Biological species concept

It is somewhat of a false dichotomy to represent the biological species concept as the only framework that is defined by the occurrence, or lack thereof, of genetic exchange. Most, if not all, concepts consider genetic exchange to be of primary concern for defining species (e.g. see Sites and Marshall 2004 for a discussion of several methodologies for delimiting species that assume reproductive isolation). As illustrated above, one has only to read Phylogenetic Systematics (Hennig 1966), the canonical treatment for the phylogenetic species concept, to see the importance given to defining reproductive communities as a litmus test for aiding in the description of species. It is more accurate to state that the biological species concept, unlike some others, emphasizes the development of barriers to genetic exchange as the paramount process by which species should be defined (Figure 2.1).

Mayr's (1942) definition that 'Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups' was based on earlier descriptions by Dobzhansky (1935, 1937). This concept thus defines species based upon the presence of reproductive isolation, with the process of speci-ation equivalent to the development of barriers to reproduction. Logically, if one substitutes the process of genetic exchange sensu lato in the place of introgression through sexual recombination, 'reproductive isolation' can be extended to include organisms that do not reproduce sexually (e.g. Arnold 2004a). I do, however, realize that the substitution of 'genetic exchange' (inclusive of, for example, introgression, lateral gene transfer, and viral recombination) in place of strict sexual recombination incorporates many processes and organisms not included by workers like Mayr and Dobzhansky. Mayr (1963, p. 28) did, however, address the perceived difficulties presented for the biological species concept by asexually reproducing organisms. He stated, 'Various proposals have been made to resolve the difficulty that asexuality raises for the biological species concept. Some authors have gone so far as to abandon the biological species concept altogether ... I can see nothing that would recommend this solution. It exaggerates the importance of asexuality. . .' Though Mayr was speaking mainly of animal taxa, he did make a most perceptive observation regarding 'asexual organisms' in general when he stated, 'Indeed clandestine sexuality appears to be rather common among so-called asexual organisms' (Mayr 1963, p. 27). If broadened to include all the various forms of genetic exchange, Mayr's conception that truly asexual organisms are very rare, or non-existent, is supported quite well by subsequent studies (e.g. Gogarten et al. 2002).

As emphasized in section 2.1, a strict application of the biological species concept negates the possibility of species exchanging genes; reproductive isolation defines species and it is thus incorrect to speak of species exchanging genetic material. Within the framework of the biological species concept, the problem of genetic exchange between well-defined species can be addressed in two ways. These two approaches were formulated by Mayr (1942, 1963) and have been used repeatedly by subsequent workers. First, Mayr (1942, 1963) argued— from the standpoint of sexual reproduction and introgression between animal taxa—that if viable, fertile hybrids were produced then one should consider the hybridizing forms to be subspecies or semispecies. Second, he allowed that species might occasionally meet and mate, but that 'The majority of such hybrids are totally sterile . . . Even those hybrids that produce normal gametes in one or both sexes are nevertheless unsuccessful in most cases and do not participate in reproduction. Finally, when they do backcross to the parental species, they normally produce genotypes of inferior viability that are eliminated by natural selection' (Mayr 1963, p. 133). In this way genetic exchange, and in particular introgressive hybridization, is reduced to a rare vagary of biology, and is seen as not worth studying.

Following Harrison (1990), I have defined natural hybridization as successful matings in nature between individuals from two populations, or groups of populations, that are distinguishable on the basis of one or more heritable characters (Arnold 1997). Furthermore, I have incorporated the effect of genetic exchange through its various mechanisms under the description of reticulate evolution (i.e. a web-like set of phylogenetic relationships reflecting genetic exchange [through lateral transfer, viral recombination, introgressive hybridization, etc.] between diverging lineages). In the context of these definitions, the potential evolutionary role of genetic

Stage 1. A uniform species with a large range

Resulting in:

Stage 2. A geographically variable species with a more or less continuous array of similar subspecies (either all subspecies are slight, or some are pronounced)

Followed by:

Process 1. Differentiation into subspecies

Followed by:

