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The species problem is the set of questions that arises when
biologist A biologist is a professional who has specialized knowledge in the field of biology, understanding the underlying mechanisms that govern the functioning of biological systems within fields such as health, technology and the Biophysical environm ...
s attempt to define what a
species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...

species
is. Such a definition is called a species concept; there are at least 26 recognized species concepts. A species concept that works well for sexually reproducing organisms such as birds may be useless for species that reproduce asexually, such as bacteria. The scientific study of the species problem has been called microtaxonomy. One common, but sometimes difficult, question is how best to decide which species an
organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological me ...

organism
belongs to, because reproductively isolated groups may not be readily recognizable, and
cryptic species In biology, a species complex is a group of closely related organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties ...
may be present. There is a continuum from ''
reproductive isolation The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, ethology, behaviors and physiology, physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensur ...
'' with no interbreeding, to ''
panmixis Panmixia (or panmixis) means random mating. A panmictic population is one where all individuals are potential partners. This assumes that there are no mating restrictions, neither genetic nor behavioural, upon the population and that therefore al ...
'', unlimited interbreeding. Populations can move forward or backwards along this continuum, at any point meeting the criteria for one or another species concept, and failing others. Many of the debates on species touch on philosophical issues, such as
nominalism In metaphysics Metaphysics is the branch of philosophy Philosophy (from , ) is the study of general and fundamental questions, such as those about reason, Metaphysics, existence, Epistemology, knowledge, Ethics, values, Philosophy of ...
and
realism Realism, Realistic, or Realists may refer to: In the arts *Realism (arts), the general attempt to depict subjects truthfully in different forms of the arts Arts movements related to realism include: *Classical Realism *Literary realism, a movem ...
, and on issues of
language A language is a structured system of communication Communication (from Latin Latin (, or , ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken in the ...

language
and
cognition Cognition () refers to "the mental action or process of acquiring knowledge and understanding through thought, experience, and the senses". It encompasses many aspects of intellectual functions and processes such as: attention, the formation of ...
. The current meaning of the phrase "species problem" is quite different from what
Charles Darwin Charles Robert Darwin (; ; 12 February 1809 – 19 April 1882) was an English naturalist Natural history is a domain of inquiry involving organism In biology, an organism () is any organic, life, living system that fu ...

Charles Darwin
and others meant by it during the 19th and early 20th centuries. For Darwin, the species problem was the question of
how new species arose
how new species arose
. Darwin was however one of the first people to question how well-defined species are, given that they constantly change.


History


Before Darwin

The idea that an organism reproduces by giving birth to a similar organism, or producing seeds that grow to a similar organism, goes back to the earliest days of farming. While people tended to think of this as a relatively stable process, many thought that change was possible. The term ''species'' was just used as a term for a sort or kind of organism, until in 1686
John Ray John Ray Fellow of the Royal Society, FRS (29 November 1627 – 17 January 1705) was a Christian England, English Natural history, naturalist widely regarded as one of the earliest of the English parson-naturalists. Until 1670, he wrote his na ...

John Ray
introduced the biological concept that species were distinguished by always producing the same species, and this was fixed and permanent, though considerable variation was possible within a species.
Carolus Linnaeus Carl Linnaeus (; 23 May 1707 – 10 January 1778), also known after his Nobility#Ennoblement, ennoblement as Carl von Linné#Blunt, Blunt (2004), p. 171. (), was a Swedish botanist, zoologist, taxonomist, and physician who formalised binomia ...

Carolus Linnaeus
(1707–1778) formalized the
taxon In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechani ...
omic rank of species, and devised the two part naming system of
binomial nomenclature In taxonomy Taxonomy (general) is the practice and science of classification of things or concepts, including the principles that underlie such classification. The term may also refer to a specific classification scheme. Originally used only ...
that we use today. However, this did not prevent disagreements on the best way to identify species. The history of definitions of the term ''species'' reveal that the seeds of the modern species debate were alive and growing long before Darwin.


From Darwin to Mayr

Charles Darwin Charles Robert Darwin (; ; 12 February 1809 – 19 April 1882) was an English naturalist Natural history is a domain of inquiry involving organism In biology, an organism () is any organic, life, living system that fu ...

