Principios de Taxonomia

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152j 6 Biological Species as a Gene-Flow Community of the taxonomist to find out why the genomes do not cooperate anymore because that lack of cooperation is one of the possible reasons that two individuals belong to two different species. A few genes have already been identified that are responsible for hybrid incompatibilities (Coyne and Orr, 2004). The difficulty of finding genes that are responsible for hybrid incompatibility is of a fundamental nature. This difficulty arises from the fact that, for example, two related Drosophila species would have to be crossed with each other to find the genetic causes of their cross-incompatibility. For a moment, this action sounds like a contradiction in itself (Wu, 1996). More recent investigations on Drosophila and other organisms indicate that, in the case of hybrid incompatibilities, the fertility of the species hybrid is more strongly affected than its vitality. In most cases, reductions in fertility belong to the restrictions that first become noticeable in the hybrid, while the hybrids vitality is not yet compromised. Only after even more incompatible crossings, vitality loss appears in the offspring as a further reduction in fitness. This remarkable difference in the appearance of fertility versus vitality reduction is apparently founded in the fact that fertility is regulated by genes that evolve at a faster rate than the genes that control vitality (see Haldane s Rule, below) (Coyne, Simeonidis, and Rooney, 1998; Wu et al., 1995). Yet one point of view is certain: the problem of hybrid incompatibility is a consequence of very specific gene expressions. It is very imprecise to state that phylogenetically distant organisms have simply drifted far apart from each other genetically and no longer match only for this reason. Even genomes that are phylogenetically distant to a relatively far extent can, in some cases, build a hybrid organism that is still vital and fertile (Turelli, Barton, and Coyne, 2001). Conversely, genomes that are phylogenetically related more closely can, in many cases, exhibit nonviable incompatibilities. Even within the same gene-flow community, and thus, in closely related organisms, there could be certain genetic incompatibilities. This scenario already appears via the numerous natural abortions within an otherwise functioning gene-flow community. In the case of humans, only a portion of the fertilized egg cells develop into a viable child (Grobstein, 1979). These data make clear that, in a gene-flow community, not all of the sperms and eggs, and thus also not all of the organisms, are reproductively compatible with each other, either factually or potentially. The frequent occurrence of reproductive incompatibilities within a species again supports the view that the species, if defined as a gene-flow community, is much more precise than the notion that the species would be a reproductive community. Many members of a species are not reproductively compatible with each other. Thus, it is not possible to test for species membership of organisms by crossing selected individuals. Postzygotic incompatibilities are, therefore, not a matter of individual, so-called statistically selected organisms. They are a property that occurs in populations and affects the majority of the organisms present, but cannot, in each case, be applied to the single individual. Even the horse and donkey, which are textbook examples of postzygotic incompatibility, can, in individual cases, be fecundly crossed with each
6.16 Haldane s Rule and the Genes for Postzygotic Incompatibilityj153 other. This already follows from the fact that there are domestic horse races, into whose bloodline donkeys have been bred. It would not be consistent to prove the species status of a population by choosing single individuals by chance, testing them for their fertility and then inferring the species status from the result (see the problem with allopatry below). Reproductive incompatibility is different from phylogenetic distance. Thus, the definition of a species as a group of organisms that is separated from another species by a specific phylogenetic distance is not the same as the definition of a species as a group of organisms that is reproductively isolated from another species (see the criticism on barcoding in Chapter 4). 6.16 Haldane s Rule and the Genes for Postzygotic Incompatibility More than 80 years ago, the British geneticist John Burdone Haldane discovered a remarkable fact. If two species are crossed with each other, then postzygotic deficits in the hybrids of the F1-generation are expressed much more strongly in the heterogametic sex than in the homogametic one. The heterogametic sex is the sex that, instead of two X chromosomes, possesses the X/Y configuration or the X/0 configuration. For this reason, the heterogametic sex produces two types of mature germ cells during meiotic division: one half are X gametes and the other are Yor 0 gametes. For this reason, it is called the heterogametic sex. These individuals are usually the males. However, in some animal groups (well-known examples are the birds and the butterflies), it is the female that is the heterogametic sex. In the latter case, the sexual chromosomes are given different designations: they are not called X and Y chromosomes but are instead called Z and W chromosomes. The introduction of these additional terms is irritating. The designations Z and W instead of X and Y are superfluous and increase confusion because the sexual chromosomes have the same meaning genetically, whether they occur in the male or in the female. Haldane discovered that in species crossings, the X/Ysons more frequently exhibit damage than the homogametic X/X daughters. This discovery entered biology as Haldane s Rule. It was then that the American geneticist Hermann Joseph Muller focused on this rule in the 1940s and offered a plausible and simple explanation for it. Because the heterogametic offspring inherits a complete set of autosomes plus an X chromosome from the one parent, but it inherits from the other parent no sex chromosome (in the case of the X/0 type) or only the Ychromosome, which contains few genes (in the case of the X/Y type), this scenario would lead to an imbalance between the autosomal genes and the sex-chromosomal genes in the X/Y or X/0 offspring. The autosomes stem half and half from both parent species; the sex chromosomes stem completely or almost completely from only one parent species. This imbalance was called X-autosomal disharmony of the heterogametic hybrid and was used for half a century as an explanation of Haldane s Rule. X-autosomal disharmony was deemed to be a plausible interpretation with regard to the postzygotic deficiencies in the heterogametic offspring of species crossings. Once again,