Process 2. (a) Isolating action of geographic barriers between some of the populations; also (b) development of isolating mechanisms in the isolated and differentiating subspecies

Resulting in:

Stage 3. A geographically variable species with many subspecies completely isolated, particularly near the borders of the range, and some of them morphologically as different as good species

Resulting in either

Stage 4. Noncrossing; that is, new species with restricted range

Stage 5. Interbreeding; that is, the establishment of a hybrid zone (zone of secondary intergradation)

Resulting in either

Stage 4. Noncrossing; that is, new species with restricted range

Biological Species Concept

Followed by: Process 3. Expansion of range of such isolated populations into the territory of the representative forms

Figure 2.1 Stages and processes associated with allopatric speciation as envisioned by Mayr (1942, p. 160).

Followed by: Process 3. Expansion of range of such isolated populations into the territory of the representative forms

Figure 2.1 Stages and processes associated with allopatric speciation as envisioned by Mayr (1942, p. 160).

or exchange is not diminished by the application of the biological species concept. Under these definitions, it does not represent a violation of the biological species concept for organisms from different evolutionary lineages to form viable, fertile hybrids, recombinants, or reassortants. For example, subspecies and semispecies are defined under the biological species concept in part by the presence of ongoing or potential gene flow (i.e. subspecies are made up of populations of a single species; Mayr

1963, p. 348). For the same reasons, given the definitions from this and my earlier work (Arnold 1997), evolutionary outcomes from genetic exchange such as adaptive trait introgression are also not explicitly discounted by the biological species concept. Yet, Mayr also argued that the results from crosses between even subspecies or semispecies are often maladaptive and transitory. He stated this viewpoint in the following manner: 'In natural populations there is usually severe selection against introgres-sion. The failure of most zones of conspecific hybridization to broaden ... shows that there is already a great deal of genetic unbalance between differentiated populations within a species' (Mayr 1963, p. 132). In contrast to these conclusions, a number of zoological examples have come to light in which natural hybridization is seen to be a frequent and, evolutionarily, important process (e.g. Dowling and DeMarais 1993; Grant et al. 2005).

One way in which to illustrate the importance of genetic exchange is to catalog its widespread occurrence in organisms as diverse as bacteria and mammals (Arnold 1997 and the present book). However, it is equally crucial to emphasize that the rarity of a particular event is not predictive of its potential evolutionary importance. If it were the case that rarity of occurrence predicted unimportance, then mutations that result in an increase in fitness—which of course form the basis for Darwinian or adaptive evolution—would be disregarded. In the same way, the frequent observation of lowered fertility and viability of F1 hybrids, relative to conspecific individuals, could lead one to conclude that these progeny will play a minor role in the evolution of a given species complex. A portion of the following quote was used in section 1.1 to exemplify how minor of a role was ascribed to genetic exchange (in the context of this quote, introgressive hybridization) in the evolution of animal groups. However, it is an excellent example of the conclusion that rarity of occurrence equals lack of importance. 'The total weight of the available evidence contradicts the assumption that hybridization plays a major evolutionary role among higher animals. To begin with, hybrids are very rare among such animals, except in a few groups with external fertilization. The majority of such hybrids are totally sterile, even where they display 'hybrid vigor.' Even those hybrids that produce normal gametes in one or both sexes are nevertheless unsuccessful in most cases and do not participate in reproduction. Finally, when they do backcross to the parental species, they normally produce genotypes of inferior viability that are eliminated by natural selection. Successful hybridization is indeed a rare phenomenon among animals' (Mayr 1963, p133). In contrast to this prediction, there are numerous instances in which, for example, the rarity of viable gametes has not prevented important evolutionary effects from F1 and later-generation hybrid formation (Arnold et al. 1999). One example from the plant literature involves the annual sunflower species Helianthus annuus and Helianthus petiolaris. Experimental crosses involving these two species produce F1 individuals that possess pollen fertilities ranging from 0 to 30% (mean approx. 14%; Heiser 1947). Additionally, experimentally produced F2 and first backcross generation plants produce a maximum seed set of 1 and 2%, respectively (Heiser et al. 1969). Notwithstanding these extremely low levels of fertility and viability in the initial hybrid generations, natural hybridization between H. annuus and H. petiolaris has resulted in at least three stabilized hybrid species (Rieseberg 1991).

