All other concepts are known to exclude diversity and none of them should be used as a primary concept of species. Importantly, many of these concepts are compatible with one another and together form a diverse net of operational guidelines that permit us to discover and recognize biodiversity consistent with the primary concept, the ESC.
General Biology/Classification of Living Things/Classification and Domains of Life
The hierarchy of concepts that I have proposed helps us achieve our objectives in biodiversity, taxonomy and systematics through a more accurate appraisal of diversity. It relieves much tension about competing concepts, views these concepts in a much more positive sense, and illustrates how these various concepts are mutually beneficial to our goals. It is important in discussions of species to keep in mind the distinctions outlined above regarding Individuals , Classes and Historical Groups.
At least 22 different concepts of species are known to exist plus some slight modifications to each. Furthermore, because of the nature of species in a philosophical sense it is likely that everyone has a slightly to significantly different perception of what a species may be. This perception differs from many reasons but most importantly because species are individuals and because your perception depends upon your.
Agamospecies ASC. Below, following a review of a couple old concepts of species, we are going to examine some of the most commonly used concepts of species today. Phylogenetic Species Concept 3 forms. Taxonomic literature contains various concepts of species. The first two described are presented primarily as an historical perspective. This concept goes back to time of Plato, Aristotle, Linnaeus and therefore has been termed the essentialist's view of species. Variation is an imperfect manifestation of the idea implicit in each species.
Rejection of Concept 1. There is no way to determine the essence of a species. Individuals of the same species can be very different on the basis of sexual dimorphism, age differences, polymorphisms, and other forms of variation. Sibling species differ hardly at all morphologically but are found to represent distinct entities. Degree of difference is not the decisive criterion in ranking a taxon as species.
Lamark, Buffon. They are mental concepts and nothing more. Species have been invented in order that we may refer to great numbers of individuals collectively. They are something for which there is no equivalent in the realm of inanimate objects. It is clear that there are discontinuities in biological diversity. This is demonstrated by both morphological, genetic, ecological, and behavioral data as well as others. Concepts of this type have limited spatial and no temporal dimension of species in question.
Thus, there is no evolutionary, phylogenetic, or lineage perspective with which one can view, perceive, or interpret descent of the taxa or their attributes eg shared plesiomorphies or apomorphies, distances , including the ability or propensity to interbreed. Concepts of this nature may appear to be more operational than those incorporating temporal and geographic components.
However, this convenience compromises both the accuracy and precision with which we are able to identify, quantify, and understand biodiversity. Finally, in this lack of accuracy we also lose our abilities to discover and understand the processes responsible for the evolution, functions, and maintenance of biodiversity. Synopsis: Thus, while the nondimensional species concept has been argued by some as a preferred operational concept of diversity, it has actually been a hindrance to the advancement of comparative and evolutionary biology.
Concepts of this type should not be consider as primary concepts for species. Interestingly, in some areas of science medicine the nondimensional concept has been perceived as grossly inferior to concepts incorporating spatial and temporal dimensions in discovering diversity Paterson, This concept provided a solution to an inanimate definition for animate and potentially changing entities that resulted with the advent of evolutionary thought. It combines the typological and nominalistic concepts in stating that species have independent reality and are typified by the statistics of populations of individuals.
It differs from both in stressing the populational concept and genetic cohesion of species, and by pointing out that it receives its reality from the historically evolved, shared information content of its gene pool This concept also attempted to solve the paradox caused by the conflict between the fixity of species of the naturalist and the fluidity of the species of the evolutionist. This conflict caused Linnaeus to basically deny a concept of evolution and for Darwin to deny the reality of species.
The biological species combines the discreteness of the local species at a given time with the ecological potential for change. This concept also is generally applicable when species are in sympatry. These three ideas raised the Biological Species Concept above the Typological concept of a class of objects.
