Organism

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An organism is any living thing that functions as an individual. [1] Such a definition raises more problems than it solves, not least because the concept of an individual is also difficult. Many criteria, few of them widely accepted, have been proposed to define what an organism is. Among the most common is that an organism has autonomous reproduction, growth, and metabolism. This would exclude viruses, despite the fact that they evolve like organisms. Other problematic cases include colonial organisms; a colony of eusocial insects is organised adaptively, and has germ-soma specialisation, with some insects reproducing, others not, like cells in an animal's body. The body of a siphonophore, a jelly-like marine animal, is composed of organism-like zooids, but the whole structure looks and functions much like an animal such as a jellyfish, the parts collaborating to provide the functions of the colonial organism.

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The evolutionary biologists David Queller and Joan Strassmann state that "organismality", the qualities or attributes that define an entity as an organism, has evolved socially as groups of simpler units (from cells upwards) came to cooperate without conflicts. They propose that cooperation should be used as the "defining trait" of an organism. This would treat many types of collaboration, including the fungus/alga partnership of different species in a lichen, or the permanent sexual partnership of an anglerfish, as an organism.

Etymology

The term "organism" (from the Ancient Greek ὀργανισμός , derived from órganon, meaning 'instrument, implement, tool', 'organ of sense', or 'apprehension') [2] [3] first appeared in the English language in the 1660s with the now-obsolete meaning of an organic structure or organization. [3] It is related to the verb "organize". [3] In his 1790 Critique of Judgment , Immanuel Kant defined an organism as "both an organized and a self-organizing being". [4] [5]

Whether criteria exist, or are needed

One criterion proposes that an organism cannot be divided without losing functionality. This basil plant cutting is however developing new adventitious roots from a small bit of stem, forming a new plant. Basilikumwurzling.jpg
One criterion proposes that an organism cannot be divided without losing functionality. This basil plant cutting is however developing new adventitious roots from a small bit of stem, forming a new plant.

Among the criteria that have been proposed for being an organism are:

Other scientists think that the concept of the organism is inadequate in biology; [13] that the concept of individuality is problematic; [14] and from a philosophical point of view, question whether such a definition is necessary. [15] [16] [8]

Problematic cases include colonial organisms: for instance, a colony of eusocial insects fulfills criteria such as adaptive organisation and germ-soma specialisation. [17] If so, the same argument, or a criterion of high co-operation and low conflict, would include some mutualistic (e.g. lichens) and sexual partnerships (e.g. anglerfish) as organisms. [18] If group selection occurs, then a group could be viewed as a superorganism, optimized by group adaptation. [19]

Another view is that attributes like autonomy, genetic homogeneity and genetic uniqueness should be examined separately, rather than requiring that an organism possess all of them. On this view, there are multiple dimensions to biological individuality, resulting in several types of organism. [20]

Organisms at differing levels of biological organisation

A lichen consists of a body formed mainly by fungi, with unicellular algae or cyanobacteria (green) interspersed within the structure, and a bacterial microbiome. The species are mutually interdependent, like cells within a multicellular organism. Lichen cross section - heteromeric thallus unlabelled.svg
A lichen consists of a body formed mainly by fungi, with unicellular algae or cyanobacteria (green) interspersed within the structure, and a bacterial microbiome. The species are mutually interdependent, like cells within a multicellular organism.

Differing levels of biological organisation give rise to potentially different understandings of the nature of organisms. A unicellular organism is a microorganism such as a protist, bacterium, or archaean, composed of a single cell, which may contain functional structures called organelles. [22] A multicellular organism such as an animal, plant, fungus, or alga is composed of many cells, often specialised. [22] A colonial organism such as a siphonophore is a being which functions as an individual but is composed of communicating individuals. [8] A superorganism is a colony, such as of ants, consisting of many individuals working together as a single functional or social unit. [23] [17] A mutualism is a partnership of two or more species which each provide some of the needs of the other. A lichen consists of fungi and algae or cyanobacteria, with a bacterial microbiome; together, they are able to flourish as a kind of organism, the components having different functions, in habitats such as dry rocks where neither could grow alone. [18] [21] The evolutionary biologists David Queller and Joan Strassmann state that "organismality" has evolved socially, as groups of simpler units (from cells upwards) came to cooperate without conflicts. They propose that cooperation should be used as the "defining trait" of an organism. [18]

