Reciprocating Bill
Posts: 4265
Joined: Oct. 2006
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*Head in hands, weeping.*
"Oh, how can we investigate speciation empirically? How? HOW?"
Well,
| Quote | Allopatric Divergence, Secondary Contact, and Genetic Isolation in Wild Yeast Populations
Kuehne, Heidi A.1 ; Murphy, Helen A.2; Francis, Chantal A.2; Sniegowski, Paul D.2
Current Biology, Volume 17, issue 5 (March 6, 2007), p. 407-411
In plants and animals, new biological species clearly have arisen as a byproduct of genetic divergence in allopatry. However, our understanding of the processes that generate new microbial species remains limited [1] despite the large contribution of microbes to the world’s biodiversity. A recent hypothesis claims that microbes lack biogeographical divergence because their population sizes are large and their migration rates are presumably high [2, 3]. In recapitulating the classic microbial-ecology dictum that “everything is everywhere, and the environment selects” [4, 5], this hypothesis casts doubt on whether geographic divergence promotes speciation in microbes. To date, its predictions have been tested primarily with data from eubacteria and archaebacteria [6–8]. However, this hypothesis’s most important implication is in sexual eukaryotic microbes, where migration and genetic admixture are specifically predicted to inhibit allopatric divergence and speciation [9]. Here, we use nuclear-sequence data from globally distributed natural populations of the yeast Saccharomyces paradoxus to investigate the role of geography in generating diversity in sexual eukaryotic microbes. We show that these populations have undergone allopatric divergence and then secondary contact without genetic admixture. Our data thus support the occurrence of evolutionary processes necessary for allopatric speciation in sexual microbes. |
And...
| Quote | Origin, radiation, dispersion and allopatric hybridization in the chub Leuciscus cephalus
Dominique Durand, Erhan Ünlü, Ignacio Doadrio, Samvel Pipoyan, Alan R. Templeton, Jean
Proceedings: Biological Sciences, Volume 267, issue 1453 (August 22, 2000), p. 1687-1697
The phylogenetic relationships of 492 chub ( Leuciscus cephalus ) belonging to 89 populations across the species' range were assessed using 600 base pairs of cytochrome b. Furthermore, nine species belonging to the L. cephalus complex were also analysed (over the whole cytochrome b ) in order to test potential allopatric hybridization with L. cephalus sensu stricto (i.e. the chub). Our results show that the chub includes four highly divergent lineages descending from a quick radiation that took place three million years ago. The geographical distribution of these lineages and results of the nested clade analysis indicated that the chub may have originated from Mesopotamia. Chub radiation probably occurred during an important vicariant event such as the isolation of numerous Turkish river systems, a consequence of the uplift of the Anatolian Plateau (formerly covered by a broad inland lake). Dispersion of these lineages arose from the changes in the European hydrographic network and, thus, the chub and endemic species of the L. cephalus complex met by secondary contacts. Our results show several patterns of introgression, from Leuciscus lepidus fully introgressed by chub mitochondrial DNA to Leuciscus borysthenicus where no introgression at all was detected. We assume that these hybridization events might constitute an important evolutionary process for the settlement of the chub in new environments in the Mediterranean area. |
And...
