cline (biology)
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biology Biology is the scientific study of life. It is a natural science with a broad scope but has several unifying themes that tie it together as a single, coherent field. For instance, all organisms are made up of Cell (biology), cells that proce ...
, a cline (from the
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
κλίνειν ''klinein'', meaning "to lean") is a measurable
gradient In vector calculus, the gradient of a scalar-valued function, scalar-valued differentiable function of Function of several variables, several variables is the vector field (or vector-valued function) \nabla f whose value at a point p is the "d ...
in a single character (or biological trait) of a species across its geographical range. First coined by
Julian Huxley Sir Julian Sorell Huxley (22 June 1887 – 14 February 1975) was an English evolutionary biologist, Eugenics, eugenicist, and Internationalism (politics), internationalist. He was a proponent of natural selection, and a leading figure in the m ...
in 1938, the "character" of the cline referred to is usually genetic (e.g.
allele frequency Allele frequency, or gene frequency, is the relative frequency of an allele (variant of a gene) at a particular locus (genetics), locus in a population, expressed as a fraction or percentage. Specifically, it is the fraction of all chromosomes in t ...
,
blood type A blood type (also known as a blood group) is a classification of blood, based on the presence and absence of antibody, antibodies and Heredity, inherited antigenic substances on the surface of red blood cells (RBCs). These antigens may be prot ...
), or phenotypic (e.g. body size, skin pigmentation). Clines can show smooth, continuous gradation in a character, or they may show more abrupt changes in the trait from one geographic region to the next. A cline refers to a spatial gradient in a specific, singular trait, rather than a collection of traits; a single population can therefore have as many clines as it has traits, at least in principle. Additionally, Huxley recognised that these multiple independent clines may not act in concordance with each other. For example, it has been observed that in Australia, birds generally become smaller the further towards the north of the country they are found. In contrast, the intensity of their plumage colouration follows a different geographical trajectory, being most vibrant where humidity is highest and becoming less vibrant further into the arid centre of the country. Because of this, clines were defined by Huxley as being an "auxiliary taxonomic principle"; that is, clinal variation in a species is not awarded taxonomic recognition in the way
subspecies In Taxonomy (biology), biological classification, subspecies is a rank below species, used for populations that live in different areas and vary in size, shape, or other physical characteristics (Morphology (biology), morphology), but that ca ...
or
species In biology, a species is the basic unit of Taxonomy (biology), classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of ...
are. While the terms "
ecotype In evolutionary ecology Evolutionary ecology lies at the intersection of ecology and evolutionary biology. It approaches the study of ecology in a way that explicitly considers the evolutionary histories of species and the interactions betw ...
" and "cline" are sometimes used interchangeably, they do in fact differ in that "ecotype" refers to a population which differs from other populations in a number of characters, rather than the single character that varies amongst populations in a cline.


Drivers and the evolution of clines

Clines are often cited to be the result of two opposing drivers:
selection Selection may refer to: Science * Selection (biology) Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the Heredity, her ...
and
gene flow In population genetics, gene flow (also known as gene migration or geneflow and allele flow) is the transfer of genetic variation, genetic material from one population to another. If the rate of gene flow is high enough, then two populations will ...
(also known as migration). Selection causes
adaptation In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their Fitness (biology), evolutionary fitness. Secondly, it is a stat ...
to the local environment, resulting in different genotypes or phenotypes being favoured in different environments. This diversifying force is countered by gene flow, which has a homogenising effect on populations and prevents
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...
through causing
genetic admixture Genetic admixture occurs when previously diverged or isolated genetic lineages mix.⅝ Admixture results in the introduction of new Lineage (genetic), genetic lineages into a population. Examples Climatic cycles facilitate genetic admixture in col ...
and blurring any distinct genetic boundaries.


Development of clines

Clines are generally thought to arise under one of two conditions: "primary differentiation" (also known as "primary contact" or "primary intergradation"), or "secondary contact" (also known as "secondary introgression", or "secondary intergradation").