Charles Darwin
's famous book ''
On the Origin of Species ''On the Origin of Species'' (or, more completely, ''On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life''),The book's full original title was ''On the Origin of Species by Me ...
'' (1859) offered an explanation as to how species , given enough time. Although Darwin did not provide details on how species can split into two, he viewed
speciation Speciation is the evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes that are ...

speciation
as a gradual process. If Darwin was correct, then, when new ''incipient species'' are forming, there must be a period of time when they are not yet distinct enough to be recognized as species. Darwin's theory suggested that there was often not going to be an objective fact of the matter, on whether there were one or two species. Darwin's book triggered a crisis of uncertainty for some biologists over the objectivity of species, and some came to wonder whether individual species could be objectively real — i.e. have an existence that is independent of the human observer. In the 1920s and 1930s, theory of inheritance and Darwin's theory of evolution by
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
were joined in what was called the
modern synthesis Modern synthesis or modern evolutionary synthesis refers to several perspectives on evolutionary biology, namely: * Modern synthesis (20th century), the term coined by Julian Huxley in 1942 to denote the synthesis between Mendelian genetics and s ...
. This conjunction of theories also had a large impact on how biologists think about species. Edward Poulton anticipated many ideas on species that today are well accepted, and that were later more fully developed by
Theodosius Dobzhansky Theodosius Grygorovych Dobzhansky ( uk, Теодо́сій Григо́рович Добжа́нський; russian: Феодо́сий Григо́рьевич Добржа́нский; January 25, 1900 – December 18, 1975) was a prominent Ukra ...
and
Ernst Mayr Ernst Walter Mayr (; 5 July 1904 – 3 February 2005) was one of the 20th century's leading evolutionary biologists File:Francesco Redi.jpg, Francesco Redi, the founder of biology, is recognized to be one of the greatest biologists of all ti ...
, two of the architects of the modern synthesis. Dobzhansky's 1937 book articulated the genetic processes that occur when incipient species are beginning to diverge. In particular, Dobzhansky described the critical role, for the formation of new species, of the evolution of
reproductive isolation The mechanisms of reproductive isolation are a collection of evolutionary mechanisms, ethology, behaviors and physiology, physiological processes critical for speciation. They prevent members of different species from producing offspring, or ensur ...
.


Mayr's ''Biological Species Concept''

Ernst Mayr Ernst Walter Mayr (; 5 July 1904 – 3 February 2005) was one of the 20th century's leading evolutionary biologists File:Francesco Redi.jpg, Francesco Redi, the founder of biology, is recognized to be one of the greatest biologists of all ti ...
's 1942 book was a turning point for the species problem. In it, he wrote about how different investigators approach species identification, and he characterized their approaches as species concepts. He argued for what came to be called the ''Biological Species Concept'' (BSC), that a species consists of populations of organisms that can reproduce with one another and that are reproductively isolated from other populations, though he was not the first to define "species" on the basis of reproductive compatibility. For example, Mayr discusses how proposed this kind of definition of "species" in 1753.
Theodosius Dobzhansky Theodosius Grygorovych Dobzhansky ( uk, Теодо́сій Григо́рович Добжа́нський; russian: Феодо́сий Григо́рьевич Добржа́нский; January 25, 1900 – December 18, 1975) was a prominent Ukra ...
was a contemporary of Mayr and the author of a classic book about the evolutionary origins of reproductive barriers between species, published a few years before Mayr's. Many biologists credit Dobzhansky and Mayr jointly for emphasizing reproductive isolation. After Mayr's book, some two dozen species concepts were introduced. Some, such as the
Phylogenetic Species Concept In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individu ...
(PSC), were designed to be more useful than the BSC for describing species. Many authors have professed to "solve" or "dissolve" the species problem. Some have argued that the species problem is too multidimensional to be "solved" by any one concept. Since the 1990s, others have argued that concepts intended to help describe species have not helped to resolve the species problem. Although Mayr promoted the BSC for use in
systematics Biological Biology is the natural science Natural science is a branch of science Science (from the Latin word ''scientia'', meaning "knowledge") is a systematic enterprise that Scientific method, builds and Taxonomy (general), o ...
, some systematists have criticized it as not
operational An operational definition specifies concrete, replicable procedures that reliably produce a differentiated, measurable outcome. For example, an operational definition of fear often includes measurable physiologic responses such as tachycardia, galva ...
. For others, the BSC is the preferred definition of species. Many geneticists who work on
speciation Speciation is the evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes that are ...

speciation
prefer the BSC because it emphasizes the role of reproductive isolation. It has been argued that the BSC is a natural consequence of the effect of sexual reproduction on the dynamics of natural selection.


