Allopatric Speciation

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Dumbbell Exercises and Lifting Routines

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Evolutionists understood since the "Fisherian synthesis" (p. 183) how and why populations of species change over time, yet species are distinct-they do not hybridize with or blend into one another. How can one separate species arise from another (parental) species or one species split into two? Some geneticists ("salta-tionists" like W. Bateson, H. de Vries, R. Goldschmidt) believed that new species arise instantaneously by large mutations, sudden steps in which either a single character or a whole set of characters together become changed. By contrast, the naturalists were "gradualists" and believed that speciation is a populational process, a gradual accumulation of small changes often by natural selection. To solve the apparent contradiction between sympatric species of a local fauna separated by bridgeless gaps on one hand and the idea of gradual speciation on the other hand Mayr presented many intermediate "borderline cases" of geographic variation from his work on the birds of Oceania pointing to a gradual origin of new species from isolated (peripheral) groups of populations of an ancestral species (1940c, 1942e, 1951l, 1963b). The differences he observed among representative island populations were not only quantitative and continuous but often qualitative and discrete as observed among congeneric species. For these reasons he replaced the typological concept of species by a concept of species taxa as aggregates of geographically variable populations.

"What I did, basing my conclusions on a long tradition of European systematics, was to introduce the horizontal (geographical) dimension, and show that the process of geographic speciation is the method by which a gradual evolution of new species is possible, in spite of the gaps in the non-dimensional situation" (Mayr 1992i: 7).

As he stated in this quote and in other historical reviews (p. 230), the principles of the biological species concept and of allopatric speciation had long been established by the naturalists (systematists), but it was Mayr's clear and synthesizing discussion which convinced the geneticists and zoologists generally of the biological species concept and of the common occurrence of allopatric speci-ation: Either a previously continuous species range is split into two or more parts (through sea-level changes or climatic-vegetational changes) or a small isolated founder population is established by dispersal of a few individuals of a parental species across a barrier. Each isolated population then evolves independently, gradually diverging from one another. When sufficient genetic differences have accumulated, the parental and daughter populations may come in contact without hybridizing-a new (daughter) species has originated. Despite the striking gaps between the species of a local flora and fauna, the gradual evolution of new species was no longer a puzzle.

There is no case in birds or mammals that would require sympatric speciation (without geographical separation of populations) and this may also be true for many, perhaps the majority of insect families (butterflies, Carabidae, Tenebrion-idae, etc.). To appreciate the effectiveness of allopatric speciation one only needs to compare the high number of endemic species in island archipelagoes with the low number of such species in a continental region of comparable size.

Mayr summarized the various stages of the differentiation process during allopatric speciation in his "dumbbell" model (1940c: 274-275,1942e: 160) as follows [with some additions]; see Fig. 5.5:

Stage 1: A uniform species with a large range; followed by

Process 1: Differentiation into subspecies; resulting in Stage 2: A geographically variable species with a more or less continuous array of similarsubspecies(2a allsubspeciesare slight, 2bsomeare pronounced); followed by

Process 2: (a) Isolating action of geographic barriers between some of the populations; also (b) development of isolating mechanisms in the isolated and differentiating subspecies; resulting in Stage 3: A geographically variable species with many subspecies completely isolated and some of them morphologically as different as good species [note that Stage 3 does not require Process 2b, only 2a]; followed by

Process 3 (often in connection with 2): Development of intrinsic isolating mechanisms for genetical isolation, as well as the development of reproductive isolation and ecological differentiation, resulting in Stage 4: Expansion of range of such isolated populations into the territory of the representative forms; resulting in either Stage 5a: Noncrossing, that is, new species with restricted range or Stage 5b: Interbreeding, that is, the establishment of a hybrid zone (zone of secondary intergradation).