This concept has been reviewed by its strongest proponent, Mayr, in several publications and several other authors see Mayden and Wood, As recently espoused by Mayr and Ashlock and Mayr , species consist of reproductive communities wherein there is both an ecological and genetic unit.
Individuals of a species seek and recognize one another for mating and thereby maintain an intercommunicating gene pool that, " regardless of the individuals that constitute it, interacts as a unit with other species with which it shares its environment. For Mayr "each biological species is an assemblage of well balanced, harmonious genotypes and As a result, there was a high selective premium for the acquisition of mechanisms, now called isolating mechanisms, that would favour breeding with conspecific individuals and inhibit mating with non-conspecifics.
This consideration provides the true meaning of species. The species is a device for the protection of harmonious, well integrated genotypes. It is this insight on which the biological species concept is based. The word interbreeding in the definition above " indicates a propensity; a spatially or chronologically isolated population, of course, is not interbreeding with other populations but may have the propensity to do so when the extrinsic isolation is terminated " Mayr, Accordingly, speciation is the process of achieving reproductive isolation Mayr, ; The BSC specifically excludes uniparental species even though they are known to exist, and some have relegated diversity of this type to pseudospecies Dobzhansky, The concept also is viewed as being an operational definition in that "taxa of the species category can be delimited against each other by operationally defined criteria, for example, interbreeding versus noninterbreeding of populations" Mayr and Ashlock, This concept is relational because "A is a species in relation to B and C because it is reproductively isolated from them.
Finally, it is a nondimensional concept that " has its primary significance with respect to sympatric and synchronic populations. The more distant two populations are in space and time, the more difficult it becomes to test their species status in relation to each other but the more biologically irrelevant this status becomes. At least ten elements of this concept are viewed by Mayden and Wood as counterproductive toward discovering and understanding biodiversity.
The BSC has received substantial criticism in recent years for issues dealing with the following. Rejection of Concept 1 the absence of a lineage perspective. Synopsis: The nature of the unfavorable attributes inherent in the Biological Species Concept preclude it from being considered a primary species concept. A lineage concept that avoids many of the problems of the biological species concept without denying that interbreeding among sexually reproducing individuals is an important component in species cohesion.
It is compatible with a broader range of reproductive modes and with all speciation models. This concept was championed originally by Simpson , out of a general dissatisfaction with the nondimensionality of the BSC. Wiley , developed the concept further and argued for its general application to biological systems. Unlike other species concepts listed above in the table, the ESC largely was ignored, until recently. Frost and Hillis , Frost and Kluge , Mayden and Wood , Mayden , , and Wiley and Mayden reviewed or further developed the concept. These authors argue that the ESC is the only available concept with the capacity to accommodate all known types of biologically equivalent diversity.
Contrary to the perception of some eg Minelli, the ESC does not consider species as Classes or focus on species as ecological entities. While Simpson advocated a lineage concept to species and ecological and evolutionary divergence, he also condoned the delineation of artifactual successional species. Identity - difficult aspect concerning species, one aspect of estimating whether two forms have different identifies is to examine various mechanisms that promote identity.
The most important would probably be recognition systems permitting individuals to recognize others of their kind and members of other species, e. Such qualities are used by systematists to recognize species, and usually they recognize that those qualities he uses may not be the ones the species itself uses. So phenotypically similar species might be recognized by characters manifested in their breeding systems sometimes given species names such as semispecies, cryptic species. Species can lose their identities and thus become extinct.
This may happen in two ways. Reductive speciation discussed later where two species come together and produce a third. Extinction of an ancestral species or additive speciation. An ancestral species may become extinct if the two or more daughter species are subdivided in such a way that neither have the same fate or tendencies of the ancestor. Evolutionary fate - Characteristics of the individual species at any particular time will determine its evolutionary fate when exposed to future selective pressures.
Historical Tendencies - This include the "phylogenetically inherited" attributes of an individual that will determine, to some extent, how it is capable of responding to selective pressures. There are 4 corollaries that Wiley discusses that are derivatives of the ESC.
