Principios de Taxonomia

Recommendations

Info

140j 6 Biological Species as a Gene-Flow Community Dupre, 1999). All traits of an organism must function in a concerted fashion. This view considers the species as a balanced unit that is resistant to change. Selective pressure could thus be much more crucial than gene flow (Mallet, 2006). That selection could be a major factor in maintaining the homogeneity of a species along large geographical distances already follows from the fact that uniparental organisms also produce distinct groups of phenotypically similar individuals. If biparental gene flow were the main factor underlying the homogeneity of a species, then uniparental organisms could not generate large populations of similar individuals in the long term. Therefore, the constancy in phenotypic appearance of the individuals of a species must not result mainly from gene flow (Lande, 1980; Mallet, 2006). Perhaps this phenomenon is comparable to vehicles, which also remain more or less similarly shaped in the longer term because they cannot freely vary in form. Trucks, cars and motorcycles have many traits that must be aligned, as they are needed to form a functioning whole (Barton, 1993), and this integrity of form is independent of whether such a vehicle is required to function in the north or the south of Europe. In roadless terrain, however, other vehicle shapes prove superior, and four-wheel-drive, off-road vehicles and tracked vehicles are predominant. However, the assumption that selection is the main factor that maintains species homogeneity should not be overstressed. The divergent adaptations of organisms to local environments are evident. Many traits are not homogeneously distributed across the entire range of a species. If this were not the case, there would be no races (Chapter 5). 6.8 Isolation by Distance When alleles do not reach geographically distant organisms and thus evolve independently, this phenomenon is called isolation by distance. Isolation by distance must be clearly distinguished from allopatry (see below). Two populations are considered allopatric if they are completely separated by external, usually geographical barriers, so that no gene flow is possible between them. If, however, two populations were geographically distant but still clinally connected by intermediary populations, then this situation would qualify as isolation by distance. Isolation by distance is problematic because the organisms of distant populations of a species can lose their ability to crossbreed (Sterelny and Griffiths, 1999). If race A overlaps in range with race B, and race B overlaps with race C, then race C will not necessarily be cross-fertile with race A (Figure 6.2a). The ability of individuals in distant populations to crossbreed may become lost despite incremental clinal transitions along a chain of populations. An example of the decreasing ability of distant organisms of a species to crossbreed was found in the 1940s and early 1950s by the British evolutionist and lepidopterologist E. B. Ford (Ford, 1940; 1954). Ford showed by the example of the Satyrid butterfly Coenonympha tullia (Large Heath) that with an increasing geographical
6.9 A Decrease in Lateral Sexual Gene Flow, together with Local Adaptation, Creates Racesj141 distance between populations, the number of fertile offspring from crossings decreases. This early discovery should actually have had a greater influence than it did on the understanding of the species as a reproductive community in the sense of Ernst Mayr because it strongly suggests that not all individuals of a species are able to successfully crossbreed with each other. Ford s findings, however, like the discoveries by Ehrlich and Raven on the effective reach of pollen and seeds in plants (Ehrlich and Raven, 1969), have not received much consideration subsequently, as Mayr s opinion was predominant. The suggestion that geographically distant populations of a species may lose their genetic compatibility is clearly demonstrated by the case of the ring species (Figure 6.2b) (see below). Ring species are continuously connected chains of races of a species that surround a geographic region than cannot be occupied because it is a mountain massif, an ocean or the polar region. Therefore, they show a ring-shaped distribution around the inhospitable region. At the endpoints of this geographic ring, the most distant populations encounter each other secondarily. Here, they are no longer genetically compatible and are unable to crossbreed. Although they still belong to the same species, they have become genetically different because their common ancestor dates back far into the past and the mutual exchange of alleles has become scarce or has ceased entirely. Ring species are usually considered to be exceptions because geographically distant populations rarely encounter each other again under natural conditions at another geographic location. The ring species, however, is only a special case of a much more general phenomenon. That general phenomenon is isolation by distance (Figure 6.2a); the idea being expressed here is that the distant populations often are no longer compatible with each other genetically, even though a connection through incremental gene flow is still present. Distant organisms are often no longer able to crossbreed when the geographical range of the species is large. Ability to crossbreed means a long-term, stable ability to crossbreed over many generations, with the descendants competing with purebred descendants. In this sense, the ring species is not an exceptional form of the species, superspecies or even the manifestation of an incipient speciation. The ring species describes only a peculiar circumstance in which the distant populations encounter each other in nature. Apart from this peculiarity, ring species do not differ from most other widely spread species. 