There are multiple ways to define the concept
of "species". The choice of definition is dependent
on the particularities of the species concerned. For
example, some species concepts apply more readily toward
sexually reproducing organisms while others lend themselves
better toward asexual organisms. Despite the diversity
of various species concepts, these various concepts
can be placed into one of three broad philosophical
approaches: interbreeding, ecological and phylogenetic.
The biological species concept (BSC) is a classic example
of the interbreeding approach. Defined by Ernst Mayr
in 1942, the BSC states that "species are groups of
actually or potentially interbreeding natural populations,
which are reproductively isolated from other such groups".
Despite its wide and long-term use, the BSC like others
is not without controversy, for example because these
concepts cannot be applied to prokaryotes, and this
is called the species problem. Some researchers have
attempted a unifying monistic definition of species,
while others adopt a pluralistic approach and suggest
that there may be a different ways to logically interpret
the definition of a species."
Barriers to reproduction between two diverging sexual
populations are required for the populations to become
new species. Gene flow may slow this process by spreading
the new genetic variants also to the other populations.
Depending on how far two species have diverged since
their most recent common ancestor, it may still be possible
for them to produce offspring, as with horses and donkeys
mating to produce mules. Such hybrids are generally
infertile. In this case, closely related species may
regularly interbreed, but hybrids will be selected against
and the species will remain distinct. However, viable
hybrids are occasionally formed and these new species
can either have properties intermediate between their
parent species, or possess a totally new phenotype.
The importance of hybridisation in producing new species
of animals is unclear, although cases have been seen
in many types of animals, with the gray tree frog being
a particularly well-studied example.
Speciation has been observed multiple times under
both controlled laboratory conditions and in nature.
In sexually reproducing organisms, speciation results
from reproductive isolation followed by genealogical
divergence. There are four mechanisms for speciation.
The most common in animals is allopatric speciation,
which occurs in populations initially isolated geographically,
such as by habitat fragmentation or migration. Selection
under these conditions can produce very rapid changes
in the appearance and behaviour of organisms. As selection
and drift act independently on populations isolated
from the rest of their species, separation may eventually
produce organisms that cannot interbreed.
The second mechanism of speciation is peripatric
speciation, which occurs when small populations of organisms
become isolated in a new environment. This differs from
allopatric speciation in that the isolated populations
are numerically much smaller than the parental population.
Here, the founder effect causes rapid speciation after
an increase in inbreeding increases selection on homozygotes,
leading to rapid genetic change.
The third mechanism of speciation is parapatric speciation.
This is similar to peripatric speciation in that a small
population enters a new habitat, but differs in that
there is no physical separation between these two populations.
Instead, speciation results from the evolution of mechanisms
that reduce gene flow between the two populations. Generally
this occurs when there has been a drastic change in
the environment within the parental species' habitat.
One example is the grass Anthoxanthum odoratum, which
can undergo parapatric speciation in response to localised
metal pollution from mines. Here, plants evolve that
have resistance to high levels of metals in the soil.
Selection against interbreeding with the metal-sensitive
parental population produced a gradual change in the
flowering time of the metal-resistant plants, which
eventually produced complete reproductive isolation.
Selection against hybrids between the two populations
may cause reinforcement, which is the evolution of traits
that promote mating within a species, as well as character
displacement, which is when two species become more
distinct in appearance.
All forms of natural speciation have taken place
over the course of evolution; however it still remains
a subject of debate as to the relative importance of
each mechanism in driving biodiversity.
One example of natural speciation is the diversity
of the three-spined stickleback, a marine fish that,
after the last ice age, has undergone speciation into
new freshwater colonies in isolated lakes and streams.
Over an estimated 10,000 generations, the sticklebacks
show structural differences that are greater than those
seen between different genera of fish including variations
in fins, changes in the number or size of their bony
plates, variable jaw structure, and color differences.
There is debate as to the rate at which speciation
events occur over geologic time. While some evolutionary
biologists claim that speciation events have remained
relatively constant over time, some palaeontologists
such as Niles Eldredge and Stephen Jay Gould have argued
that species usually remain unchanged over long stretches
of time, and that speciation occurs only over relatively
brief intervals, a view known as punctuated equilibrium.
Allopatric speciation (from the ancient Greek allos,
"other" + Greek patra, "fatherland") or geographic speciation
is speciation that occurs when biological populations
of the same species become vicariant — isolated from
each other to an extent that prevents or interferes
with genetic interchange. This can be the result of
population dispersal leading to emigration, or by geographical
changes such as mountain formation, island formation,
or large scale human activities (for example agricultural
and civil engineering developments). The vicariant populations
then undergo genotypic or phenotypic divergence as:
(a) they become subjected to different selective pressures,
(b) they independently undergo genetic drift, and (c)
different mutations arise in the populations' gene pools.
The separate populations over time may evolve distinctly
different characteristics. If the geographical barriers
are later removed, members of the two populations may
be unable to successfully mate with each other, at which
point, the genetically isolated groups have emerged
as different species. Allopatric isolation is a key
factor in speciation and a common process by which new
species arise. Adaptive radiation, as observed by Charles
Darwin in Galapagos finches, is a consequence of allopatric
speciation among island populations.