| Quote | GENETIC EVIDENCE ON THE DEMOGRAPHY OF SPECIATION IN ALLOPATRIC DOLPHIN SPECIES
Hare, Matthew P.1; Cipriano, Frank2; Palumbi, Stephen R.3
Under a neutral model, the stochastic lineage sorting that leads to gene monophyly proceeds slowly in large populations. Therefore, in many recent species with large population size, the genome will have mixed support for monophyly unless historical bottlenecks have accelerated coalescence. We use genealogical patterns in mitochondrial DNA and in introns of four nuclear loci to test for historical bottlenecks during the speciation and divergence of two temperate Lagenorhynchus dolphin species isolated by tropical Pacific waters (an antitropical distribution). Despite distinct morphologies, foraging behaviors, and mitochondrial DNAs, these dolphin species are polyphyletic at all four nuclear loci. The abundance of shared polymorphisms between these sister taxa is most consistent with the maintenance of large effective population sizes (5.09 × 104 to 10.9 × 104) during 0.74–1.05 million years of divergence. A variety of population size histories are possible, however. We used gene tree coalescent probabilities to explore the rejection region for historical bottlenecks of different intensity given best estimates of effective population size under a strict isolation model of divergence. In L. obliquidens the data are incompatible with a colonization propagule of an effective size of 10 or fewer individuals. Although the ability to reject less extreme historical bottlenecks will require data from additional loci, the intermixed genealogical patterns observed between these dolphin sister species are highly probable only under an extended history of large population size. If similar demographic histories are inferred for other marine antitropical taxa, a parsimonious model for the Pleistocene origin of these distributions would not involve rare breaches of a constant dispersal barrier by small colonization propagules. Instead, a history of large population size in L. obliquidens and L. obscurus contributes to growing biological and environmental evidence that the equatorial barrier became permeable during glacial/interglacial cycles, leading to vicariant isolation of antitropical populations. |
And...
| Quote | Allopatric origins of microbial species
Whitaker, Rachel J.1
Philosophical Transactions of the Royal Society B: Biological Sciences, Volume 361, issue 1475 (November 29, 2006), p. 1975-1984
Although allopatric divergence is a well-accepted mechanism of speciation for eukaryotic macro-organisms, the importance of geographical barriers to divergence in microbial populations is a subject of great debate. Do geographically separated populations of micro-organisms diverge independently, or does their structure fit the often quoted Bass-Becking description ‘everything is everywhere; the environment selects’? Aided by high-resolution genetic and genomic tools, the search for ‘microbial marsupials’ has revealed that in fact both are true; some species of micro-organisms demonstrate allopatric divergence, while others do not. This discovery opens the door for comparative analyses, where questions about the differences in evolutionary and ecological mechanisms that drive divergence and speciation in different microbial species can begin to be explored. Investigating these differences in evolutionary mechanisms will greatly enhance interest in, and understanding of, the dynamic processes that create and maintain the vast diversity of the microbial world. |
And...
| Quote | RAPID ALLOPATRIC SPECIATION IN LOGPERCH DARTERS (PERCIDAE: PERCINA)
Near, Thomas J.1; Benard, Michael F.2
Evolution, Volume 58, issue 12 (December 1, 2004), p. 2798-2808
Theory predicts that clades diversifying via sympatric speciation will exhibit high diversification rates. However, the expected rate of diversification in clades characterized by allopatric speciation is less clear. Previous studies have documented significantly higher speciation rates in freshwater fish clades diversifying via sympatric versus allopatric modes, leading to suggestions that the geographic pattern of speciation can be inferred solely from knowledge of the diversification rate. We tested this prediction using an example from darters, a clade of approximately 200 species of freshwater fishes endemic to eastern North America. A resolved phylogeny was generated using mitochondrial DNA gene sequences for logperches, a monophyletic group of darters composed of 10 recognized species. Divergence times among logperch species were estimated using a fossil calibrated molecular clock in centrarchid fishes, and diversification rates in logperches were estimated using several methods. Speciation events in logperches are recent, extending from 4.20 ± 1.06 million years ago (mya) to 0.42 ± 0.22 mya, with most speciation events occurring in the Pleistocene. Diversification rates are high in logperches, at some nodes exceeding rates reported for well-studied adaptive radiations such as Hawaiian silverswords. The geographic pattern of speciation in logperches was investigated by examining the relationship between degree of sympatry and the absolute age of the contrast, with the result that diversification in logperches appears allopatric. The very high diversification rate observed in the logperch phylogeny is more similar to freshwater fish clades thought to represent examples of sympatric speciation than to clades representing allopatric speciation. These results demonstrate that the geographic mode of speciation for a clade cannot be inferred from the diversification rate. The empirical observation of high diversification rates in logperches demonstrates that allopatric speciation can occur rapidly. |
And...