Queller and Strassmann's view of organisms as cooperating entities at differing levels of biological organisation [18]
LevelExampleCompositionMetabolism,
growth,
reproduction		Co-operation
Virus Tobacco mosaic virus Nucleic acid, protein NoNo metabolism, so not living, not an organism, say many biologists; [7] but they evolve, their genes collaborating to manipulate the host [18]
Unicellular organism Paramecium One cell, with organelles e.g. cilia for specific functionsYesInter-cellular (inter-organismal) signalling [22]
Swarming protistan Dictyostelium (cellular slime mould)Unicellular amoebae YesFree-living unicellular amoebae for most of lifetime; swarm and aggregate to a multicellular slug, cells specialising to form a dead stalk and a fruiting body [18]
Multicellular organism Mushroom-forming fungusCells, grouped into organs for specific functions (e.g. reproduction)YesCell specialisation, communication [22]
Permanent sexual partnership Anglerfish Male and female permanently fastened togetherYesMale provides male gametes; female provides all other functions [18]
Mutualism Lichen Organisms of different species Yes Fungus provides structure, absorbs water and minerals; alga photosynthesises [18]
Joined colony Siphonophore Zooids joined togetherYesOrganism specialisation; inter-organism signalling [8]
Superorganism Ant colonyIndividuals living togetherYesOrganism specialisation (many ants do not reproduce); inter-organism signalling [23]

Samuel Díaz‐Muñoz and colleagues (2016) accept Queller and Strassmann's view that organismality can be measured wholly by degrees of cooperation and of conflict. They state that this situates organisms in evolutionary time, so that organismality is context dependent. They suggest that highly integrated life forms, which are not context dependent, may evolve through context-dependent stages towards complete unification. [24]

Boundary cases

Viruses

A virus such as tobacco mosaic virus is not a cell; it contains only its genetic material, and a protein coat. TMV structure simple.png
A virus such as tobacco mosaic virus is not a cell; it contains only its genetic material, and a protein coat.

Viruses are not typically considered to be organisms, because they are incapable of autonomous reproduction, growth, metabolism, or homeostasis. Although viruses have a few enzymes and molecules like those in living organisms, they have no metabolism of their own; they cannot synthesize the organic compounds from which they are formed. In this sense, they are similar to inanimate matter. [7] Viruses have their own genes, and they evolve. Thus, an argument that viruses should be classed as living organisms is their ability to undergo evolution and replicate through self-assembly. However, some scientists argue that viruses neither evolve nor self-reproduce. Instead, viruses are evolved by their host cells, meaning that there was co-evolution of viruses and host cells. If host cells did not exist, viral evolution would be impossible. As for reproduction, viruses rely on hosts' machinery to replicate. The discovery of viruses with genes coding for energy metabolism and protein synthesis fuelled the debate about whether viruses are living organisms, but the genes have a cellular origin. Most likely, they were acquired through horizontal gene transfer from viral hosts. [7]

Comparison of cellular organisms and viruses [7]
Capability Cellular organism Virus
Metabolism YesNo, rely entirely on host cell
Growth YesNo, just self-assembly
Reproduction YesNo, rely entirely on host cell
Store genetic information about themselves DNA DNA or RNA
Able to evolve Yes: mutation, recombination, natural selection Yes: high mutation rate, natural selection

There is an argument for viewing viruses as cellular organisms. Some researchers perceive viruses not as virions alone, which they believe are just spores of an organism, but as a virocell - an ontologically mature viral organism that has cellular structure. [25] Such virus is a result of infection of a cell and shows all major physiological properties of other organisms: metabolism, growth, and reproduction, therefore, life in its effective presence. [12] [26]

Organism-like colonies

Apolemia, a colonial siphonophore that functions as a single individual Apolemia sp.jpg
Apolemia , a colonial siphonophore that functions as a single individual

The philosopher Jack A. Wilson examines some boundary cases to demonstrate that the concept of organism is not sharply defined. [8] In his view, sponges, lichens, siphonophores, slime moulds, and eusocial colonies such as those of ants or naked molerats, all lie in the boundary zone between being definite colonies and definite organisms (or superorganisms). [8]

Jack A. Wilson's analysis of the similar organism-like nature of siphonophores and jellyfish [8]
FunctionColonial siphonophore Jellyfish
BuoyancyTop of colony is gas-filledJelly
Propulsion Nectophores co-ordinate to pump waterBody pulsates to pump water
Feeding Palpons and gastrozooids ingest prey, feed other zooidsTentacles trap prey, pass it to mouth
Functional structureSingle functional individualSingle functional individual
CompositionMany zooids, possibly individualsMany cells

Synthetic organisms

Insect cyborg Insect cyborg.jpg
Insect cyborg

Scientists and bio-engineers are experimenting with different types of synthetic organism, from chimaeras composed of cells from two or more species, cyborgs including electromechanical limbs, hybrots containing both electronic and biological elements, and other combinations of systems that have variously evolved and been designed. [27]

