1 Although the details may vary, this model of 1 2 speciation still holds, at least for that What’s in a name? 2 3 reproduce sexually (asexual organisms and many 3 4 plants bend the rules, but that needn’t worry is the field of study dealing with biological classification. 4 5 Lumping & Splitting birders!). Why then is the BSC under siege? Humans classify objects for many reasons, using a variety of criteria. For 5 6 example, we could classify using criteria such as ease of capture, pal- 6 7 The complexities of taxonomy The problems: atability of their flesh, etc. Such a classification would be useful if you relied 7 8 on hunting birds as a source of protein. However, it is an ‘artificial’ classifi- 8 9 Are you confused by the disjunct populations cation, because it is unlikely that it would represent the evolutionary history 9 10 of birds, and the criteria used are subjective, so that different people would 10 11 appearance of ‘new’ and hybridization arrive at different classifications. 11 12 and bemused by the disap- Taxonomists strive for ‘natural’ classifications which represent the evolu- 12 13 The greatest practical problem with the BSC tionary history of organisms, using objective criteria. We use the Linnaean 13 14 pearance of old friends with is that it fails to provide an objective measure system of classification, both for naming organisms and representing their 14 15 every new edition of your for allopatric taxa (that is, those which do not evolutionary relationships. It is hierarchical in structure, placing similar taxa 15 16 occur at the same place at the same time). How together in a nested series of categories which imply relationships, and thus 16 17 favourite field guide? Or are can we define species on the basis of inter- conveys much more than a simple list of species. Birders are at least periph- 17 18 breeding if they never come into contact? erally aware of this classification system. Scientific names, unique to each 18 19 you simply happy to accumu- has numerous examples of disjunct 19 species, are a combination of two categories in the Linnaean system, 20 populations associated with the south-west and 20 late ‘armchair ticks’? and species names. Thus the Sooty Albatross has the name Phoebetria fusca, 21 north-east arid zones. In the past these areas 21 which is the terminal portion of its full classification: 22 The Percy FitzPatrick were linked, allowing arid-country birds to 22 Kingdom: Animalia (all animals) 23 disperse throughout the eastern part of Africa. 23 Institute’s Peter Ryan explains Phylum: Chordata (animals with some form of ‘backbone’) 24 Subsequent invasion by moist 24 Subphylum: Vertebrata (the vertebrates) 25 why bird taxonomy – the throughout central East Africa has isolated the 25 Class: Aves (all birds) 26 field that names species – is arid zones, dividing the ranges of many birds. 26 27 Some of these disjunct populations have Order: Procellariiformes (tube-nosed seabirds) 27 28 undergoing rapid revisions changed very little and are still regarded as the Family: Diomedeidae (albatrosses) 28 Genus: Phoebetria (sooty albatrosses) 29 after almost half a century same species (for example, 29 30 after almost half a century Polihierax semitorquatus), while others have Species: fusca (the dark sooty albatross) 30 31 evolved regional differences and are recognized This system is sufficient to represent the evolutionary history of relatively 31 of relative stability. warwick tarboton 32 by at least some authorities as separate species undiverse taxa such as the albatrosses. However, it struggles to accommo- 32 33 he species is the basic unit by which we ‘reality’. Biological species are self-defining, (for example, Northern Eurocephalus rueppelli date more diverse groups. Evolutionary relationships within large orders such 33 34 Tclassify animals and plants. It is a concept whereas other categories are human constructs. and Southern white-crowned shrikes E. anguiti- as the can only be represented by the addition of many more hier- 34 35 central to birding – the first thing you do when This is neatly encapsulated in the adage: mens). However, there is no consensus as to archical levels (parvorder, superfamily, subfamily, tribe, and so on). 35 36 faced by a group of individual birds is to iden- ‘Subspecies are a matter of opinion, genera a how much the disjunct populations must dif- The Linnaean system started out as an artificial, phenetically-based classi- 36 37 tify them to species. But matter of convenience, but species are a mat- fer to be recognized as separate species. We fication system – after all, it was developed before Darwin and Wallace 37 38 what is a species? This is ter of fact’. enter a grey area of subjective assessment described the basic principles of evolution. Today it represents our best 38 39 the question at the heart The BSC has its origins in the ‘modern syn- where the BSC provides no assistance. understanding of evolutionary relationships. Because ongoing research 39 40 of the current revolution thesis’ of biology, which married Darwin’s The other main problem with the BSC relates reveals new and sometimes unexpected relationships between taxa, the 40 41 in bird taxonomy. Two theory of evolution through natural selection to hybridization. When isolated populations system of classification is being modified constantly. The most important 41 42 classes of species concepts with Mendel’s theory of heredity. The major 42 once again come into contact, we can apply development in recent years for birds has been the monumental DNA-DNA 43 are battling for supremacy, proponents of the BSC were ornithologists. the BSC to test whether speciation has occurred. 