Lumping & Splitting
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1 Although the details may vary, this model of 1 2 speciation still holds, at least for animals 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 Taxonomy 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 birds using criteria such as ease of capture, pal- 6 7 THE COMPLEXITIES OF BIRD 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’ species 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- Africa has numerous examples of disjunct 19 species, are a combination of two categories in the Linnaean system, genus 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 woodlands 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, Pygmy Falcon 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 passerines 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 Thrush 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 Ethiopia 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 Eritrea 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.