A group of organisms of one species that interbreed and live together within a defined area.

Definition

Species are groups of actually or potentially interbreeding populations which are reproductively isolated from other such groups[13].

The biological species concept has been prevalent in the evolutionary literature for the last several decades and is emphasized in many college-level biology courses. It is probably the species concept most familiar to biologists in diverse fields, such as conservation biology, forestry, fisheries, and wildlife management. Species defined by the biological species concept have also been championed as units of conservation[17].

Theodosius Dobzhansky, a prominent evolutionary geneticist and an important contributor to the modern evolutionary synthesis, characterized the concept of a biological species as a system of populations[7,8]. The gene exchange between these systems (species) is limited or prevented by reproductive isolating mechanisms, such as species-specific breeding behaviors, hybrid sterility, and gametic incompatibility. Thus, under the biological species concept, species are simultaneously a reproductive community, a gene pool, and a genetic system. The study of reproductive isolating mechanisms is central to the biological species concept because these mechanisms provide barriers to gene flow that define the boundaries of the reproductive community and gene pool, and preserve the integrity of the genetic system of the species. In practice, however, isolating mechanisms are rarely studied and species are usually diagnosed by differences in phenotypic (morphological) features.

Despite the long historical acceptance of the biological species concept, it has become controversial because a growing number of evolutionary biologists have found the biological species concept unworkable in a wide variety of situations. Critics of the biological species concept come from the fields of both botany and zoology. A fundamental drawback to this concept is that it is exclusively defined in terms of sexual reproduction. Asexual taxa are obviously excluded from this concept, but it is also true that many species capable of sexual reproduction cannot be easily accommodated within the framework of the biological species concept. From the viewpoint of population genetics, species capable of self-fertilization (e.g., parasitic tapeworms and some plants) and those with mandatory sibling mating are more similar to asexual than to sexually outcrossing species[20]. Species that freely hybridize (open mating systems) with one or more other species yet maintain their evolutionary identity as species also provide a serious challenge to the validity of the biological species concept. Groups of freely hybridizing species are known from plants, insects, and vertebrates[20].

Another important limitation of the biological species concept concerns speciation. The most widely accepted model of speciation is the allopatric model. Generally speaking, the allopatric model entails the geographic subdivision of a single population followed by the differentiation of the isolated subpopulations into new species. Historically, the notion of a correlation between geographic subdivision of populations and speciation grew out of the observation that the closest relatives tend to occupy separate but contiguous geographic areas. Thus, in allopatric speciation lineage independence is achieved when two or more lineages are geographically disjunct. Therefore, isolating mechanisms which are fundamental to the biological species concept have very little, if anything, to do with the process of speciation because the populations undergoing speciation are geographically disjunct from one another. Hence, the evolutionary forces responsible for allopatric speciation have nothing to do with the isolating mechanisms that are a fundamental aspect of the biological species concept. A species concept that fundamentally fails to illuminate the process of speciation cannot provide the intellectual framework for the identification of units of biodiversity. Because it is impossible to study gene flow and reproductive behavior of species known only from fossil remains, the biological species concept cannot be applied to the thousands of species known only from their fossils.

In summary, the major limitations of the biological species concept are that it is irrelevant to allopatric speciation and is inapplicable to: (1) fossil species; (2) organisms reproducing asexually or with extensive self-fertilization; and (3) sexual organisms with open mating systems (species that freely hybridize). On the other hand, the biological species concept is intuitively appealing to people because Homo sapiens fits in the narrow range of sexual reproduction that is the domain of the biological species concept.

Traditional: Species as Real Entities

Different species concepts seek to define species in mutually incompatible ways. Thus, a monophyletic species concept seems not very useful to evolutionary biologists because of difficulties with multiple gene genealogies and paraphyletic remnants. In contrast, the interbreeding concept and other concepts incorporating biological processes of species maintenance (e.g., recognition, ecological, evolutionary, and cohesion concepts) suffer in the eyes of phylogenetic systematists because they lack phylogenetic coherence and produce paraphyletic taxa, or worse. If we were to allow the basal unit of our taxonomy to incorporate paraphyly, it would be harder to justify a strict adherence to monophyly at other taxonomic levels. It is beyond the scope of this article to resolve these difficult issues, but these conceptual conflicts fuel the continued debate, and also highlight the fact that if species are indeed real, objective biological units, their unifying reality has been extremely difficult to verify.

