Cline

For clines at equilibrium, the balance between selection and dispersal can be represented at the center of a cline by[2]w2∼σ2DABrwhere r is the rate of recombination and D is the maximum level of LD between loci (A and B).

From: Encyclopedia of Ecology , 2008

Clines

E.E. Sotka , in Encyclopedia of Ecology, 2008

A cline is a gradient of a phenotypic or genetic character within a single species. The geographic distances across which characters shift can range from meters to thousands of kilometers. Though clines were formally defined as recently as 1938 by Julian Huxley, the gradual change of characters within species has been observed by naturalists for centuries. Consequently, the number of published examples of clinal variation is staggering and include clines in morphology, physiology, behavior, and genetic loci. There is strong evidence that natural selection plays a central role in maintaining clines, in part because much of the spatial variation in a given trait reflects shifts in the biotic and abiotic environments. Clines inform several contentious issues in ecology and evolution, including the degree and nature of natural selection, the process of dispersal and gene flow, historical demography, and speciation.

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Genetic Diversity

Eviatar Nevo , in Encyclopedia of Biodiversity (Second Edition), 2001

Genetic Population Structure

Geographical clines may reflect the action of natural selection on genetic polymorphism at local, regional, and global scales. In Drosophila melanogaster, several latitudinal clines occur for many characters such as allozymes, inversions, and quantitative traits. The identical nature of these clines on the various continents, in both the Northern and Southern Hemispheres, strongly suggests adaptation to specific stress factors, primarily climatic selection. Polymorphism for stress-resistance genes abounds in natural populations. The ADH polymorphism shows high frequencies of the S allele in tropical regions and this declines with latitude. The reasons for this cline are difficult to determine because of the entanglement with other polymorphisms varying with latitude. In 1977, Van Delden and Kamping reviewed the tentative connections with other polymorphisms such as α-GPDH, in (2L)t inversion, body size, and development time with respect to the possible environmental stress factors involved. They concluded from these results, and also from recent experiments, that the (2L)t plays a dominant role in resistance to high temperature and is partly responsible for the ADH cline. They are currently studying the specific selective forces acting on ADH, focusing on the physiological and life history aspects. Many plant and animal species distributed in several climatic zones—tropical, temperate, and arctic—display a decline in gene diversity, both allozyme and DNA, towards the Arctic (Nevo et al., 1984). This is highlighted by postglacial colonizing populations. Regional and local clines also abound in nature.

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Lode Gold Deposits

Ulrich Kretschmar(Late), Derek McBride , in The Metallogeny of Lode Gold Deposits, 2016

9.3.4 Cline Mine

The past-producing Cline Mine and the Island Gold Mine of Richmont Mines lie in the Goudreau–Lochalsh Greenstone Belt northeast of Wawa, Ontario. McBride has studied the Cline Mine database and walked the property ( McBride, 2009). Two mineralized horizons are present that saw mining in the late 1930s. The main mine was described by Bruce (1940). His work clearly shows the vein following the folded contact between the granodiorite (felsic volcanic rocks) and andesite (Figure 9.8). Some mining took place, associated with a second band of "granodiorite." McBride's review examined the entire property using the existing database. It found that the mineralization followed felsic rocks across the property for over 7   km and around two major folds (Figure 9.9). All of the documented mineralization is associated with the two felsic volcanic bands, and the entire system is folded by the regional deformation. The historical data do not suggest a vent system, but the fact that two parallel felsic bands have substantial areas of gold mineralization, indicates at a vent system was present and active for a considerable time period.

Figure 9.8. Block diagram from Bruce (1940), showing the main Cline Mine zone following the folded contact between felsic and mafic volcanic rocks. Most of the mining was from the A vein.

Figure 9.9. Level plan of the 500   ft (150-m) level, incorporating the old mine workings and recent drilling, illustrates the location of the old mine workings and the veins associated with the north felsic band. In the old mine, the A vein bends around to become the C vein. (McBride, 2009).

Once this pattern was recognized, the recent drill intersections demonstrated that the northern felsic band had multiple parallel mineralized bands over considerable strike and dip lengths; in addition, the extensions of the favorable horizon beyond the old mine have not been explored.

Reinterpretation using modern geological knowledge has been able to redirect exploration at this past-producing mine.