Philosophical aspects


Realism

Realism Realism, Realistic, or Realists may refer to: In the arts *Realism (arts), the general attempt to depict subjects truthfully in different forms of the arts Arts movements related to realism include: *Classical Realism *Literary realism, a movem ...
, in the context of the species problem, is the philosophical position that species are real mind-independent entities,
natural kind "Natural kind" is a label to which scholars have assigned incompatible meanings. Some treat it as a classification identifying some structure of truth and reality that exists whether or not humans recognize it. Others treat it as a human classific ...
s. Mayr, a proponent of realism, attempted to demonstrate species exist as natural, extra-mental categories. He showed for example that the New Guinean tribesman classify 136 species of birds, which Western ornithologists came to independently recognize: Mayr's argument however has been criticized: Another position of realism is that natural kinds are demarcated by the world itself by having a unique property that is shared by all the members of a species, and none outside the group. In other words, a natural kind possesses an essential or intrinsic feature (“essence”) that is self-individuating and non-arbitrary. This notion has been heavily criticized as
essentialist Essentialism is the view that every entity has a set of attributes that are necessary to its identity and function. In early Western thought, Plato's idealism held that all things have such an "essence"—an "idea" or "form". In Categories (A ...
, but modern realists have argued that while biological natural kinds have essences, these need not be fixed and are prone to change through
speciation Speciation is the evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes that are ...

speciation
. According to Mayr (1957) reproductive isolation or interbreeding "supplies an objective yardstick, a completely non-arbitrary criterion” and "describing a presence or absence relationship makes this species concept non-arbitrary". The BSC defines species as "groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups". From this perspective, each species is based on a property (reproductive isolation) that is shared by all the organisms in the species that objectively distinguishes them.


Nominalism

Some philosophical variants of
nominalism In metaphysics Metaphysics is the branch of philosophy Philosophy (from , ) is the study of general and fundamental questions, such as those about reason, Metaphysics, existence, Epistemology, knowledge, Ethics, values, Philosophy of ...
propose that species are just names that people have assigned to groups of creatures but where the lines between species get drawn does not reflect any fundamental underlying biological cut-off point. In this view, the kinds of things that people have given names to, do not reflect any underlying reality. It then follows that species do not exist outside the mind, because species are just named abstractions. If species are not real, then it would not be sensible to talk about "the origin of a species" or the "evolution of a species". As recently at least as the 1950s, some authors adopted this view and wrote of species as not being real. A counterpoint to the nominalist views in regard to species, was raised by
Michael Ghiselin Michael T. Ghiselin (born May 13, 1939) is an American biologist, and philosopher as well as historian of biology, formerly at the California Academy of Sciences. He is known for his work on sea slugs, and for his criticism of the falsification ...
who argued that an individual species is not a type, but rather an actual individual, an actual
entity An entity is something that exists as itself, as a subject or as an object, actually or potentially, concretely or abstractly, physically or not. It need not be of material existence. In particular, abstraction Abstraction in its main sense is ...
. This idea comes from thinking of a species as an evolving dynamic population. If viewed as an entity, a species would exist regardless of whether or not people have observed it and whether or not it has been given a name.


Pragmatism

A popular alternative view,
pragmatism Pragmatism is a philosophical tradition that considers words and thought as tools and instruments for prediction, problem solving, and action, and rejects the idea that the function of thought is to describe, represent, or mirror reality. Pra ...
, espoused by philosophers such as
Philip Kitcher Philip Stuart Kitcher (born 20 February 1947) is a British philosophy professor teaching at Columbia University Columbia University (also known as Columbia, and officially as Columbia University in the City of New York) is a Private un ...
and John Dupré states while species do not exist in the sense of
natural kind "Natural kind" is a label to which scholars have assigned incompatible meanings. Some treat it as a classification identifying some structure of truth and reality that exists whether or not humans recognize it. Others treat it as a human classific ...
s, they are ''
concept Concepts are defined as abstract ideas A mental representation (or cognitive representation), in philosophy of mind Philosophy of mind is a branch of philosophy that studies the ontology and nature of the mind and its relationship with the bo ...

concept
ually'' real and exist for convenience and for practical applications. For example, regardless of which definition of species one uses, one can still quantitatively compare
species diversity Species diversity is the number of different species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the ...
across regions or decades, as long as the definition is held constant within a study. This has practical importance in advancing
biodiversity Biodiversity is the biological variety and Genetic variability, variability of life, life on Earth. Biodiversity is a measure of variation at the Genetics, genetic, species, and ecosystem level. Terrestrial biodiversity is usually greater near ...

biodiversity
science and
environmental science Environmental science is an interdisciplinary Interdisciplinarity or interdisciplinary studies involves the combination of two or more academic disciplines into one activity (e.g., a research project). It draws knowledge from several oth ...
.