Many ecological and biodiversity studies of actual organisms ignore these difficulties, and assume that species are objectively real basal units. Thus, in ecology, we have theories of global species diversity. In conservation, we have the Endangered Species Act in the US, which prescribes the conservation of threatened taxa we call as species. Populations not viewed as species, particularly putative hybrid taxa (like the red wolf, Canis rufus, of the southeast US), became seen as less valuable, even if rare. How do we recognize that a taxon is hybridized? Obviously, to be a hybrid, it must be a mere intergrade between two, real, objectively identifiable entities. The Endangered Species Act viewed species as important real conservation units and hybrids as unimportant. It did this because it incorporated the species concept in vogue at the time of its enactment, that is the biological species concept, in which hybridization is seen as an unnatural breakdown in isolating mechanisms (Mayr, 1970).

Species Concepts in Phytoplankton

The species concept for most phycologists is based on the morphological characters and hence the term ‘species’ means morphospecies. On the other hand, for evolutionary biologists the term means biological species that can be defined as a reproductive community of populations (reproductively isolated from others) that occupy a specific niche in Nature. If we accept the above definition of species, any talk about biological species in groups (Cyanobacteria, Euglenophyta) where sexual reproduction has not been observed yet is meaningless. Nevertheless, recent concepts argue that it would be unproductive and inconvenient to restrict the term ‘species’ exclusively to one or the other.

Most recognized freshwater algal species appear to be cosmopolitan: they may be found all over the world if studied extensively by specialists. In terms of evolutionary biology, it means that either they have highly efficient means of dispersal or their morphological characters are very static through long evolutionary times. Some species are indeed very static in their morphological characters (‘good species’) others apparently vary within a wide range. Former taxonomic concepts weighted minor morphological differences by giving them a taxonomic, usually intraspecific, rank. This practice, in lack of a sufficient species concept, must lead to infinite separation, finally to individuals. Recent investigators tend to view species formerly considered separate as being synonymous. Without a deeper discussion of species concepts applicable for algae, it is necessary to realize that any phycogeographical discussion is ultimately dependent on the species concept applied.

Since most freshwater habitats are ephemeral, each algal species should have a genetic ability to switch from the vegetative reproduction to a sexual cycle or from vegetative to dormant stage when (or even before) the habitat becomes too unfavorable. Populations of ephemeral habitats are exposed to extremes of environmental variables. Variation of temperature, either on daily or on seasonal scale, is not significantly more moderate than that in the surrounding terrestrial habitats. They have to cope with high irradiation, including the UV wavelengths, pH and conductivity also often vary in large range. These environmental exposures (especially tolerance of high temperature variation, desiccation, and high irradiation) certainly ease transport of propagules. Algae of permanent waters (large, exorheic lakes), especially phytoplankton, do not have to develop such superior tolerance properties, which, on the other hand, may limit their resistance to transport conditions. There are evidences that desiccation and subsequent rewetting in cyanobacterial trichomes leads to lysis of cells in aquatic species while aerophytic ones are able to tolerate this treatment in combination with low or high temperature treatment.

Biological Species Concept

Species are groups of actually or potentially interbreeding populations that are reproductively isolated from other such groups (Mayr, 1942).

This biological species concept has been prevalent in the evolutionary literature for the last several decades and is emphasized in many college level biology courses. It is probably the species concept most familiar to organismal biologists in diverse fields, such as conservation biology, forestry, fisheries, and wildlife management. Species defined by the biological species concept have also been championed as units of conservation (O’Brien and Mayr, 1991).