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Biology of Oysters

B.L. Bayne , in Developments in Aquaculture and Fisheries Science, 2017

Alternative explanations for the Long Island cline have been proposed based on mixing between two established populations even though there is no species difference between these two populations. Hilbish and Koehn (1985), however, demonstrated that mixing can only explain a small proportion of the clinal change in Lap alleles. Similar clines have been described on other shores on the east coast, USA, and selection on the Lap locus is thought to operate in other species, e.g., Mytilus trossulus (McDonald & Siebenaller, 1989). This example is essentially a single-locus case study, and "… the relative roles of gene flow and multilocus epistasis in maintaining the less fit allele in either environment are unresolved. Data from a series of coding and noncoding loci in samples from these estuarine gradients would shed more light on the exact nature of the selective pressures exerted on Lap and other loci" (Schmidt et al., 2008).

Fig. P1.4. (A) The Long Island Sound Lap cline. The average frequency of the three Lap alleles is shown in the pie diagrams for each site sampled. (B) The formation and variation of the Lap allele frequency cline in Long Island Sound. Genotypes with Lap 94 are subject to higher mortality than other genotypes. This results in a cline that is initially displaced into the estuary (to the left in the diagram); however, mortality shifts the cline to the right, where it is correlated with spatial changes in salinity.

 (A: Hilbish, T. and Koehn, R. (1985). The physiological basis for selection at the Lap locus. Evolution, 39(6), pp.1302–1317; B: From Koehn, R., Zera, H., & Hall, J. (1983). Enzyme polymorphism and natural selection. In M. Nei & R. Koehn (Eds.), Evolution of genes and proteins. Sunderland, MS: Sinauer Associates).

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Functional Relationships of Freshwater Invertebrates

James H. Thorp , in Thorp and Covich's Freshwater Invertebrates (Fourth Edition), 2015

Hydrogeomorphic Patterns

The previous section assumed for the sake of simplicity that rivers are longitudinal clines, exhibiting nearly continuous and predictable changes in habitat characteristics from headwaters to the river terminus. In fact, the opposite is a more accurate portrayal. Rivers are usually hydrogeomorphically complex systems with only partially predictable, and nonclinal variation downstream in bed slope, bed composition, main channel width, and channel number and connectivity ( Thorp et al., 2006, 2008; Poole, 2010). For example, one section of a river may be constricted, another meandering, a third braided, and a fourth with relatively permanent anastomosing side channels and true backwaters. As the geomorphic structure of the river changes downstream, the diversity and types of flow habitats are altered (Poole, 2010), each with different mean and variability of flow over short to long time scales (Thorp et al., 2008). These can vary from higher velocity habitats in the central main channel, to slower slackwater areas along the margins of the main channel, and to moderate-to-zero flow habitats in the lateral parts of the riverscape. They also vary in floodscape characteristics, such as the number, size, and connectivity of usually disconnected channels (backwaters and some anabranches), oxbow lakes (=billabongs of Australia; Figure 4.1), wetlands, and normally dry terrestrial floodplains (Thorp et al., 2008). Flow habitats can vary in substrate composition and size and physicochemical conditions (e.g., O2, temperature, pH).

FIGURE 4.1. Billabong (oxbow channel) of the Murray River of Australia shown in upper left. A small portion of the main channel appears in the lower right.

 Photograph by J.H. Thorp.

Given these physical and chemical differences, ecological characteristics vary among hydrogeomorphic patches (Poole, 2002; Thorp et al., 2006, 2008) (Figure 4.2). Patches of similar type but farther apart should be more similar ecologically than adjacent, but dissimilar patches (Poole, 2002). Algal and vascular plant productivity and levels of system metabolism (cf. Dodds et al., 2013) should increase with greater habitat complexity at this spatial scale, whereas nutrient spiraling lengths should decrease. Carbon sequestration and denitrification should be maximized in complex channel patches, especially those with true backwaters characterized by periodically zero directional flow, heavy detrital buildup, and some anaerobic conditions on the bottom and in surface waters. Interactions with riparia and terrestrial environments are maximized in hydrogeomorphically complex patches. Likewise, species diversity, density, and productivity of fish and invertebrates should be greatest in these complex habitats.

FIGURE 4.2. A conceptual riverine landscape is shown depicting various functional process zones (FPZs) and their possible arrangement in the longitudinal dimension.

Not all FPZs and their possible spatial arrangements are shown. Note that FPZs are repeatable and only partially predictable in location. Information contained in the boxes next to each FPZ depicts the hypothesized hydrological and ecological conditions for that FPZ, with symbols explained in the information key at the bottom. Hydrological scales are flow regime, flow history, and flow regime, with the scale of greatest importance indicated for a given FPZ. The ecological measures (food chain length, nutrient spiraling, and species diversity) are scaled from long to short, with this translated as low to high for species diversity. The light bar within each box is the expected median, with the shading estimating the range of conditions. Size of each arrow reflects the magnitude of vertical, lateral, and longitudinal connectivity.