All organisms past and present belong to some evolutionary species. That is every organism belongs to a lineage including at least its parents or parent and thus if species are lineages, all organisms must belong to one or another species. Supraspecific taxa then are collections of lineages, whereas species are the lineages themselves. These lineages are placed in higher taxa on the basis of hypothetical past linkages. Thus, natural higher taxa are purely historical constructs whose sole existence depends on how accurately they document the historical unfolding of lineage splitting or speciation.
A phylogenetic tree is composed of evolutionary species. All terminal taxa are species and linkages between them are species. Some have asserted that terminal taxa can be higher taxa and not species. This is rejected with the evolution species concept. Every living organism belongs to some evolution species, but only the various rules of nomenclature demand that each species belong to other "mandatory" categories.
Example: If species A is the ancestor to Phylum X it does not follow that it must belong to any other sub-taxon in Phylum X. Only various nomenclature rules require that we place it in other subtaxa. Wiley asserts that these rules have no particular reality in nature and these redundant names may be biologically meaningless and perhaps misleading.
Taxonomy should proceed to group as nature has produced groups. Evolution is a process of lineage splitting and higher taxa arise with their stem species Hennig, and not as Simpson claimed, because of some subsequent evolution within the higher taxa.
Describing Species | Columbia University Press
This makes sense. No genus could have originated before the family to which it belongs could have originated. Thus, a family, order, or class at the time of its origin is a single evolutionary species. Species must be reproductively isolated from each other to the extent that this is required for maintaining their separate identities, tendencies and fates. The concept of interbreeding is rather complex. The ability to interbreed depends on at least 3 factors:. How one views the evidence of intergradation and hybridization depends heavily on how one views species and these 3 factors.
However, viewed from the standpoint of the evolutionary species concept, the important question is not whether two species hybridize, but whether two species do or do not lose their distinct ecological and evolutionary roles. If, despite some hybridization, they do not merge, then they remain separate species in the evolutionary perspective. A result of the above is that it makes one feel as if he or she is committing heresy if he or she describes an allopatric form as a species after all speciation is not "complete".
In other words, this is the test for speciation subsequent sympatry without intergradation. Typically, people describe these things as subspecies when in fact they may be different evolutionary species. This causes confusion in the evolution community in that we have some things that are called subspecies as a matter of convenience and others that are probably different lineages. From work on Drosophila and many other groups, we learn that most differences develop in allopatry thus speciation is completed in allopatry.
What then would be done with zones of intergradation or hybridization or contact zones? If the 2 forms are sister groups, then we have to assess the significance of the contact zone a bit closer. If zone is wide, then the 2 forms are probably geographic variants of the same evolutionary species. Studies of scutation scale characteristics , eye size, and color pattern in the eastern and western parts of their range resulted in data interpreted as intergradation.
Electrophoretic studies showed that each species had unique alleles for certain loci 3 in fasciata , one in sipedon. Specimen from the same study areas as Blaney and Blaney showed no evidence of polymorphism at these loci. That is, all specimens were either fixed for the unique alleles of one or the other species and there were no allelic hybrids. This indicated that here is no interbreeding between the 2 forms and that the interpretation of intermediacy from morphological characters should be called "apparent intergradation" probably a result of some residual [remaining] geographic variation in an ancestral species.
In other words, here is a case in which the contact zone should be examined a bit closer and more critically than it was at first. A zone of intergradation can be wide if the particular characters examined are not characters that reinforce species identity.
So 2 species can tolerate some gene exchange for functional genes which do not affect their basic identities. In other words those characters that do not interfere with species recognition by the species themselves probably should not be worried about from the taxonomists' standpoint.
Intergradation and Hybridization - Phylogenetic position of species. This brings up the interesting point that the significance of both hybridization and introgression depends on the phylogenetic position of the species. The ability to breed successfully is, potentially, a primitive character. Within the history of any lineage, reproductive compatibility is an attribute of the members of the ancestral species of that lineage; an attribute which is gradually diminished and ultimately lost in its descendents during geographic differentiation..