Primary differentiation

Clines produced through this way are generated by spatial heterogeneity in environmental conditions. The mechanism of selection acting upon organisms is therefore external. Species ranges frequently span environmental gradients (e.g. humidity, rainfall, temperature, or day length) and, according to natural selection, different environments will favour different
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleot ...
s or
phenotype In genetics, the phenotype () is the set of observable characteristics or phenotypic trait, traits of an organism. The term covers the organism's morphology (biology), morphology or physical form and structure, its Developmental biology, dev ...
s. In this way, when previously genetically or phenotypically uniform populations spread into novel environments, they will evolve to be uniquely adapted to the local environment, in the process potentially creating a gradient in a genotypic or phenotypic trait. Such clines in characters can not be maintained through selection alone if much gene flow occurred between populations, as this would tend to swamp out the effects of local adaptation. However, because species usually tend to have a limited dispersal range (e.g. in an isolation by distance model), restricted gene flow can serve as a type of barrier which encourages geographic differentiation. However, some degree of migration is often required to maintain a cline; without it,
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...
is likely to eventually occur, as local adaptation can cause reproductive isolation between populations. A classic example of the role of environmental gradients in creating clines is that of the peppered moth, ''Biston betularia'', in the UK. During the 19th century, when the industrial sector gained traction, coal emissions blackened vegetation across northwest England and parts of northern Wales. As a result of this, lighter morphs of the moth were more visible to predators against the blackened tree trunks and were therefore more heavily predated relative to the darker morphs. Consequently, the frequency of the more cryptic melanic morph of the peppered moth increased drastically in northern England. This cline in morph colour, from a dominance of lighter morphs in the west of England (which did not suffer as heavily from pollution), to the higher frequency of melanic forms in the north, has slowly been degrading since limitations to sooty emissions were introduced in the 1960s.


Secondary contact

Clines generated through this mechanism have arisen through the joining of two formerly isolated populations which differentiated in allopatry, creating an intermediate zone. This secondary contact scenario may occur, for example, when climatic conditions change, allowing the ranges of populations to expand and meet. Because over time the effect of gene flow will tend to eventually swamp out any regional differences and cause one large homogenous population, for a stable cline to be maintained when two populations join there must usually be a selective pressure maintaining a degree of differentiation between the two populations. The mechanism of selection maintaining the clines in this scenario is often intrinsic. This means that the fitness of individuals is independent of the external environment, and selection is instead dependent on the genome of the individual. Intrinsic, or endogenous, selection can give rise to clines in characters through a variety of mechanisms. One way it may act is through heterozygote disadvantage, in which intermediate genotypes have a lower relative fitness than either homozygote genotypes. Because of this disadvantage, one allele will tend to become fixed in a given population, such that populations will consist largely of either ''AA'' (
homozygous Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mo ...
dominant) or ''aa'' (homozygous recessive) individuals. The cline of hetero
zygote A zygote (, ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual ...
s that is created when these respective populations come into contact is then shaped by the opposing forces of selection and gene flow; even if selection against heterozygotes is great, if there is some degree of gene flow between the two populations, then a steep cline may be able to be maintained. Because instrinsic selection is independent of the external environment, clines generated by selection against hybrids are not fixed to any given geographical area and can move around the geographic landscape. Such
hybrid zone A hybrid zone exists where the ranges of two interbreeding species or diverged intraspecific lineages meet and cross-fertilize. Hybrid zones can form ''in situ'' due to the evolution of a new lineage but generally they result from secondary contact ...
s where hybrids are a disadvantage relative to their parental lines (but which are nonetheless maintained through selection being counteracted by gene flow) are known as "tension zones". Another way in which selection can generate clines is through
frequency-dependent selection Frequency-dependent selection is an evolutionary process by which the fitness (biology), fitness of a phenotype or genotype depends on the phenotype or genotype composition of a given population. * In positive frequency-dependent selection, the fit ...
. Characters that could be maintained by such frequency-dependent selective pressures include warning signals (
aposematism Aposematism is the Advertising in biology, advertising by an animal to potential predation, predators that it is not worth attacking or eating. This unprofitability may consist of any defences which make the prey difficult to kill and eat, suc ...
). For example, aposematic signals in ''
Heliconius ''Heliconius'' comprises a colorful and widespread genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viru ...
'' butterflies sometimes display steep clines between populations, which are maintained through positive frequency dependence. This is because heterozygosity,
mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...
s and recombination can all produce patterns that deviate from those well-established signals which mark prey as being unpalatable. These individuals are then predated more heavily relative to their counterparts with "normal" markings (i.e. selected against), creating populations dominated by a particular pattern of warning signal. As with heterozygote disadvantage, when these populations join, a narrow cline of intermediate individuals could be produced, maintained by gene flow counteracting selection. Secondary contact could lead to a cline with a steep gradient if heterozygote disadvantage or frequency-dependent selection exists, as intermediates are heavily selected against. Alternatively, steep clines could exist because the populations have only recently established secondary contact, and the character in the original allopatric populations had a large degree of differentiation. As genetic admixture between the population increases with time however, the steepness of the cline is likely to decrease as the difference in character is eroded. However, if the character in the original allopatric populations was not very differentiated to begin with, the cline between the populations need not display a very steep gradient. Because both primary differentiation and secondary contact can therefore give rise to similar or identical clinal patterns (e.g. gently sloping clines), distinguishing which of these two processes is responsible for generating a cline is difficult and often impossible. However, in some circumstances a cline and a geographic variable (such as humidity) may be very tightly linked, with a change in one corresponding closely to a change in the other. In such cases it may be tentatively concluded that the cline is generated by primary differentiation and therefore moulded by environmental selective pressures.