43 hybridization study by Charles Sibley and Jon Ahlquist. The results of this 44 the outcome of which will Two deserve specific mention: David Lack, for However, unless the populations have diverged 44 work still have not been accepted fully by the ornithological establishment, 45 have profound conse- his brilliant explanation of variation and spe- to the point where they no longer perceive 45 in part due to a reluctance to countenance major changes. However, it 46 quences for birders. ciation among Darwin’s Finches, and Ernst each other as potential mates, first contact is a 46 seems that it will only be a matter of time before bird books are produced 47 In the beginning there Mayr, the chief architect of the BSC. messy process. Often some degree of inter- 47 in the ‘new sequence’ which places woodpeckers and parrots ahead of 48 was the Biological Species The Biological Species Concept has been breeding or hybridization takes place. If hybrids 48 49 Concept (BSC), the formal successful because it is in accord with the are disadvantaged (because of divergence penguins and raptors! 49 50 name for the species con- most plausible mechanism for speciation. between the genetic composition of the parent How many bird species? 50 51 cept with which most Using bird examples, Lack and Mayr showed species), birds that select the ‘right’ partner will 51 ian sinclair 52 people are familiar. It that speciation occurs when populations be favoured, and species boundaries reinforced. The number of described bird species increased steadily from around 3 000 52 53 The isolated race states that species are groups of individuals are isolated and no longer share genes. Alternatively, if hybrids are not disadvantaged, species in 1800, to more than 6 000 species by 1850 and peaked at almost 53 54 of Groundscraper that more or less freely interbreed, and that are Adaptation to local conditions or simply the two populations will coalesce and form a 19 000 species in the early 1900s. However, with the advent of the 54 55 (simensis) (above) reproductively isolated from other such groups. random drift (coupled with the random sam- single species. This process takes many genera- Biological Species Concept, many of the Linnaean ‘morphospecies’ were 55 56 occurring in The key feature is that individuals share genes: pling that occurs when isolated populations tions to play out, and during this time the tax- re-interpreted as subspecies, and considerable lumping took place. By 1950 56 57 and southern is species comprise populations that interbreed form) results in heritable, genetic differences onomic status of the populations is hard to the number of recognized species stabilized at around 8 600, although fur- 57 58 sufficiently different in and thus have a common evolutionary history. between populations. These differences accu- resolve. ther field collecting had increased the total number of taxa (including sub- 58 59 call and behaviour from Of all the taxonomic categories devised to clas- mulate over several generations, and when Strict adherents of the BSC lump all taxa species) to 28 500. Recent trends to recognize allopatric forms as distinct 59 60 the southern African bird sify birds and other organisms – species, gen- the populations meet again they no longer that hybridize. However, many well-defined species has reversed some of the earlier lumping, and the latest list pro- 60 61 (top) that it possibly era, families, orders, etc. (see box on page 65) – interbreed and hence have become separate bird species interbreed at least occasionally – duced by Charles Sibley stands at 9 946 species. Clearly there is no defini- 61 62 warrants full species status. only the species has an objective biological evolutionary entities, or species.  a recent review found that about 10 per  tive bird list, which is why birding is much more dynamic than stamp col- 62 lecting!  