Language and the role of human investigators

The nominalist critique of the view that kinds of things exist, raises for consideration the role that humans play in the species problem. For example, Haldane suggested that species are just mental abstractions. Several authors have noted the similarity between "species", as a word of ambiguous meaning, and points made by
Wittgenstein Ludwig Josef Johann Wittgenstein ( ; ; 26 April 1889 – 29 April 1951) was an Austrian Austrian may refer to: * Austrians, someone from Austria or of Austrian descent ** Someone who is considered an Austrian citizen, see Austrian nationalit ...
on
family resemblance Family resemblance (german: Familienähnlichkeit, link=no) is a philosophical idea made popular by Ludwig Wittgenstein Ludwig Josef Johann Wittgenstein ( ; ; 26 April 1889 – 29 April 1951) was an Austrian Austrian may refer to: * Austri ...
concepts and the indeterminacy of language. Jody Hey described the species problem as a result of two conflicting motivations by biologists: # to categorize and identify organisms; # to understand the evolutionary processes that give rise to species. Under the first view, species appear to us as typical natural kinds, but when biologists turn to understand species evolutionarily they are revealed as changeable and without sharp boundaries.
Hey Hey or Hey! may refer to: Music * Hey (band), a Polish rock band Albums * Hey (Andreas Bourani album), ''Hey'' (Andreas Bourani album) or the title song (see below), 2014 * Hey! (Julio Iglesias album), ''Hey!'' (Julio Iglesias album) or the ti ...
argued that it is unrealistic to expect that one definition of "species" is going to serve the need for categorization and still reflect the changeable realities of evolving species.


Pluralism and monism

Many approaches to the species problem have attempted to develop one single common conception of what species are and of how they should be identified. It is thought that, if such a
monistic Monism attributes oneness or singleness (Greek: μόνος) to a concept e.g., existence. Various kinds of monism can be distinguished: * ''Priority monism'' states that all existing things go back to a source that is distinct from them; e.g ...
description of species could be developed and agreed upon, then the species problem would be solved. In contrast, some authors have argued for pluralism, claiming that biologists cannot have just one shared concept of species, and that they should accept multiple, seemingly incompatible ideas about species.
David Hull David Lee Hull (15 June 1935 – 11 August 2010) was an American philosopher who was most notable for founding the field philosophy of biology. In addition to his academic prominence, he was well known as a gay man who fought for the rights of ot ...
however argued that pluralist proposals were unlikely to actually solve the species problem.


Quotations

"No term is more difficult to define than "species," and on no point are zoologists more divided than as to what should be understood by this word." Nicholson (1872). "Of late, the futility of attempts to find a universally valid criterion for distinguishing species has come to be fairly generally, if reluctantly, recognized" Dobzhansky (1937). "The concept of a species is a concession to our linguistic habits and neurological mechanisms" Haldane (1956). "An important aspect of any species definition whether in
neontology Neontology is a part of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, ph ...
or
palaeontology Paleontology (), also spelled palaeontology or palæontology, is the scientific study of life that existed prior to, and sometimes including, the start of the Holocene epoch (geology), epoch (roughly 11,700 years before present). It includes th ...
is that any statement that particular individuals (or fragmentary specimens) belong to a certain species is an hypothesis (not a fact)" Bonde (1977). "The species problem is the long-standing failure of biologists to agree on how we should identify species and how we should define the word 'species'." Hey (2001). "First, the species problem is not primarily an empirical one, but it is rather fraught with philosophical questions that require — but cannot be settled by — empirical evidence." Pigliucci (2003). "We show that although discrete phenotypic clusters exist in most
lant Lant is aged urine Urine is a liquid by-product A by-product or byproduct is a secondary product derived from a production process, process or ; it is not the primary product or service being produced. A by-product can be useful and marketa ...
/nowiki> genera (> 80%), the correspondence of taxonomic species to these clusters is poor (< 60%) and no different between plants and animals. ... Contrary to conventional wisdom, plant species are more likely than animal species to represent reproductively independent lineages." Rieseberg et al. (2006).


See also

*
Evolutionarily significant unitAn evolutionarily significant unit (ESU) is a population of organism In biology, an organism (from Ancient Greek, Greek: ὀργανισμός, ''organismos'') is any individual contiguous system that embodies the Life#Biology, properties of ...
(ESU) *
Ring species In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanisms, ...

Ring species


References


External links

{{Wikispecies, Species
A catalogue of species conceptions

Curiosities of Biological Nomenclature
Species Unsolved problems in biology