During the course of time, isolated populations differentiate as members of the following microtaxonomic categories: local population-subspecies-species-superspecies-species group with numerous cases of intermediate differentiation

Arbol Genealogico Template
Fig.5.5. Stages of geographical (allopatric) speciation (from Mayr 1942e, Fig. 16)

found which were difficult to classify (see Table 4.2 on p. 177). However, such "intermediate" forms illustrated for Mayr the process of speciation graphically. Strictly speaking it is not the subspecies but the geographically isolated population which may reach species status. However, little was known about the genetic basis of the speciation process itself which both Dobzhansky (1937) and Mayr (1942e) felt certain occurred as a continuation of microevolution. In a superspecies the member species are not ecologically compatible, for which reason the respective species populations, if their ranges abut, compete along the contact zones and exclude each other geographically without (or only rarely) hybridizing. The closely related members of a species group may be sympatric in parts of their ranges. In ring species the overlapping populations are so strongly differentiated that they no longer hybridize and would be considered species if their direct intergradation through the "ring" of subspecies were to be interrupted (an example are the Asian greenish warblers Phylloscopus trochiloides, Irwin et al. 2005). According to the "Wallace-Dobzhansky model" isolating mechanisms of neospecies originate due to selection forces in sympatry, whereas according to the "Darwin-Muller-Mayr model" isolating mechanisms arise as incidental, pleiotropic by-products of divergence in allopatry (but see also under sympatric speciation, p. 223). David Lack (1944, 1949, 1971) added important data on ecological aspects of the speciation process, as acknowledged by Mayr (1982d: 274).

Besides the "dumbbell" model of geographic speciation, Mayr (1942e) developed an alternative speciation model: a new species originates from jump dispersal of a few individuals forming a small strongly isolated peripheral founder population (founder principle, 1942e: 237; 1954c; 1963b: 539, 554; peripatric speciation, 1982l). This principle relates the reduced variability of small founder populations not to accidental gene loss, but to the fact that the entire population was started by a single pair, a few individuals or even by one fertilized female which contained only a fraction of the gene pool of the parent population. These "founders" carried with them only a very small proportion of the genetic variability of the parent population. This idea goes back to an early formulation by Rensch (1939: 184) who, however, did not appreciate its importance. He emphasized (transl.) that "the gene pool of a few individuals [establishing a new island population] never corresponds to the total gene pool of the numerous individuals of the parent population. In such cases the newly developing island race is not characterized by an increase but by a decrease of genes" (italics in the original).

In a later article on "Change of genetic environment and evolution" (which he considered as one of his most important publications) Mayr (1954c) discussed the genetic processes during speciation of peripherally isolated founder populations. His main idea was that in the limited gene pool the selective value of the genes would be different from the original condition in the parent population particularly owing to inbreeding and a rise in homozygosity. Later the population will enlarge and gradually build up its depleted genetic variability. Rapid evolution in founder populations may lead to the development of strongly differentiated bizarre peripheral isolates (p. 158). Such forms, however, have low evolutionary success, when subsequently exposed to competition with mainland stocks. Mayr did not claim, however, that every founder population speciates, that every genetic change in a founder population is a genetic revolution or that speciation occurs only in founder populations. On the other hand, he was already fully aware of the consequences of his theory pointing out that geographical isolation and the small size of speciating founder populations may explain the phenomenon of lack of documentation of speciation in the fossil record.

As he stated, "Many paleontologists have postulated various kinds of typo-strophic 'saltations' in order to explain the absence of crucial steps from the fossil record. If these changes have taken place in small peripheral isolated populations, it would explain why they are not found by paleontologists" (1954c: 210).

Many years later the paleontologists Eldredge and Gould (1972) based their theory of "punctuated equilibrium" on Mayr's observations. Although they did refer to his relevant publications, this intellectual debt got almost lost in later years. In developing his model of genetic revolutions during the speciation process Mayr (1954c) applied the "new genetics" of Dobzhansky, Wallace and others who like Wright emphasized the interaction of genes and their varying selective values depending on their "genetic environment" or "genetic background." Mayr (l.c.) introduced these latter terms which were widely appreciated and applied in discussions of the integration of the gene pool or of coadaptation among genes (Williams 1966:59). Mayr got the idea of "genetic reorganization" (or "genetic revolution") in peripherally isolated populations when visiting Naples, Italy during the summer of 1951 and lectured on it already in Oxford during September of that year. At that time, E. B. Ford asked him to contribute a paper on this topic to a festschrift for Julian Huxley which only appeared 3 years later. Until then Mayr was "mortally afraid that someone else would get ahead of me."

