.gif)
Cline (population genetics)
Encyclopedia
In biology
, an ecocline or simply cline (Greek: κλίνω = to possess or exhibit gradient, to lean) describes an ecotone
in which a series of biocommunities
display continuous gradient. The term was coined by the English evolutionary biologist Julian Huxley
in 1938.
s or forms
of species that exhibit gradual phenotypic
and/or genetic differences over a geographical area, typically as a result of environmental heterogeneity.
Genetically, clines result from the change of allele frequencies
within the gene pool
of the group of taxa
in question. Clines may manifest in time and/or space.
where the abundance and distribution of organisms is rendered by sinusoidal curves on the plane. From these curves can be extracted that populations occupy zones of maximum and minimum presence, according to their special needs and tolerances imposed by their environment.
Typically, a well-marked cline does not allow for a delineation of subspecies
, as it is then impossible, by definition, to draw any further clear dividing lines between populations. In population genetics
, a cline could include a spectrum of subspecies
, as allele
and haplotype
frequencies tend to vary over a larger space; moreover, in evolution
, genetic lineage sorting usually lags behind the establishment of locally-differentiated phenotypes. Regardless, in neither case will such a variation yield different species, as long as the populations, though geographically spread, can interbreed one with another.
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. Ring species present an interesting problem for those who seek to divide the living world into discrete species
, as chain species are closely related to speciation
(in this case, parapatric).
In the case of Larus
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.
However a recent study (Liebers et al., 2004) has provided genetic evidence that this example is far more complicated than presented here, and likely does not constitute a typical ring species.
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
, an ecocline or simply cline (Greek: κλίνω = to possess or exhibit gradient, to lean) describes an ecotone
Ecotone
An ecotone is a transition area between two biomes but different patches of the landscape, such as forest and grassland. It may be narrow or wide, and it may be local or regional...
in which a series of biocommunities
Community
The term community has two distinct meanings:*a group of interacting people, possibly living in close proximity, and often refers to a group that shares some common values, and is attributed with social cohesion within a shared geographical location, generally in social units larger than a household...
display continuous gradient. The term was coined by the English evolutionary biologist Julian Huxley
Julian Huxley
Sir Julian Sorell Huxley FRS was an English evolutionary biologist, humanist and internationalist. He was a proponent of natural selection, and a leading figure in the mid-twentieth century evolutionary synthesis...
in 1938.
Introduction
More technically, clines consist of ecotypeEcotype
In evolutionary ecology, an ecotype,Greek: οίκος = home and τύπος = type, coined by Göte Turesson in 1922 sometimes called ecospecies, describes a genetically distinct geographic variety, population or race within species , which is adapted to specific environmental conditions.Typically, ecotypes...
s or forms
Polymorphism (biology)
Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
of species that exhibit gradual phenotypic
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...
and/or genetic differences over a geographical area, typically as a result of environmental heterogeneity.
Genetically, clines result from the change of allele frequencies
Allele frequency
Allele frequency or Gene frequency is the proportion of all copies of a gene that is made up of a particular gene variant . In other words, it is the number of copies of a particular allele divided by the number of copies of all alleles at the genetic place in a population. It can be expressed for...
within the gene pool
Gene pool
In population genetics, a gene pool is the complete set of unique alleles in a species or population.- Description :A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can survive bouts of intense selection...
of the group of taxa
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...
in question. Clines may manifest in time and/or space.
Gradient analysis
In ecology, spatial clines have led to gradient analysisGradient analysis
Gradient analysis is an empirical analytical method used in plant community ecology to relate the abundances of various species in a plant community to various environmental gradients by ordination or by weighted averaging...
where the abundance and distribution of organisms is rendered by sinusoidal curves on the plane. From these curves can be extracted that populations occupy zones of maximum and minimum presence, according to their special needs and tolerances imposed by their environment.
Typically, a well-marked cline does not allow for a delineation of subspecies
Subspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...
, as it is then impossible, by definition, to draw any further clear dividing lines between populations. In population genetics
Population genetics
Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...
, a cline could include a spectrum of subspecies
Subspecies
Subspecies in biological classification, is either a taxonomic rank subordinate to species, ora taxonomic unit in that rank . A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one...