Reproductive compatibility is not necessarily evidence of close relationship. In swordtails Teleostei, Poeciliidae of the genus Xiphophorus this potentiality has become a reality. Rosen has studied both the phylogenetic relationships and interbreeding potential of four species of Xiphophorus. These species and their interrelationships are shown in the figure below "X. Xiphophorus signum is the most derived species of the group and is reproductively isolated from X. Xiphophorus signum is also reproductively isolated from X. But, X. We may conclude that X.
Their ability to interbreed is due to the retention of plesiomorphic features, not to the fact that they are closest relatives. One should be aware of the assumption that the ability to interbreed denotes a close phylogenetic relationship. In this case, that criterion would have led to a mistake in analysis. Phylogenetic relationships among four species of swordtails Xiphophorus.
Intergrades are found between X. Xiphophorus sp. Data from Rosen, Evolutionary species may or may not exhibit recognizable phenetic differences and an investigation may over or under estimate diversity. Inadequate data may lead to an over estimate of diversity. But inadequate data may also lead to under estimate. No presumed separate, single, evolutionary lineage may be subdivided into a series of ancestral and descendant species. This practice is common in paleontology. Each subdivision is recognition of a series of ancestor-descendant populations undergoing anagenesis. Referred to as paleospecies , chronospieces, successive, or successional species.
Division here is arbitrary. Unsuitable for evolutionary studies because they receive the same range as "good" species and one will not know which are good and which are paleospecies. Likewise, one cannot know when character evolution is the result of speciation and which is anagenesis. Recognition of phyletic species is arbitrary. Arbitrary species produce arbitrary mechanisms. In accepting phyletic speciation, we must give up any hope of reaching a general synthesis of the evolution of biotas because speciation in one group cannot be compared to speciation in another group.
Evolutionary species concept provides the theoretical basis for interpreting branching diagrams as evolutionary trees. All terminal branches are species and they are connected by common ancestral species. Each branch is therefore the result of a speciation event. The ESC is not considered an operational concept.
It is a lineage concept that is non-relational. Thus, the attributes and patterns of species can be correctly interpreted with respect to their unique descent. The ESC accommodates uniparentals, species formed by hybridization, and ancestral species. There is no threshold for particular attributes needed for the existence of a species. Finally, reproductive isolation, is considered a derived attribute from the plesiomorphic status of reproductive compatibility; reproductive success is thus largely uninformative. Synopsis: The ESC is the most theoretically significant of the species concepts; it accommodates all "types" of species known to date and thus has the greatest applicability.
As such, the ESC can serve as a primary concept.
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The reliance upon criteria such as "substantially reproductively isolated" and "evolutionary legacy" incorporates attributes traditionally viewed as qualities of species from other concepts. These components are nothing more than the "identities" of cohesive groups of organisms through time and over space, possessing their own independent evolutionary fate and historical tendencies advocated in the ESC.
While the ESU has been proposed as a concept targeted at revealing "distinct" populations within species Waples, , , the distinction between "distinct" populations and species as natural, evolutionary entities is not made clear. Synopsis: This concept excludes known biodiversity, thereby unduly biasing our perception of process. Incorrect assumptions about diversity targeted for protection, brought about by misconceived formulations, only obstructs efforts to understand and preserve it.
While basically a lineage concept, its emphasis on genetics and isolation preclude its use as primary concept. This is probably considered the most sensible and commonly used "method" of species definition by taxonomists, general biologists, and laypersons alike.
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Because in the vast majority of situations involving allopatric populations little or no information is available regarding reproductive independence, morphological distinctiveness serves only as a surrogate to lineage independence. This concept also bridges a decided gap inherent in some other concepts between sexual and asexual species, so long as morphological distinctiveness is heritable and is representative of lineage independence.