6.9 A Decrease in Lateral Sexual Gene Flow, together with Local Adaptation, Creates Races Local geographical populations that diverge with regard to their traits and that therefore are clearly diagnostically different are called races or (synonymously) subspecies (Chapter 5). Races originate from different adaptations to the environmental conditions at different geographical locations. Races develop in the course of evolution, emerging when the homogenizing forces of lateral sexual gene flow become weaker than the force of selection driving adaptation to local
Page 2 and 3:
Werner Kunz Do Species Exist?
Page 4 and 5:
Werner Kunz Do Species Exist? Princ
Page 6 and 7:
Contents Foreword XI Preface XV Col
Page 8 and 9:
5.5 Are Carrion Crow and Hooded Cro
Page 10 and 11:
7.8 Gene Trees are not Species Tree
Page 12 and 13:
XIIj Foreword formed out in various
Page 14 and 15:
Preface What is water? You would no
Page 16 and 17:
Color Plates Do Species Exist? Prin
Page 18 and 19:
Color PlatesjXIX
Page 20 and 21:
Color PlatesjXXI
Page 22 and 23:
Color PlatesjXXIII
Page 24 and 25:
Color PlatesjXXV
Page 26 and 27:
Color PlatesjXXVII
Page 28 and 29:
Color PlatesjXXIX
Page 30 and 31:
Color PlatesjXXXI
Page 32 and 33:
Color PlatesjXXXIII
Page 34 and 35:
230j Scientific Terms Autapomorphy
Page 36 and 37:
232j Scientific Terms Gene-flow com
Page 38 and 39:
234j Scientific Terms Monophylum A
Page 40 and 41:
236j Scientific Terms barriers prev
Page 42 and 43:
238j Scientific Terms Universal Uni
Page 44 and 45:
2j Introduction would be sufficient
Page 46 and 47:
4j Introduction A number of the col
Page 48 and 49:
6j 1 Are Species Constructs of the
Page 50 and 51:
2 Why is there a Species Problem? 2
Page 52 and 53:
2.2 Can Species be Defined and Deli
Page 54 and 55:
2.3 What Makes Biological Species s
Page 56 and 57:
These conclusions indeed sound para
Page 58 and 59:
2.4 Species: To Exist, or not to Ex
Page 60 and 61:
If the species concept is, or even
Page 62 and 63:
2.6 The Constant Change in Evolutio
Page 64 and 65:
2.7 Can a Scientist Work with a Spe
Page 66 and 67:
2.8 The Species as an Intuitive Con
Page 68 and 69:
Contrary to this, it is always auto
Page 70 and 71:
2.9 Taxonomy s Status as a Soft or
Page 72 and 73:
2.11 Sociological Consequences of a
Page 74 and 75:
Races are not populations of indivi
Page 76 and 77:
2.12 Species Pluralism: How Many Sp
Page 78 and 79:
2.12 Species Pluralism: How Many Sp
Page 80 and 81:
2.13 It is One Thing to Identify a
Page 82 and 83:
2.14 The Dualism of the Species Con
Page 84 and 85:
2.14 The Dualism of the Species Con
Page 86 and 87:
3 Is the Biological Species a Class
Page 88 and 89:
3.2 Class Formation and Relational
Page 90 and 91:
3.3 Is the Biological Species a Uni
Page 92 and 93:
3.4 The Difference Between a Group
Page 94 and 95:
3.4 The Difference Between a Group
Page 96 and 97:
3.5 Artificial Classes and Natural
Page 98 and 99:
3.6 The Biological Species Cannot b
Page 100 and 101:
3.7 The Biological Species as a Hom
Page 102 and 103:
3.9 The Linnaean System is Based on
Page 104 and 105:
3.10 Comparison of the System of Or
Page 106 and 107:
3.11 The Relational Properties of t
Page 108 and 109:
68j 4 What are Traits in Taxonomy?
Page 110 and 111:
Page 112 and 113:
Page 114 and 115:
Page 116 and 117:
Page 118 and 119:
Page 120 and 121:
Page 122 and 123:
Page 124 and 125:
Page 126 and 127:
Page 128 and 129:
Page 130 and 131: 90j 4 What are Traits in Taxonomy?
Page 132 and 133: 92j 4 What are Traits in Taxonomy?
Page 134 and 135: 94j 5 Diversity within the Species:
Page 140 and 141: 100j 5 Diversity within the Species
Page 168 and 169: 128j 6 Biological Species as a Gene
Page 226 and 227: 7 The Cohesion of Organisms Through
Page 228 and 229: 7.2 The Problem of Displaying the P
Page 230 and 231:
level of organisms with the next-hi
Page 232 and 233:
Figure 7.4 Classification of organi
Page 234 and 235:
7.4 How is a Cladistic Bifurcation
Page 236 and 237:
7.5 Descent is not the Same Thing a
Page 238 and 239:
Figure 7.7 Paraphyletic classificat
Page 240 and 241:
7.6 Why are Paraphyla used Despite
Page 242 and 243:
Figure 7.8 Phylogenetic trees at di
Page 244 and 245:
Figure 7.10 Gene trees are not spec
Page 246 and 247:
7.9 The Concepts of Monophyly and P
Page 248 and 249:
7.10 Paraphyly and Anagenesis are M
Page 250 and 251:
7.11 The Cladistic Bifurcation of a
Page 252 and 253:
7.12 The Phylocode j213 Thus, if th
Page 254 and 255:
7.12 The Phylocode j215 Figure 7.13
Page 256 and 257:
8 Outlook What is a species? The an
Page 258 and 259:
Index a aberration 96 Acinonyx juba
Page 260 and 261:
essence 45, 46, 56, 58, 64, 65 esse
Page 262 and 263:
Old World Ruddy Duck 150 Old World
Page 264:
v vague boundaries 21, 184 vaguenes
show all

Principios de Taxonomia

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?