In retrospect Mayr commented on his idea of "genetic revolutions" during speciation in an interview as follows: "I still think that this is an important idea. The key thing is that the smaller the population, the more important the chance factor becomes because it no longer involves a very strong selection for just one kind of thing. By pure chance a lot of genes and gene combinations simply drop out and so in small populations, improbable combinations are quite frequent and they may be the starting point of something very interesting which in a large population would never happen. Among the classical population geneticists, the only one who saw that was Sewall Wright. Fisher and Haldane never saw it and they would say that small and large populations are one and the same thing. In fact, Fisher said something that was completely and utterly wrong, he said 'the larger a population, the faster it will evolve.' We now know that the truth is exactly the opposite. The larger a population, the more inert it becomes because it is difficult for any change to penetrate through a large population. That is why Wright had his shifting balance theory by which a little piece of a population went outside, in the spirit of my 1954 paper. Then as a unit, it goes back inside again and spreads into the large population. The critics of Wright, in particular J. Coyne [et al. 1997], claim that this whole thing of Wright's is quite impossible, it just can't happen for many reasons. But you have to give Wright credit, that at least he saw there was a genuine problem there, which Fisher and Haldane didn't see" (see Wilkins 2002: 966).11

Current evidence provides little support for founder effect speciation and it appears that selection is more important for speciation than genetic drift, which also plays little part in morphological evolution (Coyne and Orr 2004). One recent case study of founder effects in silvereyes (Zosterops lateralis) on Pacific islands appears to be inconclusive (Grant 2002b).

In his review of different patterns and theories of speciation, Mayr (1982l) refuted the theory of stasipatric speciation and discussed the weakness of the theories of parapatric and sympatric speciation. Chromosomal changes and geographical isolation occur simultaneously. Similarly, Mayr and O'Hara (1986a) refuted an earlier claim that parapatric speciation had occurred in West African birds.

Mayr summarized the various speciation models in the following table (1987a: 311-312): New species originate:

(A) Through a speciation event

(a) Instantaneous (e.g., polyploidy, stabilized hybrid)12

(b) Very rapid (peripatric speciation, conceivably sympatric speciation)

11 Provine (2005) criticized (1) Mayr's concept of genetic revolution as being "devoid of genetic content" and (2) his usage of terms like gene pool and homeostasis of gene pools as being "biological nonsense." However, Futuyma (2006) rejected these criticisms pointing out that (ad 1) "given what Mayr had learned from his population geneticist colleagues, his hypothesis [...] had as much genetic content, I believe, as Wright's shifting balance theory" and (ad 2) that "Mayr was taught genetic homeostasis in one of the major schools of evolutionary genetics of his day." Moreover, unlike Darwin, Mayr has shown "that species of sexually reproducing organisms are real and that they exist by virtue of reproductive isolation rather than of phenotypic distinctiveness" and often originate in allopatry.

12 New evidence indicates that hybrid speciation occurs not only in plants but also in animals and is more frequent than previously thought (O. Seehausen, Trends in Ecology and Evolution 19: 198-207, 2004 and J. Mallet, Nature 446: 279-283, 2007).

(B) Without a speciation event (parental species transformed)

(c) Dichopatric speciation

(split by a geographic barrier with gradual divergence)

(d) Gradual phyletic transformation of a single lineage.

The crucial process in allopatric speciation is geographic isolation (not selection). This statement only refers to the geographic position and separation of the speciating populations. It still remains completely unknown as to what happens genetically during speciation. Possibly rather different genetic mechanisms are involved in the speciation of different kinds of organisms and under different circumstances. Of course, the speciating populations remain well adapted through natural selection which is always present.

Some authors have suggested that Sewall Wright's writings have influenced Mayr's thinking and that in particular, his model of the "adaptive landscape" had given Mayr the idea of geographic speciation.13 Nothing could be further from the truth. Geographic speciation was discussed among naturalists since the 19th century (p. 230) and was standard thinking in Erwin Stresemann's department in Berlin; Mayr described geographical speciation already in 1927(f) in his paper on the Rosy-Finches (Leucosticte). Moreover, Wright's model of the "adaptive landscape" was an abstract model, not one of a three-dimensional landscape; it has very little to do with speciation, but refers mainly to "the evolution of the species as a whole" (quote in Mayr 1992i: 11). As Mayr (1999k: XXIV) pointed out, S. Wright was never particularly interested in problems of speciation and, although he published reviews of all the major works on evolution, the only one he omitted was Mayr's Systematics and the Origin of Species (1942e).