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Werner Kunz Do Species Exist?
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Werner Kunz Do Species Exist? Princ
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Contents Foreword XI Preface XV Col
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5.5 Are Carrion Crow and Hooded Cro
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7.8 Gene Trees are not Species Tree
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XIIj Foreword formed out in various
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Preface What is water? You would no
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Color Plates Do Species Exist? Prin
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230j Scientific Terms Autapomorphy
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232j Scientific Terms Gene-flow com
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234j Scientific Terms Monophylum A
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236j Scientific Terms barriers prev
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238j Scientific Terms Universal Uni
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2j Introduction would be sufficient
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4j Introduction A number of the col
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6j 1 Are Species Constructs of the
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2 Why is there a Species Problem? 2
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2.2 Can Species be Defined and Deli
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2.3 What Makes Biological Species s
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These conclusions indeed sound para
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2.4 Species: To Exist, or not to Ex
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If the species concept is, or even
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2.6 The Constant Change in Evolutio
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2.7 Can a Scientist Work with a Spe
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2.8 The Species as an Intuitive Con
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Contrary to this, it is always auto
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2.9 Taxonomy s Status as a Soft or
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2.11 Sociological Consequences of a
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Races are not populations of indivi
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2.12 Species Pluralism: How Many Sp
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2.12 Species Pluralism: How Many Sp
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2.13 It is One Thing to Identify a
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2.14 The Dualism of the Species Con
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2.14 The Dualism of the Species Con
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3 Is the Biological Species a Class
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3.2 Class Formation and Relational
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3.3 Is the Biological Species a Uni
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3.4 The Difference Between a Group
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3.4 The Difference Between a Group
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3.5 Artificial Classes and Natural
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3.6 The Biological Species Cannot b
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3.7 The Biological Species as a Hom
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3.9 The Linnaean System is Based on
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3.10 Comparison of the System of Or
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3.11 The Relational Properties of t
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68j 4 What are Traits in Taxonomy?
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94j 5 Diversity within the Species:
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100j 5 Diversity within the Species
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Figure 7.8 Phylogenetic trees at di
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Figure 7.10 Gene trees are not spec
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7.9 The Concepts of Monophyly and P
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7.10 Paraphyly and Anagenesis are M
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7.11 The Cladistic Bifurcation of a
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7.12 The Phylocode j213 Thus, if th
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7.12 The Phylocode j215 Figure 7.13
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8 Outlook What is a species? The an
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Index a aberration 96 Acinonyx juba
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essence 45, 46, 56, 58, 64, 65 esse
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Old World Ruddy Duck 150 Old World
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v vague boundaries 21, 184 vaguenes
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Principios de Taxonomia

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