An evolved organism takes its form by the partially understood mechanisms of evolutionary developmental biology, in which the genome directs an elaborated series of interactions to produce successively more elaborate structures. The existence of chimaeras and hybrids demonstrates that these mechanisms are "intelligently" robust in the face of radically altered circumstances at all levels from molecular to organismal. [27]

Synthetic organisms already take diverse forms, and their diversity will increase. What they all have in common is a teleonomic or goal-seeking behaviour that enables them to correct errors of many kinds so as to achieve whatever result they are designed for. Such behaviour is reminiscent of intelligent action by organisms; intelligence is seen as an embodied form of cognition. [27]

Related Research Articles

<span class="mw-page-title-main">Evolution</span> Gradual change in the heritable traits of organisms

Evolution is the change in the heritable characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, resulting in certain characteristics becoming more or less common within a population over successive generations. The process of evolution has given rise to biodiversity at every level of biological organisation.

<span class="mw-page-title-main">Life</span> Matter with biological processes

Life is a quality that distinguishes matter that has biological processes, such as signaling and self-sustaining processes, from matter that does not. It is defined descriptively by the capacity for homeostasis, organisation, metabolism, growth, adaptation, response to stimuli, and reproduction. All life over time eventually reaches a state of death, and none is immortal. Many philosophical definitions of living systems have been proposed, such as self-organizing systems. Viruses in particular make definition difficult as they replicate only in host cells. Life exists all over the Earth in air, water, and soil, with many ecosystems forming the biosphere. Some of these are harsh environments occupied only by extremophiles.

<span class="mw-page-title-main">Carl Woese</span> American microbiologist (1928–2012)

Carl Richard Woese was an American microbiologist and biophysicist. Woese is famous for defining the Archaea in 1977 through a pioneering phylogenetic taxonomy of 16S ribosomal RNA, a technique that has revolutionized microbiology. He also originated the RNA world hypothesis in 1967, although not by that name. Woese held the Stanley O. Ikenberry Chair and was professor of microbiology at the University of Illinois Urbana–Champaign.

<span class="mw-page-title-main">Colony (biology)</span> Living things grouping together, usually for common benefit

In biology, a colony is composed of two or more conspecific individuals living in close association with, or connected to, one another. This association is usually for mutual benefit such as stronger defense or the ability to attack bigger prey.

<span class="mw-page-title-main">Superorganism</span> Group of synergistic organisms

A superorganism, or supraorganism, is a group of synergetically-interacting organisms of the same species. A community of synergetically-interacting organisms of different species is called a holobiont.

<span class="mw-page-title-main">Multicellular organism</span> Organism that consists of more than one cell

A multicellular organism is an organism that consists of more than one cell, unlike unicellular organisms. All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- and partially multicellular, like slime molds and social amoebae such as the genus Dictyostelium.

<span class="mw-page-title-main">Evolutionary biology</span> Study of the evolution of life

Evolutionary biology is the subfield of biology that studies the evolutionary processes such as natural selection, common descent, and speciation that produced the diversity of life on Earth. In the 1930s, the discipline of evolutionary biology emerged through what Julian Huxley called the modern synthesis of understanding, from previously unrelated fields of biological research, such as genetics and ecology, systematics, and paleontology.

<span class="mw-page-title-main">Evolution of sexual reproduction</span>

Evolution of sexual reproduction describes how sexually reproducing animals, plants, fungi and protists could have evolved from a common ancestor that was a single-celled eukaryotic species. Sexual reproduction is widespread in eukaryotes, though a few eukaryotic species have secondarily lost the ability to reproduce sexually, such as Bdelloidea, and some plants and animals routinely reproduce asexually without entirely having lost sex. The evolution of sexual reproduction contains two related yet distinct themes: its origin and its maintenance. Bacteria and Archaea (prokaryotes) have processes that can transfer DNA from one cell to another, but it is unclear if these processes are evolutionarily related to sexual reproduction in Eukaryotes. In eukaryotes, true sexual reproduction by meiosis and cell fusion is thought to have arisen in the last eukaryotic common ancestor, possibly via several processes of varying success, and then to have persisted.

<span class="mw-page-title-main">Unit of selection</span> Biological entity within the hierarchy of biological organization

A unit of selection is a biological entity within the hierarchy of biological organization that is subject to natural selection. There is debate among evolutionary biologists about the extent to which evolution has been shaped by selective pressures acting at these different levels.

A metasystem transition is the emergence, through evolution, of a higher level of organization or control.

