the same way the catalogue of genes that will come forth as a result of the "Human Genome Project" will not tell you what a person is or why you differ from me. It will not even tell you how a person is built. To understand that, you must understand how the genes function and interact within the human system.

For a useful discussion of emergent properties see: http://alf.nbi.dk/~emmeche/coPubl/97e.EKS/emerg.html

Life's structural hierarchy

An important biological hierarchy is the Structural Hierarchy.
    Individual Organism

Physiologists, anatomists, morphologists and our text add several levels:
    Individual Organism
      Organ System

In contrasting these two versions of the structural hierarchy it is useful to think about making the distinction between levels that might be considered individuals with some level of independence and strong emergent properties.

It is easy to see the structural links between individual organisms that can be physically subdivided into classes of parts, but it is harder to see the liks upward into populations and species. These seem different because populations and species are not as spaciotemporally closed as, say cells within an individual organism - although the distinction is far from as clean as it might seem.

We can recognize individual organisms and we can recognize a variety of classes into which they fall. Some of these classes will have emergent properties that make them individuals as well at a different level in the structural hierarchy.

A peculiar example of this jumping of levels is seem amongst some members of group of "sea squirts" or tunicates (Phylum Chordata, Subphylum Urocordata), called thaliacians (Class Thaliacea). Thaliacians are planktonic sea squirts that filter nutritive particles and organisms out of seawater as their barrel-like bodies float along. drawing of doliolid

We would have no problem recognizing most thaliacian individuals as standard individual organisms. They consist of cells, tissues, and organs with a definite body plan. They develop from the union of gametes, grow, mature, reproduce and die. The doliolids and salps are good examples. See Drawing of a doliolid tunicate at right; mouth is on upper left (based on photograph in Pearse et al., 1987).

However, some thaliacians the pyrosomes, produce giant barrel-shaped colonies that float through the water. These pyrosome "colonies" can in excess of 10 m long. See drawing at left of a 10 m long pyrosome (based on photographs in Pearse et al., 1987.) The colony propels itself through the water by means of cilia that pump water through the "bodies" of what would be called the individual tunicates. However, the colony responds to mechanical, chemical, and light stimulus by moving and by spectacular bioluminescent displays. Thus, the colony acts much like an individual salp or doliolid. What is the individual here?

Perhaps a more fundamental example is that of the Eukaryotic cell itself.

What is a species?

Amongst organismal biologists this is one of the most difficult and contentious issues. Reams of publications deal with the question. The answer has great practical import, more and more so, especially with our biodiversity crisis (e. g. Mayden and Wood, 1995).

A large numbers of definitions have been proffered for what a species is, however, in virtually all biologists minds a species is the fundamental unit of life.

Here is a summary of a some definitions of species grouped as different species concepts.

Typological species concept: A species is a group of individuals expressing an underlying unitary ideal in which the variation seen among the individuals imperfect manifestations of the "type." Its conceptual origins lie with Plato and Aristotle.

Morphological species concept: A species is a group of individuals united by common morphology, different than other such groups. This is operational definition used by most systematists.

Biological species concept: Species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. (Mayr 1942).

Evolutionary species concept: A species is a lineage (an ancestor descendant sequence) of populations or organisms that maintains its identity from other such lineages and which has its own evolutionary tendencies and historical fete. (Wiley 1978).

Phylogenetic species concept: A species is the smallest monophyletic group of common ancestry. (de Queiroz and Donoghue 1990). An irreducible cluster of organisms that is diagnosably distinct from other such clusters, and within which there is a parental pattern of ancestry and descent. (Cracraft 1989).

Genotypic cluster species concept: A species is a common genetic pool that are identified by genetic differences from other groups. This is new definition based on differences in DNA sequences.

Recognition species concept: A species is the most inclusive population of individual biparental organisms that share a common fertilization system. (Paterson 1985).

Cohesion species concept: A species is the most inclusive population of individuals having the potential for phenotypic cohesion through intrinsic cohesion mechanisms. (Templeton 1989).

Ecological species concept: A species is a lineage (or a closely related set of lineages) that occupies an adaptive zone minimally different from that of any other lineage in its range and which evolves separately from all lineages outside its range. (Van Valen 1976).

Nominalist species concept: A species is an arbitrary class or cluster of organisms given a name as a handle.

Taxonomic species concept: A species is what a good taxonomist says it is (Simpson, 1961).

The huge variety of definitions reflect, not just changing theory, but different purposes to which the species are used by us.

In any case, species have a temporal extent that is obviously much larger than an individual organism. Species have a continuity of genetic information and are the units (along with populations) that evolve. Species are probably good individuals with emergent properties, although this is also hotly debated.

Probably the most useful definitions, and the ones we will concentrate on here are the Morphological, Biological, and Evolutionary species. The specific mechanisms by which species originate and are maintained will be discussed in Part 2 of this course.

For more discussion of species concepts see these links:




2. The Linnean Hierarchy

LinneausCarolus Linnaeus (Carl von Linné) (1706-1778) (portrait at right) was a Swedish naturalist who took it upon himself to classify the entire natural world.

He became popularly known as "God's Registrar" Linnaeus recognized that interbreeding showed that there was a natural break between organisms that would freely interbreed and those that would not. This was a fact known well by breeders of dogs and other domestic animals.

Linnaeus saw that there was a "unity of type". He showed that the basic unit of natural classification was not the individual but the "species", and that they were recognizable by the fact that the individuals within a species would freely interbreed. This definition was close to our biological species concept.