It can be seen from the above discussion that the application of the biological species concept has almost always led to the conclusion that genetic exchange, particularly natural hybridization, produces relatively unimportant evolutionary consequences. Yet, the definition of species in terms of reproductive barriers led Dobzhansky to view the process as extremely important. In this case, however, the ascription of importance was once again merely a reflection of the assumption of a uniformly maladaptive outcome from genetic exchange. Dobzhansky (1940, 1970) argued that the significant evolutionary effect from natural hybridization was to finalize the construction of prezygotic barriers to reproduction (i.e. 'reinforcement'; Blair 1955). Reinforcement was hypothesized to occur when individuals from previously allopatric, and genetically divergent, populations came back together spatially and mated, resulting in less fit hybrid offspring and thus selection favoring those individuals that mated with conspecifics (Dobzhansky 1940, 1970; Noor 1995; Hoskin et al.

2005; Lukhtanov et al. 2005). As seen through the window of the biological species concept, such instances reflect the role of natural hybridization as merely a means for finalizing speciation. Though of possibly large evolutionary effect for certain groups of organisms (see Servedio and Noor 2003 for review), reinforcement reflects once again an emphasis on hybridization/genetic exchange as being maladaptive.

Reinforcement is a model describing how natural hybridization can act as a mechanism by which reproductive isolation (and within the biological species concept the process of speciation) is completed. Under this model, natural hybridization plays a key, though secondary, role in 'biological' speciation. Similarly, the majority of zoological, and some botanical, treatments of hybridization have utilized this process as a tool for identifying how barriers to reproduction accumulate (e.g. Ramsey et al. 2003; Britton-Davidian et al. 2005). In many cases, the origin of barriers to reproduction has been inferred using hybrid zones as indicators of incipient speciation. The framework of the biological species concept suggests that speciation is an extended, gradual process occurring in an allopatric setting (Mayr 1942). As an extension of these assumptions, hybridizing taxa are considered to be at some genetically, evolutionarily, and ecologically intermediate point between conspecific and specific status. Studying hybrid zones is thus seen as an adjutant to viewing steps in early stages of speciation (i.e. before reproductive barriers are completed). The inferred utility of hybrid zones for studying 'biological speciation' is reflected in their description as a 'window' (Harrison 1990) through which an evolutionary biologist may peer, or a 'natural laboratory' (Hewitt 1988) in which to experiment. In the context of the biological species concept, hybrid zones allow the opportunity to determine the processes that have been causal in the origin of barriers to one mechanism for genetic exchange; that is, introgressive hybridization.

As stated at the outset of this section, the estimate of the strength of reproductive barriers as the litmus test for species and speciation is not unique to the biological species concept. However, the fact that this species concept has been widely applied has resulted in an emphasis on cessation of gene flow as the necessary and sufficient component for speciation. In a similar way, the strict application of the phylogenetic species concept is also problematic for considerations of evolutionary effects from genetic exchange. However, it is also the case that some work within the context of the phylogenetic species concept has taken into account the possibility of reticulate, rather than simply bifurcating, evolutionary networks (e.g. Baum and Donoghue 1995). As with the biological species concept, and indeed any species concept, applying the phyloge-netic species concept does not need to limit studies that test the importance of reticulate evolution. Furthermore, if the application of any concept limits the study of evolutionary processes, then that species concept must be modified until it reflects biological reality.

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Responses

  • svetlana
    Do hybrids violate the biological species concept?
    3 years ago
  • aatifa
    Does hybridization aplicable to biological species concept?
    3 years ago
  • jayden
    Is hybridisation Applicable to Biological concept species?
    2 years ago
  • ausonia longo
    What is biological speies concerpt?
    2 years ago
  • Jean
    Do hybrids fit the biological speciation concept?
    9 months ago
  • SENAY
    Why the application of the biological species concept is difficult on natural hybridization?
    1 month ago

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