Principios de Taxonomia

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180j 6 Biological Species as a Gene-Flow Community 6.29 Gene theft between two Species of Green Frogs (Pelophylax ridibunda and P. lessonae) The well-known European Water Frog (Pelophylax esculenta) is a taxonomically problematic case whose status as a species is disputed. The Water Frog is not a distinct species; it is not reproductively isolated from the two species to which it is related, the Marsh Frog (P. ridibunda) and the Pool Frog (P. lessonae) (Color Plate 8). The Water Frog is diagnostically distinguished from the Marsh Frog and the Pool Frog, but it does not form a separate gene-flow community that is isolated from the other two species. Pelophylax esculenta results from a hybridization of two species, the Marsh Frog and the Pool Frog. No doubt, the Water Frog is a hybrid. The Water Frog, however, exists only as an F1 product and not as an F2 or even an F3 product. The Water Frog, as an F1 hybrid with the genomes of the Marsh Frog and the Pool Frog, does not continue into a second generation. Instead, the Water Frog must be repeatedly recreated. The hybrid status of the Water Frog initially resembles the many known cases of species hybrids, which result from accidents without evolutionary importance, because they do not reproduce among themselves any further but die off again at the end of their individual lives (see above). The Water Frog, however, is not an exceptional transient species. Instead, it is found permanently in most areas. Water Frogs reproduce very well among themselves, as can be observed in the garden pond. They are vital and completely fertile, but if two Water Frogs reproduce with each other, then only part of the offspring of this reproduction are Water Frogs; other offspring are again pure Marsh or Pool Frogs. The offspring of the Water Frog do not continue a distinct line of Water Frogs. The remarkable hybridization between the Marsh Frog and the Pool Frog does not produce a new line of organisms that would then exist separate from Marsh and Pool Frogs because there is no autonomous Water Frog genome that exists separately from the genomes of the Marsh and Pool Frogs and could pursue a separate evolutionary line. The Water Frog is not a third species alongside Marsh and Pool Frogs that evolved hybridogenically. Instead, the Water Frog evolves again and again anew. How is this explained? It starts with a hybridization between the two species, Marsh Frog and Pool Frog. The Marsh and Pool Frog mate with each other unrestrictedly; there is no prezygotic mating barrier. In doing so, they do not forfeit their identities. The hybrid, the Water Frog, is vital and fertile. The zygote resulting from the hybridization, as well as all the Water Frog s somatic cells, are allodiploid; they contain a genome of the Marsh Frog and a genome of the Pool Frog. Its germ line cells are allodiploid, but only for a certain time in the ontogenetic development of an individual Water Frog. Before meiosis starts in the testes of the male or in the ovaries of the female, that is to say, before the preliminary germ cells start to differentiate into spermatogonia or oogonia, one of the two genomes is completely removed from the preliminary germ cells. Accordingly, the premeiotic germ cells only contain one of the two parental genomes. Only the somatic cells of the Water Frog are equipped with
6.30 How many Genes Must Mutate for the Origin of New Species?j181 both genomes. Only the somatic cells are true hybrid cells, but these die when the frog dies. Consequently, a genetic recombination between the two parental species cannot occur in the germ line during the subsequent meiosis because at the beginning of meiosis, only one of the two genomes is still present. When the Water Frog produces mature germ cells, these sperm or eggs always contain either pure Marsh Frog or pure Pool Frog genomes. Therefore, a male Water Frog ready for mating either produces Marsh Frog or Pool Frog sperm, but no Water Frog sperm, and similarly, the Water Frog female only produces Marsh Frog or Pool Frog eggs. The Water Frog can mate with whomever it wants: (1) a Pool Frog or (2) a Marsh Frog or even with another (3) Water Frog . The offspring are in any case (1) Pool Frogs or (2) Marsh Frogs or again (3) hybrids, that is, Water Frogs. This is an unusual situation. While the Water Frog is again and again recreated, it is nevertheless not an autonomous new species, as in the hybridogenic speciation of many plants. In the long run, Marsh and Pool Frogs keep their identities, although they continue to intermingle (Schr€oer and Greven, 1999). The reason for this is that no genetic recombination occurs between the genomes during the hybrid s meiosis. The Marsh and Pool Frog genomes remain preserved unblended. Water Frogs are effectively reproductive parasites. To secure their continued existence, the egg of the Water Frog must steal the genome from another species in the course of insemination to build the somatic cells of its body. However, this stolen foreign genome is not used for the meiotic recombination. The produced offspring does not contain recombined genomes. For this reason, the Water Frog is also termed a kleptogamic form or a kleptospecies (from Greek klepto ¼ to steal) (Dubois and G€unther, 1982). Now, what is the situation regarding the species status of the Marsh Frog P. ridibunda and the Pool Frog P. lessonae? They cannot be races, for races do not occur permanently syntopically in the same geographical region without intermingling with each other (Chapter 5). What is more important, however, is that no genes flow from the P. ridibunda to the P. lessonae gene pool via the Water Frog or vice versa. This means that Marsh and Pool Frogs stay clearly separated. Consequently, P. ridibunda and P. lessonae are each, for good reasons, distinct species. This example provides another good argument to justify the position that the species concept of a reproductive community is not precise and should be replaced by the more accurate term gene-flow community (see above). 6.30 How many Genes Must Mutate for the Origin of New Species? It is often assumed that for speciation, the cumulative alteration of many genes is necessary and that only this can lead to the divergence of populations. On the contrary, gene flow barriers can already be created by alterations in only a few genes. Single-gene speciation is possible (Orr, 1991). The two Hawaiian Drosophila species
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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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128j 6 Biological Species as a Gene
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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
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Principios de Taxonomia

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