 Figure reproduced from a revision of Figure 1.1 in Thorp et al. (2008).

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Geochemistry of Mineral Deposits

W.C. Pat ShanksIII, in Treatise on Geochemistry (Second Edition), 2014

13.3.5.6 Precious-Metal Deposits: Epithermal, Carlin-Type, and Orogenic Au

Precious-metal deposits have been studied widely for decades using stable isotope techniques ( Cline et al., 2005; Goldfarb et al., 2005; Simmons et al., 2005; Taylor, 2007). In this section, a diverse group of deposits, including orogenic Au deposits, epithermal precious- and base-metal deposits, and Carlin-type and volcanic-hosted disseminated Au deposits (see Chapter 13.15 ), are treated together because, in some respects, they have similar stable isotope characteristics. All of these deposit types display highly variable δD, δ 18O, and δ 34S values that show variable contributions of meteoric water and magmatic water-rich volatiles and are very strongly affected by w/r reaction with country rocks. Orogenic gold deposits differ in deriving some or most of their ore fluid from metamorphic waters.

Epithermal deposits form in relatively near-surface (<   1.5   km) settings and generally are strongly affected by magmatism that drives meteoric water circulation (Henley and Ellis, 1983; Taylor, 2007). Magmatic volatiles and boiling on ascent are important factors that control stable isotope characteristics. Oxygen and hydrogen isotope systematics of epithermal deposits ( Figure 25 ) show a huge range of values, with meteoric and magmatic waters playing major roles. Many deposits display evidence of mixing between magmatic waters and meteoric waters, whereas others are dominated by one of the end-members. In particular, data for El Indio, McLaughlin, Summitville, Hishikari, and Ladolam overlap the magmatic water fields. Nansatsu and Ladolam show distinct mixing trends from local meteoric water to magmatic water. The Ladolam deposit on Lihir Island, Papua New Guinea, is a world-class gold deposit in a still-active geothermal system. Simmons and Brown (2006) have shown that downhole fluid samples from a deep drill hole have nearly 100% magmatic water containing ~   15   ppm Au, which is enough to deposit the ~   1300   tons of gold in the deposit in only ~   55   000   years. The Summitville data display a classic water–rock interaction trajectory (Taylor, 1997) wherein meteoric waters exchange with rocks during hydrothermal alteration, causing a shift to higher δ 18O values followed by mixing with magmatic water and a shift to higher δD and δ 18O values in the magmatic water field.

Figure 25. Plot of δ 18O versus δD for present-day meteoric waters and for waters in equilibrium with gangue minerals in selected epithermal Au deposits, illustrating the origin and oxygen isotope enrichment of meteoric waters in many epithermal vein systems, after Taylor (2007). Magmatic water compositions defined by Taylor (1987, 1992): Volcanic waters are from Giggenbach (1992). Mixing of magmatic fluids and local meteoric water in the Nansatsu deposit, Japan, and the Ladolam deposit is shown by mixing lines with round symbols for fluid inclusion isotope data from individual samples. The Summitville field of waters and process arrows illustrate meteoric waters that evolved by water/rock reaction followed by mixing with magmatic waters.

Volcanic-hosted and Carlin-type disseminated gold deposits represent two other major classes of epithermal precious-metal deposits. These types of deposits are best-known in the western United States and southwest China and are characterized by having subtle alteration of fine-grained carbonaceous clastic or carbonate rocks or volcanic rocks and finely disseminated gold deposited with pyrite. Oxygen and hydrogen isotope studies of alteration and gangue minerals and of fluid inclusions in Carlin-type deposits generally show meteoric water signatures ( Figure 25 ) with most calculated fluids shifted to higher δ 18O values due to water–rock interaction. However, some deposits have fluid isotope values that suggest mixing of evolved meteoric water with magmatic or metamorphic waters.

Sulfur isotope studies of Carlin-type deposits have a large range of values for syngenetic and diagenetic pyrite in the host rocks (Cline et al., 2005). Calculated values of δ S H 2 S 34 in the mineralizing fluids also show large variability, with fluids falling in the range of 5–10‰. The Getchell deposit stands as a sole exception, with δ S H 2 S 34 values close to magmatic values. In general, sulfur is derived from host rocks. Similar conclusions can be reached for most epithermal deposits and orogenic gold deposits.