Theodosius Dobzhansky, a prominent evolutionary geneticist and an important contributor to the modern evolutionary synthesis, characterized the concept of a biological species as a system of populations (Dobzhansky, 1935, 1970). The gene exchange between these systems (species) is limited or prevented by reproductive isolating mechanisms, such as species-specific breeding behaviors, hybrid sterility, and gametic incompatibility. Thus, under the biological species concept, species are simultaneously a reproductive community, a gene pool, and a genetic system. The study of reproductive isolating mechanisms is central to the biological species concept because these mechanisms provide gene flow barriers that define the boundaries of the reproductive community and gene pool thereby preserving the integrity of the species’ genetic system. In practice, however, isolating mechanisms are rarely studied and species are usually diagnosed by differences in morphological features.

A fundamental drawback to the biological species is the concept is that it is exclusively defined in terms of sexual reproduction. Asexual and cyclically parthenogenetic taxa are obviously excluded from this concept, but it is also true that many species capable of sexual reproduction cannot be easily accommodated within the framework of the biological species concept. Species capable of self-fertilization (e.g., parasitic tapeworms and some plants) and those with mandatory sibling mating are more similar to asexual than to sexually outcrossing species (Templeton, 1989) from the viewpoint of population genetics. Species that freely hybridize (open mating systems) with one or more other species yet maintain their evolutionary identity as species also provide a serious challenge to the validity of the biological species concept. Freely hybridizing species are known from plants, insects, and vertebrates (Templeton, 1989).

Another important limitation of the biological species concept concerns speciation. The most widely accepted model of speciation is the allopatric model. Generally speaking, the allopatric model entails the isolation of a subpopulation from the main population followed by the differentiation of the isolated subpopulations into new species. Historically, the notion of a correlation between geographic subdivision of populations and speciation grew out of the observation that the closest relatives tend to occupy separate but contiguous geographic areas. However, isolation mechanisms can be geographical, temporal, or behavioral. In each case, it is a barrier to reproduction and gene flow. Hence, the evolutionary forces responsible for allopatric speciation may not be influenced by the isolating mechanisms that are a fundamental aspect of the biological species concept.

Because it is impossible to study gene flow and reproductive behavior of species known only from fossil remains, the biological species concept cannot be applied to the thousands of species known only from their fossils.

In summary, the major limitations of the biological species concept are that it is inapplicable to: (1) fossil species; (2) organisms reproducing asexually or with extensive self-fertilization; and (3) sexual organisms with open mating systems (species that freely hybridize).

II.B.4. Phylogenetic, Evolutionary, and Genealogical Species Concepts

Other more recently derived species concepts rely more heavily on a retrospective view by defining a species in a strictly historical sense as a separate evolutionary lineage that is internally connected through time (i.e., the “phylogenetic species concept” (Cracraft article in Otte and Endler, 1989; Nixon and Wheeler, 1990), the “evolutionary species concept” (Wiley, 1978), and the “genealogical species concept” (Baum and Shaw, 1995); for a good overview, see Harrison article in Howard and Berlocher, 1998). These species concepts are less oriented toward process and identifying specific biological traits that maintain cohesion within species or promote divergence between species and instead are more oriented toward the final evolutionary result—lineage divergence. That is not to say that process cannot be usefully inferred from looking at historical patterns of lineage sorting and splitting (see The Cohesion Species Concept below and Section VI.A), but the emphasis in these particular species concepts is clearly on pattern rather than on trying to incorporate a variety of biological processes into the species definitions themselves.

Phylogenetic, Evolutionary, and Genealogical Species Concepts

Other more recently derived species concepts rely more heavily on a retrospective view by defining a species in a strictly historical sense as a separate evolutionary lineage that is internally connected through time (i.e., the “phylogenetic species concept” (Cracraft article in Otte and Endler, 1989; Nixon and Wheeler, 1990), the “evolutionary species concept” (Wiley, 1978), and the “genealogical species concept” (Baum and Shaw, 1995); for a good overview, see Harrison article in Howard and Berlocher, 1998; likewise, de Queiroz, 2007). These species concepts are less oriented toward process and identifying specific biological traits that maintain cohesion within species or promote divergence between species and instead are more oriented toward the final evolutionary result – lineage divergence. That is not to say that process cannot be usefully inferred from looking at historical patterns of lineage sorting and splitting (see The Cohesion Species Concept and Incorporation of Genealogical Models), but the emphasis in these particular species concepts is clearly on pattern rather than on trying to incorporate a variety of biological processes into the species definitions themselves.