, as allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
and haplotype
Haplotype
A haplotype in genetics is a combination of alleles at adjacent locations on the chromosome that are transmitted together...
frequencies tend to vary over a larger space; moreover, in evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
, genetic lineage sorting usually lags behind the establishment of locally-differentiated phenotypes. Regardless, in neither case will such a variation yield different species, as long as the populations, though geographically spread, can interbreed one with another.
Ring species
Ring speciesRing species
In biology, a ring species is a connected series of neighboring populations, each of which can interbreed with closely sited related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed, though there is a potential gene...
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. Ring species present an interesting problem for those who seek to divide the living world into discrete species
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...
, as chain species are closely related to speciation
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
(in this case, parapatric).
In the case of Larus
Larus
Larus is a large genus of gulls with worldwide distribution . Many of its species are abundant and well-known birds in their ranges...
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.
However a recent study (Liebers et al., 2004) has provided genetic evidence that this example is far more complicated than presented here, and likely does not constitute a typical ring species.
Types of clines
- ThermoclineThermoclineA thermocline is a thin but distinct layer in a large body of fluid , in which temperature changes more rapidly with depth than it does in the layers above or below...
- A cline based on difference in temperatureTemperatureTemperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
, - ChemoclineChemoclineA chemocline is a cline caused by a strong, vertical chemistry gradient within a body of water. A chemocline is analogous to a thermocline, the border at which warmer and cooler waters meet in an ocean, sea, lake, or other body of water...
- A cline based on difference in water chemistryChemistryChemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
, - HaloclineHaloclineIn oceanography, a halocline is a subtype of chemocline caused by a strong, vertical salinity gradient within a body of water. Because salinity affects the density of seawater, it can play a role in its vertical stratification...
- A cline based on difference in water salinitySalinitySalinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
, - PycnoclinePycnoclineA pycnocline is the cline or layer where the density gradient is greatest within a body of water. An ocean current is generated by the forces such as breaking waves, terms of temperature and salinity differences, wind, Coriolis effect, and tides caused by the gravitational pull of the Moon and the...
- A cline based on difference in water densityDensityThe mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
.
See also
- ClineClineCline, as spelled, is an Irish surname with historical roots in County Roscommon.People bearing the name include:* Catherine Ann Cline , American historian and author* Edward Cline , American screenwriter and director...
- Isolation by distanceIsolation by distanceIsolation by distance describes the tendency of individuals to find mates from nearby populations rather than distant populations. As a result of this tendency, populations that live near each other are genetically more similar than populations that live further apart. Isolation by distance results...
- Ring speciesRing speciesIn biology, a ring species is a connected series of neighboring populations, each of which can interbreed with closely sited related populations, but for which there exist at least two "end" populations in the series, which are too distantly related to interbreed, though there is a potential gene...
- Race (classification of human beings)
- Human skin colorHuman skin colorHuman skin color is primarily due to the presence of melanin in the skin. Skin color ranges from almost black to white with a pinkish tinge due to blood vessels underneath. Variation in natural skin color is mainly due to genetics, although the evolutionary causes are not completely certain...
- SpeciationSpeciationSpeciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
- Polymorphism (biology)Polymorphism (biology)Polymorphism in biology occurs when two or more clearly different phenotypes exist in the same population of a species — in other words, the occurrence of more than one form or morph...
- EcotypeEcotypeIn evolutionary ecology, an ecotype,Greek: οίκος = home and τύπος = type, coined by Göte Turesson in 1922 sometimes called ecospecies, describes a genetically distinct geographic variety, population or race within species , which is adapted to specific environmental conditions.Typically, ecotypes...
- Allele frequencyAllele frequencyAllele frequency or Gene frequency is the proportion of all copies of a gene that is made up of a particular gene variant . In other words, it is the number of copies of a particular allele divided by the number of copies of all alleles at the genetic place in a population. It can be expressed for...
- Allometric engineeringAllometric engineeringAllometric engineering is the process of experimentally shifting the scaling relationships of a population. More specifically, the process of experimentally breaking the tight covariance evident among component traits of a complex phenotype by altering the variance of one trait relative to another....