Given that humans are a vision-oriented species, it is readily appealing as an operational concept. Kornet considers morphology in its widest sense wherein "similarity between organisms may thus be perceived in macromorphology as well as in gene-structure, and may range from shared 'sets of independent characters' for classical taxonomists to shared 'unique combinations of character states' for pattern cladists The only real problem with a morphological concept involves instances of sibling or cryptic species, or the retention of plesiomorphic morphologies.
Here, little or no morphological divergence has accompanied the acquisition of lineage independence and two or more different species may appear "similar. Another potential problem with this concept is the inherent tendency to require an arbitrary level of morphological divergence. By employing such a criterion the researcher assumes that all morphological traits, especially those traditionally employed in a taxon, evolve at a constant rate of divergence. This is an unjustified assumption and is falsified by the observation that even within a taxonomic group morphological divergence is largely random.
Synopsis : This is a nondimensional concept that treats species as Classes, defining them on the basis of particular essential morphological attributes. Possession of these essential attributes provides for membership in the species. As such it does not allow the researcher to treat species as historical entities forming lineages. As individuals, the definition of every species will necessarily change as the essential attributes of a species at t 1 will be different from t 2 through descent. While this concept has served as a traditional method for "identifying" species it is fatally flawed as a primary concept.
This is a nondimensional and strictly operational concept that may be likened to any concept where overall similarity is the primary criterion for the existence of species. Operationally, where variation in a set of characters is less "within" a group than "between" groups the entity is recognized as a distinct taxon. Species are treated as Classes under this concept; they do not exist as lineages and, if a species changes through descent, then the diagnosis will have to be revised.
Synopsis: While essentially the methodology employed by taxonomists, the barren theoretical nature of this concept precludes its use as a primary concept. Currently at least three different concepts of species are identified as "phylogenetic. Some argue that with the growing popularity of phylogenetics it is critical to have a definition to identify the smallest units suitable for analysis boundary between toko- and phylogenetic processes.
For some, species is the smallest unit appropriate for analysis, and infraspecific units are inappropriate in this context Nixon and Wheeler, ; Wheeler and Nixon, This same perspective holds that species diversity must be understood before a phylogenetic analysis is performed. Others defend the position that hierarchical patterns exist within species and phylogenetic methods are appropriate de Queiroz and Donoghue, , ; McKitrick and Zink, Species are thus the biological entities and unit product of natural selection and descent.
Consequently, subspecies, fraught with ambiguities between convenience and naturalness, is not an appropriate evolutionary unit and has no ontological status Cracraft, ; McKitrick and Zink, ; Warren, The different PSCs form three general Classes; one emphasizing monophyly, one emphasizing diagnosability, and one emphasizing both. Note: this uniqueness cannot be part of the definition because ancestors cannot have unque traits and therefore cannot be part of a species.
This Class of definitions emphasizes the a priori diagnosability of species, irrespective of a criterion of monophyly. There are two purported benefits of this perspective. First, process is not invoked before pattern is observed. Second, phylogenetic methodologies are argued to be applicable only to genealogical relationships of species and supraspecific taxa, not below the level of integration of species wherein tokogenetic relationships of infraspecific entities are the norm sensu Wheeler and Nixon, ; Nixon and Wheeler, To conduct a phylogenetic analysis below the level of species would confuse the reticulate tokogenetic relationships with the usual non-reticulate phylogenetic relationships.
For proponents of this concept, monophyly, paraphyly, and polyphyly apply only at a level of organization above species. Species are delimited by the distributions of fixed, diagnostic characters across populations. Where variability exists in an attribute within the taxon this attribute is considered inappropriate for that level of analysis where only tokogenetic, not phylogenetic, relationships exist.
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CABI Bioscience Introduction 1. Introduction 2. Biological Nomenclature II. Recognizing Species 3. Species and Their Discovery Species Concepts 4. Establishing Identity: The Literature Search 5.