3.1 The Case for Pluralism

Let us start by introducing three prominent species concepts in biology. There are many more prominent species concepts, but introducing three is sufficient for providing the argument for pluralism. The most common species concept in the biological literature is Mayr's [1970] Biological Species Concept. The Biological Species Concept defines a species taxon as a group of organisms that can successfully interbreed and produce fertile offspring. According to that concept, a species’ integrity is maintained by interbreeding within a species as well as by reproductive barriers between organisms in different species. The Ecological Species Concept defines a species taxon as a lineage of organisms maintained and segmented by ecological forces [Van Valen, 1976]. Stabilizing selection maintains a species’ integrity, while disruptive selection can lead to new species. The Phylogenetic Species Concept (which has multiple versions) defines a species taxon as a basal monophyletic lineage [Mishler and Brandon, 1987]. A monophyletic lineage contains all and only the descendants of a common ancestor. Because monophyletic lineages occur up and down the Linnaean hierarchy, species are defined as basal monophyletic lineages — the smallest lineages represented in Linnaean classifications.

These species concepts, the biological, ecological, and phylogenetic, not only provide different definitions of ‘species,’ their use gives rise to different classifications of the organic world. This is confirmed by numerous empirical investigations. The most glaring discrepancy is between the Biological Species Concept (BSC) and the other two concepts. BSC requires that the organisms of a species exchange genetic information through interbreeding. That requires sexual reproduction. BSC does not require that every member of a species successfully interbreed, but it does require that a sufficient number of the organisms sexuality reproduce to maintain a species’ integrity. The problem is that most of life on this planet does not reproduce sexually but asexually, through cloning or vegetative means. Asexual organisms do not form species according to the BSC. Nevertheless, asexual organisms do form species according to the Phylogenetic Species Concept (PSC) and Ecological Species Concept (ESC). For the PSC, species are defined genealogically, independent of mode of reproduction. For the ESC, species are defined as lineages of organisms maintained by selection forces. PSC's and ESC's classifications of the organic world include asexuals, while BSC's classifications exclude asexuals. These species concepts carve up the world in different ways.

Other cases of species pluralism are more complicated. For example, the BSC and the PSC sort the very same organisms into different species. Consider the case of ancestral species. Many supporters of the BSC believe that a standard form of speciation occurs when a population of a species becomes isolated from the main body of a species and undergoes a ‘genetic revolution.’ The parental species, or ‘ancestral species,’ remains intact. For proponents of the BSC, two species are present in such cases: the ancestral species consisting of A and B, and the new species C, see Figure 3.1.

However, the PSC cannot allow the existence of two species in this case. Recall for the PSC, a species must contain all and only the descendents of a common ancestor. The ancestral species consisting of A and B violates that requirement on species taxa: some of its descendents belong to the new species C. Thus, according to the PSC, either there is one species present (the combination of A, B, and C), or there are three species (the ancestral species A, which went extinct, and two new species, B and C). Either way, the PSC and the BSC cross-classify the very same group of organisms. Take an organism, X, in B. According to the BSC, X belongs to a species consisting of A and B. According to the PSC, X either belongs to a species containing only B or a species containing A, B, and C. Each species concept places X into two different taxa.

The above examples are just the tip of the iceberg of examples where species concepts provide different classifications of the same group of organisms. Generalizing from these examples, different species concepts give rise to different classifications of the organic world. Pluralists believe that examples like these show that we should take a pluralistic approach to biological classification: different species concepts provide different but equally legitimate classifications. Monists disagree. Before turning to monist responses, let us focus on the various brands of pluralism in the literature. This will help further articulate the pluralist's argument.