Orogenic gold deposits (Goldfarb et al., 2005) generally occur in metamorphic terranes within or adjacent to deep crustal faults. Mineralizing fluids are believed to be driven along pressure gradients related to seismic and orogenic events. Gold mineralization typically occurs at 250–400   °C. Oxygen and hydrogen isotope studies of fluid inclusions and hydrous silicate minerals in the deposits indicate that mineralizing fluids typically are metamorphic waters ( Figure 26 ). But orogenic gold deposits have formed from ore fluids with very diverse sources: some deposits have magmatic water components and some clearly include a meteoric water component. Sulfur isotope values of sulfides generally cluster between 0‰ and 10‰, but many higher and lower values are observed, suggesting the dominance of crustal rather than magmatic sulfur (Goldfarb et al., 2005).

Figure 26. Hydrogen and oxygen isotope variations of mineralizing fluids in orogenic gold deposits. Western Nevada disseminated Au deposit fluid values (Cline et al., 2005) show mostly w/r reaction of meteoric waters, but some values may show magmatic water addition.

 Modified from Goldfarb R, Baker T, Dubé B, Groves D, Hart C, and Gosselin P (2005) Distribution, character, and genesis of gold deposits in metamorphic terranes. Economic Geology 100th Anniversary Volume: 407–450.

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GENETICS AND GENETIC RESOURCES | Genecology and Adaptation of Forest Trees

S.N. Aitken , in Encyclopedia of Forest Sciences, 2004

Genecological Trends

For widely distributed tree species in temperate and boreal regions, most species have broad genetic clines associated with gradients in mean annual temperature, growing-season length (i.e., frost-free period) and, to a lesser extent, total and growing-season precipitation. In mild test environments, overall growth is generally highest for populations with the mildest source environments, and lowest for those from particularly cold (or dry) locations. In harsher test environments, populations from warmer source environments often suffer higher mortality, while populations from similarly cold or dry environments have higher survival and good growth rates for those particular environments.

While local provenances in general are the safest to use for reforestation in the absence of good provenance data, as they have higher survival and productivity than provenances from afar, there are two common exceptions to this pattern. For a number of species, superior provenances have been identified, trees from which have higher than expected growth rates and perform well above the norm for the genetic cline over a wide range of test environments. The second trend is that for many western North American species, the most rapidly growing genotypes with comparable survival and health to local provenances are from slightly milder environments than the test site, e.g., 1–2°   S, or 100–300   m lower in elevation. This may reflect adaptational lag, that is, the local adaptation of populations to past rather than current environments, given the long generation interval of trees, or it may reflect a lack of extreme climatic events as agents of natural selection since the provenance trials were planted.

The steepness of genetic clines varies with trait assessed (Figure 2). The steepest genetic clines often exist for phenological traits and cold-hardiness. The period of active primary growth from bud break (or growth initiation for indeterminate species) to bud set (or growth cessation) varies with annual frost-free period of source environments. There is typically more variation within species for timing of growth cessation (or bud set) than for timing of growth initiation (or bud burst). Populations within species typically differ in the timing of growth cessation and initiation of cold acclimatization in autumn, or in the timing of dehardening in the spring, rather than in the level of maximum cold-hardiness achieved mid-winter. It should be noted that autumn and spring cold-hardiness are really different traits from a genetic standpoint, as variation in these traits is relatively uncorrelated. In Douglas fir, genetic mapping of quantitative trait loci (QTL) controlling cold-hardiness has revealed that autumn and spring cold-hardiness are controlled by largely independent sets of genes. These processes have different cues: acclimatization (hardening) in the autumn is triggered by photoperiod, while first sufficient chilling, then exposure to warm temperatures, initiates dehardening.

Areas with late summer drought generally have populations with earlier growth initiation and cessation, and greater allocation of biomass below ground (higher root/shoot ratios) than locations with more summer precipitation. The mean total growth of trees in populations tends to be correlated with length of the growing season (period between primary growth initiation and cessation), which explains at least part of the lower growth potential of populations from colder or drier source environments, even under favorable conditions. Populations adapted to dry environments are often phenotypically similar to those adapted to frost-prone locations.

Drought-avoidance mechanisms such as a shorter, earlier growing season, preemptive stomatal closure (resulting in cessation of photosynthesis at a higher water potential), and greater allocation of biomass to roots (as opposed to increasing photosynthetic leaf area) tend to decrease net growth; thus provenances from drier regions often have a lower inherent growth capability. Tolerance mechanisms include higher water-use efficiency (less water used per unit of photosynthesis) and a lower vulnerability to cavitation (the water potential at which xylem water columns embolize). Significant interprovenance variation has been observed for all of these drought-related traits in genecological studies of temperate forest trees, with changes in growth phenology and biomass allocation being the best documented.