63 64 64 bird taxonomy africa – birds & birding 1997 – volume 2, number 6 65 1 Left The Kori Bustard is 1 2 Glossary of jargon ‘Subspecies are a matter another bird of semi-arid 2 3 of opinion, genera a matter of savanna with disjunct 3 Scientists delight in coining new terms for the concepts 4 convenience, but species are populations in north- 4 5 they devise. The main benefit of creating this jargon is that it forms eastern and south-western 5 6 a convenient shorthand for discussing ideas, but all too often a matter of fact’. Africa. The two popula- 6 7 it becomes a shield to exclude the uninitiated. The following brief tions are recognized as 7 8 glossary should help you negotiate the minefield of terms that cent of birds had been recorded to hybridize different subspecies on the 8 9 litter the taxonomic literature. with other species. Sometimes these pairings basis of subtle differences 9 10 don’t involve closest relatives, and it would in plumage pattern. They 10 11 Allopatric (allopatry): taxa whose Lineage: a sequence of populations be ludicrous to lump the species involved. may represent different 11 12 ranges do not overlap (cf. sympatric through time, from ancestors to Once again we get into a subjective argument phylogenetic species, but 12 13 and parapatric). descendants. as to how much hybridization is too much. it is debatable whether 13 peter steyn 14 An allied problem is that many widespread each ‘species’ is as impor- 14 15 Allospecies: allopatric species Parapatric (parapatry): taxa whose species evolve regional differences. These are Above More than 20 subspecies of Spike- tant from a conservation 15 16 that together form a superspecies, ranges have a common boundary typically recognized as subspecies if there are heeled Lark have been described, with perspective as is a well- 16 17 assumed to have evolved from a but do not overlap (cf. allopatric regions where the local variants ‘intergrade’ 11 taxa currently recognized. How many defined biological species 17 18 single, geographically variable species. and sympatric). or blend into each other. However, some well- of these subspecies represent phylogenetic such as Ludwig’s Bustard. 18 19 defined geographic forms have very distinct species requires further research, although 19 20 Analogous character: a character Phenetic species concept: a species characters (often associated with specific hab- many appear to be merely clinal colour 20 shared between taxa as a result of is a group of organisms with similar 21 itat types), and contact zones between them variants. However, recent observations of 21 convergence, not as a result of being appearance. 22 are narrow, with limited interbreeding. the isolated population in northern 22 present in a common ancestor 23 Lumping these forms conceals important bio- suggest it may be a distinct biological 23 (cf. homologous character). Phenotype: an individual’s 24 logical diversity. Another classic example of and phylogenetic species. 24 physical appearance and charac- richard du toit 25 25 Biological species concept: a teristics, which results from the the problem of geographic variation is posed 26 species is a group of freely interbreed- interaction between its genotype by ‘ring species’. Herring Larus argentatus and 26 27 ing organisms. and the environment in which Lesser Black-backed gulls L. fuscus are good defining criterion. This sounds great, but the have regional variation in a variety of charac- 27 28 it lives. biological species in Europe, where they co- PSC is not without its problems. One implica- ters, but unless the boundaries coincide they 28 29 Clade: a group of taxa descended occur without interbreeding. However, they tion is that every time speciation occurs, two should not be treated as species. This applica- 29 30 from a common ancestor (also termed Phylogenetic species concept: are linked by a ring of interbreeding popula- new species form and the old species disap- tion of the PSC results in taxa roughly equiva- 30 31 a monophyletic group). a species is a lineage between tions around the rim of the Arctic Ocean pears. This is disconcerting if speciation occurs lent to those identified by the BSC, but has the 31 32 speciation events. Operationally, a which show that the two forms are merely as a result of colonization of an offshore advantage of handling situations where the 32 33 Cline (clinal variation): a geographic phylogenetic species is characterized the extremes of a continuum. island, and there is no detectable change in BSC cannot be applied. It is the use of this type 33 34 gradient in average character state by a common derived character. the mainland population. of argument that has resulted in the recent rec- 34 35 (e.g. wing length or plumage colour). This is also known as the cladistic spe- An alternative approach A more crucial problem is that species lose ognition of geographically discrete ‘subspecies’ 35 36 cies concept. their special status as self-defining entities. as full species. 