Ecosystem Considerations

MSY is traditionally a single-species concept, and it conflicts with multispecies or ecosystem science and management. It is generally not possible to simultaneously achieve the MSY for multiple species that are caught in the same fishery. Even if the maximum combined yield of all species is obtained, the species that are most vulnerable will be overfished while those that are least vulnerable will be underfished relative to their single species MSY. Moreover, MSY does not consider interactions among species and the impact of removal of fish on ecosystem dynamics and functioning. The yields obtained by fishing each population at FMSY based on single-species analyses are likely to be much different than predicted from the single-species analyses.

Environmental and ecosystem variability can impact MSY. MSY is an equilibrium concept that has historically been viewed in a deterministic setting. However, even in the absence of fishing, populations vary over time. Therefore, MSY should be calculated as an average over time rather than using equilibrium deterministic calculations. Analyses have shown that average MSY in a stochastic setting is generally less than in a deterministic setting. In fact, fishing using a constant catch based on a deterministic MSY will cause a population to go extinct in the presence of population variability. Therefore, MSY-related quantities that use constant catch in a stochastic environment and define biological risk have been developed (e.g., the concept of maximum constant yield (MCY) used in fisheries management in New Zealand). There has been a recent trend toward reflecting environmental variability in MSY-related quantities. For example, rather than calculating a single equilibrium BMSY value, dynamic BMSY is calculated for each time period as the biomass under FMSY, which takes into consideration the variation in recruitment.

Distinguishing Similar Species

Most modern taxonomists subscribe to the biological species concept in which total reproductive isolation between organisms is taken as an indication of species status. Although the concept has considerable merit, several problems are associated with implementation. For example, most taxonomists work primarily with preserved museum specimens, and reproductive isolation cannot be tested in museum preserved material. Biological control workers, with laboratory and insectary facilities available, are better equipped than most museum taxonomists to carry on reproductive isolation studies.

The confirmation of reproductive isolation through hybridization studies in some cases has led to reevaluation of the comparative morphology of sibling species, which, in turn, has elicited minor but consistent anatomical differences. Examples of hybridization studies with closely related taxa pertinent to biological control include Muscidifurax (Kogan & Legner, 1970), Aphytis (Rao & DeBach, 1969a, 1969b, 1969c) and Trichogramma (Nagarkatti and Nagaraja, 1977). It is important to emphasize that the taxonomy of the cultures and species involved must be carefully researched before taxonomic decisions are made based on hybridization work.

The extent of reproductive isolation has been shown to vary among organisms. Hybridization experiments with natural enemies for use in biological control projects often yield living samples of closely related natural enemies from geographically and ecologically diverse localities. These can provide the raw material for the basic hybridization studies needed to clarify the taxonomic status of similar entomophagous forms.

Not all organisms reproduce sexually. In so-called uniparental organisms, the biological species concept cannot be used to test reproductive isolation because males do not exist or exist at very low percentages of the offspring and may not be functional. The phenomenon of female-only species is called thelytoky by workers in biological control. Unfortunately for biological control workers, thelytoky is common among natural enemies of agricultural pests and presents an obstacle to accurate identification. In the absence of tests for reproductive isolation, morphometric analysis may provide clues to identity of closely related or morphologically nearly identical forms.

Parlatoria pergandii Comstock (chaff scale) represents a problem on citrus in Texas. Aphytis hispanicus (Mercet) and A. comperei (DeBach & Rosen) are among the natural enemies found on chaff scale. Both species are thelytokous and similar (cryptic species). Key anatomical characters used to distinguish the species overlap. However, Woolley and Browning (1987) have used principal component analysis and canonical variate analysis to distinguish between the species. These and other statistical techniques may be used by museum taxonomists when electrophoretic analysis is not possible.

1 Introduction

Chemical substances are the central kinds of chemistry, as important to understanding chemistry as the species concept—or concepts—is to understanding the biological sciences. Chemical substances are elements, compounds or mixtures: elements are just those substances that have no others as components. Elements in this sense are the building blocks of chemical composition. Here are three claims about elements: (i) in the 18th century, long before any direct investigation of atomic structure, chemists used element names with determinate extensions; (ii) membership of those extensions was conferred by having atoms with particular nuclear charges; (iii) the chemical facts that make all this so were unknown until the twentieth century, so if they are known now they must have been discovered.