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Geochemistry of Mineral Deposits

R.J. Bodnar , ... M. Steele-MacInnis , in Treatise on Geochemistry (Second Edition), 2014

13.5.9.2 Pressure and Depth of Formation

The depth range for Carlin deposits determined from FI is typically on the order of a few kilometers. Hofstra and Cline (2000) report that Carlin deposits form at depths <   2   km, but some earlier studies of Carlin deposits (Lamb and Cline, 1997; Osterburg, 1990) proposed greater depths of formation. More recent evaluation of those data suggests that the inclusions used for geobarometry in those studies do not correspond to the mineralization stage. Cline et al. (2005) and Cline and Hofstra (2000) report that some premineralization FI in Carlin systems may have formed at depths of up to 7   km.

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Plant Hybrids

Robert S. Fritz , in Encyclopedia of Biodiversity, 2004

II.A. Types of Hybrid Zones

Hybrid zones are locations where hybrids between species, subspecies, or races are found. Typically, hybrid zones are described as clines, spatial gradients in traits or alleles across a geographic transect where two taxa meet. The balance between selection on hybrids and dispersal of genes determines cline width. Clines are expected to be narrow if there is strong selection against hybrids, if gene flow is limited, or if there are steep environmental gradients. Clines will be wider if selection is weak, gene flow is more extensive, or environmental gradients are gradual. Cline shape is predicted to be a smooth, sigmoid curve if selection acts on single genes or on quantitative traits, but linkage disequilibrium combined with dispersal of genes can distort the smooth shape of the cline, creating a stepped cline. In stepped clines most of the change in allele frequency or trait expression occurs in a narrow range in the middle of a cline. Allele frequency, linkage disequilibrium with other alleles or traits, and gene flow can be used to infer the form by which selection acts in these clines. A clinal hybrid zone is one type of spatial pattern of plant genetic diversity that can influence biodiversity and spatial distribution of animal and pathogen species.

Narrow hybrid zones 10   m wide are found between the sedges Carex canescens and C. mackenziei at the edge of water along the Bothnian coast in northern Sweden. Oak hybrid zones approximately 30   m wide between the oaks Quercus depressipes and Q. rugosa are found on steep slopes in northern Mexico. A much wider hybrid zone extending about 20   km occurs in the same area between Q. coccolobifolia and Q. rugosa.

In contrast to the traditional clinal model of hybrid zones, mosaic hybrid zones occur when habitats of the hybridizing taxa are patchily distributed. Hybridization may occur where the different habitat patches abut, leading to the patchy distribution of hybrids. In contrast to the more or less discrete location of hybrids geographically in the clinal hybrid zones, mosaic hybrid zones can be as widely distributed as the distribution of habitats and parental species. Therefore, the impact of mosaic hybrid zones on the distribution of plant genetic variation and its effects on diversity of communities of herbivores and pathogens are geographically more widespread.

Louisiana irises Iris fulva, I. brevicaulis, and I. hexagona fit the mosaic model since species and hybrid genotypes are associated with different, interspersed habitats. For example, I. fulva is associated with maple-dominated forest habitats, I. brevicaulis is associated with black oak forests, and I. hexagona is found at the edge of freshwater marshes. Hybrid genotypes are either not strongly associated with specific habitats or occupy intermediate or novel habitats. More dispersed mosaic hybrid zones occur where broadly sympatric species hybridize, either occasionally or extensively, where their habitats mix. The sunflowers Helianthus annuus and H. petiolaris exemplify this type of hybrid zone. These sunflower species have overlapping ranges and form local hybrid swarms in the western United States. Hybrids between Salix sericea and S. eriocephala also fit the mosaic model, with hybrids being found throughout the sympatric range of these species in eastern North America.

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Climate Change

J.S. Griffiths , M.W. Kelly , in Encyclopedia of the Anthropocene, 2018

Molecular Genetics Through Space and Time

Climate change can cause shifts in genotypes or allele frequencies, such as a shift toward genotypes favored in warmer conditions. Genetic clines that are driven by latitudinal variation in temperature can shift through time as the climate warms. An example of this was observed in the fly Drosophila subobscura (Balanyá et al., 2006). The frequency of chromosomal inversion ( Box 1 ) changes with latitude, and thus with climate, across three continents—North America, South America, and Europe. Balanyá et al. (2006) scored the frequencies of inversions from 1997 to 2004 from the same populations across their latitudinal ranges and observed genetic shifts equivalent to 1° of latitude toward the equator on all three continents. Balanyá et al. (2006) also observed increases in temperature from weather stations nearby their study sites, providing strong support for climate change acting as the major driver of shifts in the frequencies of inversions.

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