Cheating is a term used in behavioral ecology and ethology to describe behavior whereby organisms receive a benefit at the cost of other organisms. Cheating is common in many mutualistic and altruistic relationships. A cheater is an individual who does not cooperate but can potentially gain the benefit from others cooperating. Cheaters are also those who selfishly use common resources to maximize their individual fitness at the expense of a group. Natural selection favors cheating, but there are mechanisms to regulate it. The stress gradient hypothesis states that facilitation, cooperation or mutualism should be more common in stressful environments, while cheating, competition or parasitism are common in benign environments.

Enquiry into the evolution of ageing, or aging, aims to explain why a detrimental process such as ageing would evolve, and why there is so much variability in the lifespans of organisms. The classical theories of evolution suggest that environmental factors, such as predation, accidents, disease, and/or starvation, ensure that most organisms living in natural settings will not live until old age, and so there will be very little pressure to conserve genetic changes that increase longevity. Natural selection will instead strongly favor genes which ensure early maturation and rapid reproduction, and the selection for genetic traits which promote molecular and cellular self-maintenance will decline with age for most organisms.

In evolutionary biology, function is the reason some object or process occurred in a system that evolved through natural selection. That reason is typically that it achieves some result, such as that chlorophyll helps to capture the energy of sunlight in photosynthesis. Hence, the organism that contains it is more likely to survive and reproduce, in other words the function increases the organism's fitness. A characteristic that assists in evolution is called an adaptation; other characteristics may be non-functional spandrels, though these in turn may later be co-opted by evolution to serve new functions.

<span class="mw-page-title-main">Living systems</span> Multiple interactions and regulation of life forms with their environment

Living systems are life forms treated as a system. They are said to be open self-organizing and said to interact with their environment. These systems are maintained by flows of information, energy and matter. Multiple theories of living systems have been proposed. Such theories attempt to map general principles for how all living systems work.

<span class="mw-page-title-main">Eusociality</span> Highest level of animal sociality a species can attain

Eusociality is the highest level of organization of sociality. It is defined by the following characteristics: cooperative brood care, overlapping generations within a colony of adults, and a division of labor into reproductive and non-reproductive groups. The division of labor creates specialized behavioral groups within an animal society, sometimes called castes. Eusociality is distinguished from all other social systems because individuals of at least one caste usually lose the ability to perform behaviors characteristic of individuals in another caste. Eusocial colonies can be viewed as superorganisms.

<span class="mw-page-title-main">Species</span> Basic unit of taxonomic classification, below genus

A species is a population of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. Other ways of defining species include their karyotype, DNA sequence, morphology, behaviour, or ecological niche. In addition, palaeontologists use the concept of the chronospecies since fossil reproduction cannot be examined. The most recent rigorous estimate for the total number of species of eukaryotes is between 8 and 8.7 million. About 14% of these had been described by 2011. All species are given a two-part name, called a "binomial". The first part of a binomial is the genus to which the species belongs. The second part is called the specific name or the specific epithet. For example, Boa constrictor is one of the species of the genus Boa, with constrictor being the species' epithet.

Evolution of cells refers to the evolutionary origin and subsequent evolutionary development of cells. Cells first emerged at least 3.8 billion years ago approximately 750 million years after Earth was formed.

<span class="mw-page-title-main">Allorecognition</span>

Allorecognition is the ability of an individual organism to distinguish its own tissues from those of another. It manifests itself in the recognition of antigens expressed on the surface of cells of non-self origin. Allorecognition has been described in nearly all multicellular phyla.

<span class="mw-page-title-main">Outline of evolution</span> Overview of and topical guide to change in the heritable characteristics of organisms

The following outline is provided as an overview of and topical guide to evolution:

<span class="mw-page-title-main">Holobiont</span> Host and associated species living as a discrete ecological unit

A holobiont is an assemblage of a host and the many other species living in or around it, which together form a discrete ecological unit through symbiosis, though there is controversy over this discreteness. The components of a holobiont are individual species or bionts, while the combined genome of all bionts is the hologenome. The holobiont concept was initially introduced by the German theoretical biologist Adolf Meyer-Abich in 1943, and then apparently independently by Dr. Lynn Margulis in her 1991 book Symbiosis as a Source of Evolutionary Innovation. The concept has evolved since the original formulations. Holobionts include the host, virome, microbiome, and any other organisms which contribute in some way to the functioning of the whole. Well-studied holobionts include reef-building corals and humans.

References

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  27. 1 2 3 Clawson, Wesley P.; Levin, Michael (1 January 2023). "Endless forms most beautiful 2.0: teleonomy and the bioengineering of chimaeric and synthetic organisms". Biological Journal of the Linnean Society. 138 (1): 141. doi: 10.1093/biolinnean/blac116 . ISSN   0024-4066.
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