No selection (drift/migration balance)

While selection can therefore clearly play a key role in creating clines, it is theoretically feasible that they might be generated by
genetic drift Genetic drift, also known as allelic drift or the Wright effect, is the change in the Allele frequency, frequency of an existing gene variant (allele) in a population due to random chance. Genetic drift may cause gene variants to disappear co ...
alone. It is unlikely that large-scale clines in genotype or phenotype frequency will be produced solely by drift. However, across smaller geographical scales and in smaller populations, drift could produce temporary clines. The fact that drift is a weak force upholding the cline however means that clines produced this way are often random (i.e. uncorrelated with environmental variables) and subject to breakdown or reversal over time. Such clines are therefore unstable and sometimes called "transient clines".


Clinal structure and terminology

The steepness, or gradient, of a cline reflects the extent of the differentiation in the character across a geographic range. For example, a steep cline could indicate large variation in the colour of plumage between adjacent bird populations. It has been previously outlined that such steep clines may be the result of two previously allopatric populations with a large degree of difference in the trait having only recently established gene flow, or where there is strong selection against hybrids. However, it may also reflect a sudden environmental change or boundary. Examples of rapidly changing environmental boundaries like this include abrupt changes in the heavy metal content of soils, and the consequent narrow clines produced between populations of ''
Agrostis ''Agrostis'' (bent or bentgrass) is a large and very nearly Cosmopolitan distribution, cosmopolitan genus of plants in the Poaceae, grass family, found in nearly all the countries in the world. It has been bred as a Genetically modified organis ...
'' that are either adapted to these soils with high metal content, or adapted to "normal" soil. Conversely, a shallow cline indicates little geographical variation in the character or trait across a given geographical distance. This may have arisen through weak differential environmental selective pressure, or where two populations established secondary contact a long time ago and gene flow has eroded the large character differentiation between the populations. The gradient of a cline is related to another commonly referred to property, clinal width. A cline with a steep slope is said to have a small, or narrow, width, while shallower clines have larger widths.


Types of clines

According to Huxley, clines can be classified into two categories; continuous clines and discontinuous stepped clines. These types of clines characterise the way that a genetic or phenotypic trait transforms from one end of its geographical range of the species to the other.


