In
zoology
Zoology ( , ) is the scientific study of animals. Its studies include the anatomy, structure, embryology, Biological classification, classification, Ethology, habits, and distribution of all animals, both living and extinction, extinct, and ...
, intergradation is the way in which two distinct
subspecies
In Taxonomy (biology), biological classification, subspecies (: 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), morpholog ...
are connected via areas where
populations
Population is a set of humans or other organisms in a given region or area. Governments conduct a census to quantify the resident population size within a given jurisdiction. The term is also applied to non-human animals, microorganisms, and pl ...
are found that have the characteristics of both. There are two types of intergradation: primary and secondary intergradation.
Primary intergradation
This occurs in cases where two subspecies are connected via one or more intermediate populations, each of which is in turn intermediate to its adjacent populations and exhibits more or less the same amount of
variability as any other population within the
species
A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...
. Adjacent populations and subspecies are subject to
cline
Cline may refer to:
Science
* Cline (biology), a measurable gradient in a single trait in a species across its geographical range
* Cline (hydrology), a fluid layer with a property that varies
* Cline (mathematics) or generalised circle, a ci ...
intergradation, and in these situations it is usually taken for granted that the clines are causally related (by
natural selection
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, heritable traits characteristic of a population over generation ...
) to
environmental gradients.
[Mayr E, Ashlock PD (1991). ''Principles of Systematic Zoology''. Second edition. McGraw-Hill. .]
Secondary intergradation
When contact between a geographically isolated subspecies is reestablished with the main body of the species or with another isolate subspecies, interbreeding takes place as long as the isolate has not yet evolved an effective set of isolating mechanisms. Consequently, a relatively distinct zone or belt of
hybridization will develop depending on the degree of
genetic and
phenotypic
In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology (physical form and structure), its developmental processes, its biochemical and physiological propert ...
difference that was achieved by the previously isolated
subspecies
In Taxonomy (biology), biological classification, subspecies (: 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), morpholog ...
.
See also
*
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 ...
References
{{Reflist
Hybridisation (biology)
Population genetics
Evolutionary biology