| Quote | Comparison of allopatric scallop populations in northerb Chile
Jollan, Luis; von Brand, Elisabeth; Winkler, Federico
Aquaculture, Volume 137, issue 1-4 (December 1, 1995), p. 50-51
Many species, with a wide geographic distribution and subdivided into populations located at different latitudes, show differences in phenotypic characters like morphology, physiology and behaviour (Mayr, 1963; Vernberg, 1962; Marcus, 1977) and can show differences in genotypes. In many marine invertebrates, the existence of a planctonic phase in their life cycle allows dispersion and genetic exchange between geographically distant populations, reducing or avoiding the genetic differentiation process (Hedgecock, 1986). The purpose of the present study was to determine if genetic differences exist in quantitative traits between allopatric populations of the chilean scallop Argopecten purpuratus (Lamarck 1819), and determine whether these differences can be detected by starch gel electrophoresis (Beaumont, 1991).
Adult scallops were randomly collected from four natural populations located in northern Chile: Iquique (20°08?S, 70°08?W), Bahia Mejillones (23°03?S, 70°22?W) Bahia Inglesa (27°07?S, 70°53 W) and Bahia Tongoy (30°17?S, 71°34?W) and brought to the facilities of the Universidad Catolica del Norte at Coquimborwhere they were acclimatized. Simultaneous spawning and fertilization was performed for the four populations using seven or more individuals from each broodstock, following methods described by DiSalvo et al. (1984). The progeny were reared in tanks and growth and survival data of larvae and juveniles were taken. Starch gel electrophoresis was performed using samples of progeny from the different broodstocks with modified methods described by Fujio (1984).
Significant differences were found for oocyte diameter, larval survival, growth in Iength, juvenile survival, growth in shell height and total weight. From the nine isozymes used to compared the populations, only three were found to be polymorphic (MDH, PGM and ?-GPD) and no significant differences were found in allele frequencies between populations.
We can conclude that genetic differences exist for quantitative traits between scallop populations in northern Chile (Jollan, 1992), but a latitudinal differentiation pattern was neither detected at each trait separately, nor in the electrophoretic analysis. |
And...
| Quote | Impact of ancestral populations on postzygotic isolation in allopatric speciation
Hayashi, Takehiko I.1 ; Kawata, Masakado1,2
Population Ecology, Volume 48, issue 2 (April 2006), p. 121 - 130
Postzygotic isolation evolves due to an accumulation of substitutions (potentially deleterious alleles in hybrids) in populations that have become geographically isolated. These potentially deleterious alleles might also be maintained in ancestral populations before geographic isolation. We used an individual-based model to examine the effect of the genetic state of an ancestral population on the evolution of postzygotic isolation after geographic isolation of a population. The results showed that the number of loci at which degenerative alleles are fixed in an ancestral population at equilibrium significantly affects the evolutionary rates of postzygotic isolation between descendant allopatric populations. Our results suggest that: (1) a severe decrease in population size (e.g., less than ten individuals) is not necessary for the rapid evolution of postzygotic isolation (e.g., <10,000 generation); (2) rapid speciation can occur when there is a large difference in the equilibrium number of accumulated degenerative alleles between ancestral and descendant populations; and (3) in an ancestral population maintained at a small effective population size for a long period of time, postzygotic isolation rarely evolves if back mutations that restore the function of degenerative alleles are limited. |
And...