Continuous clines

In continuous clines, all populations of the species are able to interbreed and there is gene flow throughout the entire range of the species. In this way, these clines are both biologically (no clear subgroups) and geographically (contiguous distribution) continuous. Continuous clines can be further sub-divided into smooth and stepped clines. *Continuous smooth clines are characterised by the lack of any abrupt changes or delineation in the genetic or phenotypic trait across the cline, instead displaying a smooth gradation throughout. Huxley recognised that this type of cline, with its uniform slope throughout, was unlikely to be common. *Continuous stepped clines consist of an overall shallow cline, interspersed by sections of much steeper slope. The shallow slope represents the populations, and the shorter, steeper sections the larger change in character between populations. Stepped clines can be further subdivided into horizontally stepped clines, and obliquely stepped clines. **Horizontally stepped clines show no intra-population variation or gradation in the character, therefore displaying a horizontal gradient. These uniform populations are connected by steeper sections of the cline, characterised by larger changes in the form of the character. However, because in continuous clines all populations exchange genetic material, the intergradation zone between the groups can never have a vertical slope. **In obliquely stepped clines, conversely, each population also demonstrates a cline in the character, albeit of a shallower slope than the clines connecting the populations together. Huxley compared obliquely stepped clines to looking like a "stepped ramp", rather than taking on the formation of a staircase as in the case of horizontally stepped clines.


Discontinuous stepped clines

Unlike in continuous clines, in discontinuous clines the populations of species are allopatric, meaning there is very little or no gene flow amongst populations. The genetic or phenotypic trait in question always shows a steeper gradient between groups than within groups, as in continuous clines. Discontinuous clines follow the same principles as continuous clines by displaying either *Horizontally stepped clines, where intra-group variation is very small or non-existent and the geographic space separating groups shows a sharp change in character *Obliquely stepped clines, where there is some intra-group gradation, but this is less than the gradation in the character between populations


Clines and speciation

It was originally assumed that geographic isolation was a necessary precursor to speciation (
allopatric speciation Allopatric speciation () – also referred to as geographic speciation, vicariant speciation, or its earlier name the dumbbell model – is a mode of speciation that occurs when biological populations become geographically isolated from ...
). The possibility that clines may be a precursor to speciation was therefore ignored, as they were assumed to be evidence of the fact that in contiguous populations gene flow was too strong a force of homogenisation, and selection too weak a force of differentiation, for speciation to take place. However, the existence of particular types of clines, such as
ring species In biology, a ring species is a connected series of neighbouring populations, each of which interbreeds with closely sited related populations, but for which there exist at least two "end" populations in the series, which are too distantly relate ...
, in which populations did not differentiate in allopatry but the terminal ends of the cline nonetheless do not interbreed, cast into doubt whether complete geographical isolation of populations is an absolute requirement for speciation. Because clines can exist in populations connected by some degree of gene flow, the generation of new species from a previously clinal population is termed
parapatric speciation In parapatric speciation, two subpopulations of a species evolve reproductive isolation The mechanisms of reproductive isolation are a collection of evolution Evolution is change in the heredity, heritable Phenotypic trait, characteri ...
. Both extrinsic and intrinsic selection can serve to generate varying degrees of
reproductive isolation The mechanisms of reproductive isolation are a collection of evolution Evolution is change in the heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the ...
and thereby instigate the process of
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...
. For example, through environmental selection acting on populations and favouring particular allele frequencies, large genetic differences between populations may accumulate (this would be reflected in clinal structure by the presence of numerous very steep clines). If the local genetic differences are great enough, it may lead unfavourable combinations of genotypes and therefore to hybrids being at a decreased fitness relative to the parental lines. When this hybrid disadvantage is great enough, natural selection will select for pre-zygotic traits in the homozygous parental lines that reduce the likelihood of disadvantageous hybridisation - in other words, natural selection will favour traits that promote assortative mating in the parental lines. This is known as
reinforcement In behavioral psychology, reinforcement is a consequence applied that will strengthen an organism's future behavior whenever that behavior is preceded by a specific antecedent stimulus. This strengthening effect may be measured as a higher fr ...
and plays an important role in parapatric and
sympatric speciation Sympatric speciation is the evolution of a new species from a surviving ancestral species while both continue to inhabit the same geographic region. In evolutionary biology Evolutionary biology is the subfield of biology that studies the ...
.