36 37 Convergence: the evolution of com- Where do we go from here? One possibility is Moreover, the criterion of a common derived Another solution may be to adopt a trino- 37 38 mon characters in unrelated organisms, Phylogeny: the evolutionary to adopt a new species concept, and the main character can be taken to absurd lengths – for mial system to recognize both biological and 38 39 often as a result of adaptation to similar history of a group of taxa, often contender is the Phylogenetic Species example, defining species on the basis of a phylogenetic species. For example, the Purple 39 40 environmental pressures (for example, represented as a branching tree Concept (PSC). Unlike the BSC, which defines single genetic substitution that doesn’t even Heron Ardea purpurea is a biological species 40 41 wings in birds and bats). (termed a phylogenetic tree). species at a specific moment in time, the PSC result in a change in protein structure. comprising two phylogenetic species, the 41 Further reading 42 views species as parent-ancestor lineages Applying this standard would vastly inflate widespread Purple Heron A. [ p.] purpurea of 42 Disjunct distribution: geographically Recognition species concept: 43 between speciation events. It has its origins the numbers of species and the frequency of Africa and the Palearctic, and the Cape Verde For a more complete 43 isolated; normally applied to a species a species is a group of organisms review of this subject, 44 among systematists who are interested in the extinctions, as many small species would be Purple Heron A. [ p.] bournei, restricted to a 44 with two or more isolated populations. that recognize each other as see the June 1997 edi- 45 relationships between groups of organisms created and expire as a result of chance alone. single island in the Cape Verdes. Such a sys- 45 potential mates as a result of a tion of the Bulletin of the 46 and attempt to reconstruct evolutionary his- Although strictly correct in terms of the PSC tem necessitates the scrapping of subspecies 46 Ecological species concept: a shared mate recognition system. British Ornithologists’ Club 47 (vol. 117, no. 2). Jeremy 47 species is a group of organisms tories. They see species as the terminal definition, this approach would result in ‘spe- for clinal variants. Greenwood (pp. 85-96) 48 adapted to a specific set of resources Sibling species: species which branches of the evolutionary ‘tree’. cies’ being very different from our current Ultimately we can’t expect to pigeonhole all 48 49 What does this mean in terms of identifying perception (not to mention making life all populations into neatly-defined species bound- provides an introduction 49 (a niche) in the environment. are so similar phenotypically to taxonomic principles, 50 species at any given time? Phylogenetic species but impossible for birders!) aries – evolution is an ongoing, dynamic pro- 50 that they are hard to distinguish. followed by well-rea- 51 Genotype: the sum of the genetically- are defined as groups of organisms character- cess, with populations splitting and coalescing soned arguments for and 51 52 stored information within an individual Speciation: the process by which ized by at least one common derived character So what’s the solution? over geological time. We need a classification against the Phylogenetic 52 53 (cf. phenotype). Because birds have two new species form. (that is, they differ consistently from related system that is both biologically accurate and Species Concept by Bob 53 54 copies of all genes (one set derived from species in at least one feature). This objective The scientific world remains divided on the useful. Species remain the primary currency for Zink (pp. 97-109) and 54 55 each parent), not all the components of Sympatric (sympatry): taxa whose criterion avoids the problem of disjunct popu- issue of species concepts. Systematists have setting conservation priorities, and it is impor- David Snow (pp. 110- 55 56 the genotype are necessarily expressed. ranges overlap, at least in part lations – if the isolated populations differ con- forged ahead applying the PSC, while more tra- tant that we do not devalue the species catego- 121), respectively. Finally, 56 Nigel Collar considers 57 (cf. allopatric and parapatric). sistently, they are separate species. It is also ditional taxonomists still support the BSC. As ry to the point that people are no longer con- 57 the implications for Homologous character: a character 58 more forgiving with regard to hybridization. in many debates, it seems likely that some cerned by the threat of species extinctions. At conservation of whole- 58 59 trait that is shared between taxa Taxon (pl. taxa): any named As long as the core of a population differs con- form of middle ground will be found. Moderate the same time, we must guard against allowing heartedly embracing the 59 60 as a result of common ancestry taxonomic group, from subspecies sistently from adjacent populations, it doesn’t adherents of the PSC already argue that there distinctive allopatric populations from going PSC (pp. 122-136). 60 61 (cf. analogous character). to kingdom. matter if some hybridization occurs, because must be concordant variation in a number of extinct because they are currently perceived to 61 62 reproductive compatibility is no longer the  characters to recognize a species. Many species be ‘merely subspecies’.  62

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