Species originate through "splitting" and "budding" (Fig. 5.6). "Budding" occurs when, e.g., a derivative population of a widespread mainland species reached species status on a nearby island. This speciation event had no effect on the parental biospecies (no. 3, Fig. 5.6) on the mainland from which neospecies 4 has budded off. The mainland species is real in the sense that it represents a biological unit characterized by close genetic-reproductive and ecological relations among its component subspecies taxa. The cladistic analyses schematically illustrated in Fig. 5.6 (if feasible at that intraspecific level) yield relevant phylogenetic ("vertical") and biogeographical data on the origin and relationships of the various groups of taxa. Mayr (e.g., 2000f: 164) feels that most new species originate by budding, splitting is less common.

One may ask the question why Mayr, in 1942, preferred the dichopatric model of speciation (the "dumbbell" model), although the island populations of the Polynesian and Melanesian birds he studied in detail certainly had originated through jump dispersal (and peripatric speciation). When he studied a particular aberrant peripheral island population, the question foremost in his mind at that

13 Thus M. Ruse (1999: 118) wrote: "Mayr, who in later years put distance between himself and Wright, based his original picture of the evolutionary process on the hypothesis" (the shifting balance hypothesis of S. Wright).

Fig. 5.6. Speciation through splitting (A) and budding (B) resulting in monophyletic biospecies 1 and 2 (consisting of 3 and 2 subspecies, respectively) and paraphyletic biospecies 3 (three subspecies). Species 4 which budded off from species 3 is monotypic and may demonstrate its species status by reinvading the ranges of some or all subspecies of species 3. Shading indicates genetic cohesion and intergradation of subspecies along contact zones. From Haffer (1992)

Fig. 5.6. Speciation through splitting (A) and budding (B) resulting in monophyletic biospecies 1 and 2 (consisting of 3 and 2 subspecies, respectively) and paraphyletic biospecies 3 (three subspecies). Species 4 which budded off from species 3 is monotypic and may demonstrate its species status by reinvading the ranges of some or all subspecies of species 3. Shading indicates genetic cohesion and intergradation of subspecies along contact zones. From Haffer (1992)

time was a taxonomic one, whether this form should be ranked as a subspecies, species or as a monotypic genus. The evolutionary questions why and how these aberrant forms originated on isolated islands were not yet foremost in his mind.

Another question is to what extent his fieldwork in New Guinea and the Solomon Islands influenced Mayr's ideas as developed and promoted in his 1942 volume and in later publications. He gave the following answer in an interview:

"The thing is that the influence [of my fieldwork] was only limited because I already had most of the basic ideas. They had been developed further in recent years by Stresemann and several other workers in this area including in Russia. Naturally, everything I had done and observed in New Guinea and the Solomon Islands fitted exactly on these basic ideas of the European workers, but they never had developed all the ideas in these areas: What are the possibilities? What are the methods to be used? What are the mistakes you might make?" (Bock and Lein 2005, video CD-ROM).

Mayr's 1942 book had an enormous influence on the development of evolutionary studies worldwide, but various general questions still remain open: What is the percentage of species of major taxonomic units (e.g., birds or mammals) that originated through peripatric ("budding") and dichopatric speciation ("splitting"), respectively? To what extent did peripatric speciation take place on continents? How many of the taxonomic species currently recognized are, in a cladistic sense, monophyletic and how many are paraphyletic?14 And how important is sympatric speciation?

14 Monophyletic species contain all descendants of a common ancestor and paraphyletic species contain some, but not all descendants of a common ancestor. Coyne and Orr (2004:472) conclude: "Because there is no unitary genetic history at the population level, it is almost impossible to recognize true paraphyly among closely related taxa using genetically based phylogenies."

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Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

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