In other words a species is held together by sex. In a monumental work on the sexual system in plants Linnaeus showed that the "biological definition of a species" holds true just as well for animals.

Nonetheless, in most cases species are recognized by the fact that members of a species tend to look much more like each other than they do to members of other species (i. e., the morphological species concept).

In 1758 Linnaeus published his grand opus: SYSTEMA NATURAE

In this work he outlined not only the known species of animals and plants, but also what has become to be known as the "Linnean Hierarchy" of Taxonomic levels.

Linnaeus noticed that while the fundamental unit was the species, that species could be grouped by similarity of structure into larger groups: genera, families, orders, classes, phyla, and kingdoms (and others in between). We call any entity within this hierarchy a taxon (plural - taxa). Thus, a species is a taxon as is a family. This is an inclusive hierarchy.

In other words, kingdoms consist of and contain classes, classes consist of and contain orders, orders consist of and contain families, families consist of and contain genera, genera consist of and contain species.

In this "Linnean Hierarchy" species are handled specially - a fact you need to know and be familiar with.

Here is the Linnean Hierarchy for us:

  Phylum Chordata
    Class Mammalia
      Order Primates
        Family Hominidae
          Genus Homo
            Species Homo sapiens

The Linnean Hierarchy is more than just a classificatory tool for a pharmacopoeia or a gene catalogue. It can reflect the evolutionary relationships of organisms, as will be discussed later in the course and in this week's lab.

In the Cladistic methodology, the levels in the Linnean Hierarchy reflect the evolutionary acquisition of novel characters within the common ancestor (a species) of a group.

Within the Linnean Hierarchy, while the species has characteristics that make it an individual, higher taxonomic levels do not; they are groups of species, united by common descent.

3. The Phylogenetic Hierarchy

The Linnean Hierarchy has a relatively fixed number of levels and was developed by many different philosophical means, orginally without reference to evolution. Species grouped into higher level taxa according to their phylogenetic relationships (from a common ancestor) form a special kind of hierarchy termed a phylogenetic hierarchy.

Most systematists now use some sort of phylogenetic hierarchy and use the names for the holons used within the Linnean Hierarchy.

4. The Ecological Hierarchy

While the most fundamental units of life might be individual organisms and the species and these united within a structural hierarchy, and evolution results in a "natural" order of life reflected in phylogenetic and Linnean hierarchies, there are systems of interacting organisms that transcend all of these.

Examination of any single organisms or patch of environment reveals interacting parts comprising systems, some of which may have emergent properties and may be individuals.

Ecology is the field that deals with the interactions between organisms and between organisms and their environment. Various ecological hierarchies have been introduced for different purposes. For our purposes we will use the following:

        Individual Organisms


A term originally coined by geologist Eduard Suess (1875) meaning the totality of living beings on Earth, as well as those parts of the oceans, atmosphere, and lithosphere with which living organisms interact. It was much elaborated on by the Russian biogeochemist Vladimir Vernadsky (see Vernadsky, 1997). See http://www.kheper.net/gaia/biosphere/biosphere.htm for a discussion of the concept and origin of the term. A term is more recently being used synonomously with the Biosphere, but with considerably more "intellectual baggage", is James Lovelock's Gaia.


Term introduced by ecological botanist Sir Arthur George Tansley (1871-1955) (portrait at right) in 1935 to include. "... the whole system ... including not only the organism-complex, but also the whole complex of physical factors forming what we call the environment ...". He meant it to be the basic unit of ecology. To some, ecosystems are individuals with strong emergent properties.


An assemblage of organisms that share a habitat.

Note that individual organisms are shared in this hierarchy with the structural hierarchy above.

The ecological hierarchy radically departs from the structural, phylogenetic, and Linnean hierarchies in that the flow of material and energy is inherently emphasized. The levels within the ecological hierarchy are complex systems and impossible to understand without considering their dynamic behavior.

A useful example of an ecosystem is a lake, because it is small enough to understand and study, and is relatively well bounded.


Cracraft, J., 1989, The empirical consequences of alternative species concepts for understanding patterns and processes of differentiation. in Otte, D. and Endler, J. A. (eds.) Speciation and its Consequences, Sinauer, Sunderland, p. 28-59.

Hull, D. L., 1992, Individual. in Keller, E. F. and Lloyd, E. A. (eds.) Keywords in Evolutionary Biology, Harvard University Press, Cambridge, p. 180-187.

Lovelock, J. E., 1979, Gaia; A New Look at Life on Earth. Oxford Univ. Press, Oxford.

Mayden R. and Wood, R., 1995. Systematics, species concepts, and the evolutionarily significant unit in biodiversity and conservation biology. American Fisheries Society Symposium, v. 17, p. 58-113

Mayr, E., 1942, Systematics and the Origin of Species. Columbia University Press, New York.

Simpson, G. G., 1961, Principles of Animal Taxonomy. Columbia University Press, New York.

Suess, E., 1875, Die Enstchung der Alpen. W. Braunmuller, Vienna.

Tansley, A., 1935, The use of vegitational concepts and terms. Ecology, v. 16, p. 284-307.

Vernadsky, V. I., The Biosphere. Copernicus (Springer-Verlag), New York.

Volk, T., 1995, Metapatterns, Columbia University Press. 296 p.

Wiley, E. O., 1978, The evolutionary species concept reconsidered. Systematic Zoology. v. 27, p. 17-26.

Updated January 19, 2006
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