Clinal maps

Clines can be portrayed graphically on maps using lines that show the transition in character state from one end of the geographic range to the other. Character states can however additionally be represented using isophenes, defined by Ernst Mayr as "lines of equal expression of a clinally varying character". In other words, areas on maps that demonstrate the same biological phenomenon or character will be connected by something that resembles a contour line. When mapping clines therefore, which follow a character gradation from one extreme to the other, isophenes will transect clinal lines at a right angle.


Examples of clines

Although the term "cline" was first officially coined by Huxley in 1938, gradients and geographic variations in the character states of species have been observed for centuries. Indeed, some gradations have been considered so ubiquitous that they have been labelled ecological "rules". One commonly cited example of a gradient in morphology is Gloger's Rule, named after Constantin Gloger, who observed in 1833 that environmental factors and the pigmentation of avian
plumage Plumage ( "feather") is a layer of feathers that covers a bird and the pattern, colour, and arrangement of those feathers. The pattern and colours of plumage differ between species and subspecies and may vary with age classes. Within species, ...
tend to covary with each other, such that birds found in arid areas near the Equator tend to be much darker than those in less arid areas closer to the Poles. Since then, this rule has been extended to include many other animals, including flies, butterflies, and wolves. Other ecogeographical rules include
Bergmann's Rule Bergmann's rule is an ecogeographical rule that states that within a broadly distributed taxonomic clade, populations and species of larger size are found in colder environments, while populations and species of smaller size are found in warmer r ...
, coined by Carl Bergmann in 1857, which states that homeotherms closer to the Equator tend to be smaller than their more northerly or southerly conspecifics. One of the proposed reasons for this cline is that larger animals have a relatively smaller surface area to volume ratio and therefore improved heat conservancy – an important advantage in cold climates. The role of the environment in imposing a selective pressure and producing this cline has been heavily implicated due to the fact that Bergmann's Rule has been observed across many independent lineages of species and continents. For example, the
house sparrow The house sparrow (''Passer domesticus'') is a bird of the Old World sparrow, sparrow family Passeridae, found in most parts of the world. It is a small bird that has a typical length of and a mass of . Females and young birds are coloured pale ...
, which was introduced in the early 1850s to the eastern United States, evolved a north-south gradient in size soon after its introduction. This gradient reflects the gradient that already existed in the house sparrow's native range in Europe.
Ring species In biology, a ring species is a connected series of neighbouring populations, each of which interbreeds with closely sited related populations, but for which there exist at least two "end" populations in the series, which are too distantly relate ...
are a distinct type of cline where the geographical distribution in question is circular in shape, so that the two ends of the cline overlap with one another, giving two adjacent populations that rarely interbreed due to the cumulative effect of the many changes in phenotype along the cline. The populations elsewhere along the cline interbreed with their geographically adjacent populations as in a standard cline. In the case of ''
Larus ''Larus'' is a large genus of gulls with worldwide distribution (by far the greatest species diversity is in the Northern Hemisphere). Many of its species are abundant and well-known birds in their ranges. Until about 2005–2007, most gulls ...
'' gulls, the habitats of the end populations even overlap, which introduces questions as to what constitutes a species: nowhere along the cline can a line be drawn between the populations, but they are unable to interbreed. In humans, clines in the frequency of blood types has allowed scientists to infer past population migrations. For example, the Type B blood group reaches its highest frequency in Asia, but become less frequent further west. From this, it has been possible to infer that some Asian populations migrated towards Europe around 2,000 years ago, causing genetic admixture in an isolation by distance model. In contrast to this cline, blood Type A shows the reverse pattern, reaching its highest frequency in Europe and declining in frequency towards Asia.


References

{{DEFAULTSORT:Cline (Biology) Biology terminology Evolutionary biology Genetic genealogy Kinship and descent Landscape ecology Modern human genetic history Population genetics Biological classification Species