| Quote | A demonstration of allopatric sibling species within the Gymnophallidae (Digenea)
Bowers, E. A.1; Bartoli, P.2; James, B. L.3
Systematic Parasitology, Volume 17, issue 2 (October 1990), p. 143 - 152
Meiogymnophallus minutus and Meiogymnophallus fossarum n. comb. both use the oystercatcher Haematopus ostralegus as their final host and Scrobicularia plana as the primary host. M. minutus uses only the estuarine cockle Cerastoderma edule as its second host, where it occurs enclosed in mantle epithelium in a wedge-shaped cavity beneath the umbo and causes no apparent damage to the host. In contrast, the metacercariae of M. fossarum occur in a variety of microhabitats in a wide variety of lagoon lamellibranchs, including Cerastoderma glaucum. In this host it occurs free in the extrapallial fluid usually beneath the periostracum along the edges of the two valves, where it severely inhibits shell growth, and also beneath the umbo. Comparative morphology, experimental cross infections, second host and microhabitat selection, together with ecological and geographical distribution, suggest that Meiogymnophallus minutus and M. fossarum n. comb. are allopatric sibling species. The former occurs in estuaries in northern and western Europe and the latter from lagoons along the French Mediterranean coast. |
And...
| Quote | Strong assortative mating between allopatric sticklebacks as a by-product of adaptation to different environments
Vines, Timothy H.1; Schluter, Dolph1
Proceedings of the Royal Society B: Biological Sciences, Volume 273, issue 1589 (April 22, 2006),
1. University of British Columbia Department of Zoology Vancouver, BC V6T 1Z4, Canada
Speciation involves the evolution of reproductive isolation between populations. One potentially important mechanism is the evolution of pre- or postzygotic isolation between populations as a by-product of adaptation to different environments. In this paper, we tested for assortative mating between allopatric stickleback populations adapted to different ecological niches. Our experimental design controlled for interpopulation interactions and non-adaptive explanations for assortative mating. We found that prezygotic isolation was surprisingly strong: when given a choice, the majority of matings occurred between individuals from similar environments. Our results indicate that the by-product mechanism is a potent source of reproductive isolation, and likely contributed to the origin of sympatric species of sticklebacks. |
And...
| Quote | FUNCTIONAL INCOMPATIBILITY BETWEEN THE FERTILIZATION SYSTEMS OF TWO ALLOPATRIC POPULATIONS OF CALLOSOBRUCHUS MACULATUS (COLEOPTERA: BRUCHIDAE)
Brown, Denise V.1; Eady, Paul E.1
Evolution, Volume 55, issue 11 (November 1, 2001), p. 2257-2262
Recent studies indicate that postcopulatory sexual selection may represent an important component of the speciation process by initiating reproductive isolation via the evolutionary divergence of fertilization systems. Using two geographically isolated populations of the polyandrous beetle Callosobruchus maculatus, we investigated divergence in fertilization systems by determining the extent of postcopulatory functional incompatibility. Through reciprocal, cross-population matings we were able to separately estimate the effects of male and female population origin and their interaction on the extent of last-male sperm precedence, female receptivity to further copulation and female oviposition. Our results indicate partial incompatibility between the fertilization systems of the two populations at all three functional levels. Males derived from the same population as females outcompete rival, allopatric males with respect to sperm preemption, sperm protection, and ability to stimulate female oviposition. This pattern is reciprocated in both populations indicating that postcopulatory, prezygotic events represent important mechanisms by which between-population gene flow is reduced. We suggest the partial gametic isolation observed is a by-product of the coevolution of male and female fertilization systems by a process of cryptic female choice. Our results are consistent with a mechanism akin to conventional mate choice models although they do not allow us to reject antagonistic sexual coevolution as the mechanism of cryptic female choice. |
And...I could go on and on. But formatting these things is a lot of work.
I would say "you get the picture," Daniel, but I am confident that you don't.
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Myth: Something that never was true, and always will be.
"The truth will set you free. But not until it is finished with you."
- David Foster Wallace
"Here’s a clue. Snarky banalities are not a substitute for saying something intelligent. Write that down."
- Barry Arrington
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