Allopatric speciation may occur when a species is
subdivided into genetically isolated populations. Such
separation commonly is referred to as vicariance. Allopatric
and allopatry are terms from biogeography, referring
to organisms whose ranges are entirely separate such
that they do not occur in any one place together. If
these organisms are closely related (e.g. sister species),
such a distribution is usually the result of allopatric
speciation. Separation may be attributed to either geological
processes or population dispersal.
Geological processes can fragment a population through
such events as emergence of mountain ranges, canyon
formation, glacial processes, the formation or destruction
of land bridges, or the subsidence of large bodies of
water. On a global scale, plate tectonics are major
geological factors leading to separation of populations
and the resulting distribution of species.
Approximately 50,000 years ago, the Death Valley
region of the western United States had a rainy climate
which produced an interconnecting system of freshwater
rivers and lakes. Climatic changes resulted in a drying
trend that has continued for the last 10,000 years.
As the lakes and rivers shrank, fish populations became
geologically isolated. The few remaining (separated)
springs are currently home to a variety of fish, many
sharing a close common ancestor; yet each has uniquely
adapted to its own particular pool.
The extent to which a geological barrier can effectively
isolate a population correlates to the mobility of the
organism or its offspring. For example physical barriers
such as canyons may effectively block migration and
dispersal of small mammals; however, have little impact
on flying birds or wind-borne seeds.
Population dispersal is used to describe migratory
events, either in the form of range expansion (natural
movement away from parents) or jump dispersal (crossing
of barriers), which may lead to genetic isolation. If
the smaller population fragment becomes genetically
isolated from the parental group, it may be subjected
to its own unique mutations, selection forces, and genetic
drift effects; thus, it will follow its own evolutionary
pathway. Migrations or accidental relocations (such
as birds being blown off course) may lead to population
fragments; whereby groups merely become separated by
distance. Once gene flow between the two groups is disrupted,
speciation becomes a possibility.
The African Elephant has always been regarded as
a single species but, because of morphological and DNA
differences, some scientists classify them into three
subspecies. Researchers at the University of California,
San Diego have argued that divergence due to geographical
isolation has gone further, and the elephants of West
Africa should be regarded as a separate species from
both the savanna elephants of Central, Eastern and Southern
Africa, and the forest elephants of Central Africa.
A similar situation exists with the Asian Elephant,
which has four distinct living sub-species.
Other cases arise where two populations that are
quite distinct morphologically, and are native to different
continents, have been classified as different species;
but when members of one species are introduced into
the other's range, they are found to interbreed freely,
showing that they were in fact only geographically isolated
subspecies. This was found to be the case when the Mallard
was introduced into New Zealand and interbred freely
with the native Grey Duck, which had been classified
as a separate species. It is controversial whether its
specific status can now be retained. Mallards interbreed
similarly and aggressively with the Southern African
Island genetics, the tendency of small, isolated genetic
pools to produce unusual traits, has been observed in many
circumstances, including insular dwarfism and the radical
changes among certain famous island chains, for example
on Komodo. The Galápagos islands are particularly famous
for their influence on Charles Darwin. During his five weeks
there he heard that Galápagos tortoises could be identified
by island, and noticed that Finches differed from one island
to another, but it was only nine months later that he reflected
that such facts could show that species were changeable.
When he returned to England, his speculation on evolution
deepened after experts informed him that these were separate
species, not just varieties, and famously that other differing
Galápagos birds were all species of finches. Though the
finches were less important for Darwin, more recent research
has shown the birds now known as Darwin's finches to be
a classic case of adaptive evolutionary radiation.
Peripatric and peripatry are terms from biogeography,
referring to organisms whose ranges are closely adjacent
but do not overlap, being separated where these organisms
do not occur – for example a wide river or a mountain range.
Such organisms are usually closely related (e.g. sister
species), their distribution being the result of peripatric
Peripatric speciation is a form of speciation, the formation
of new species through evolution. In this form, new species
are formed in isolated peripheral populations; this is similar
to allopatric speciation in that populations are isolated
and prevented from exchanging genes. However, peripatric
speciation, unlike allopatric speciation, proposes that
one of the populations is much smaller than the other. One
possible consequence of peripatric speciation is that a
geographically widespread ancestral species becomes paraphyletic,
thereby becoming a paraspecies. The concept of a paraspecies
is therefore a logical consequence of the Evolutionary Species
Concept, by which one species give rise to a daughter species.
The evolution of the polar bear from the brown bear is a
well-documented example of a living species that gave rise
to another living species through the evolution of a population
located at the margin of the ancestral species' range.
Peripatric speciation was originally proposed by Ernst
Mayr, and is related to the founder effect, because small
living populations may undergo selection bottlenecks. Genetic
drift is often proposed to play a significant role in peripatric
- ·Mayr bird fauna
- ·The Australian bird Petroica multicolor
- ·Reproductive isolation occurs in populations of
Drosophila subject to population bottlenecking
Parapatric speciation is a form of speciation that occurs
due to apparition of dimorphism between populations of a
species, and simultaneous variation in the mating habits,
within a continuous geographical area. In this model, the
parent species lives in a continuous habitat, in contrast
with allopatric speciation & peripatric speciation where
subpopulations become geographically isolated, and sympatric
speciation inside the same area (and still contested).
Niches in this habitat can differ along an environmental
gradient, hampering gene flow, and thus creating a cline.
In parapatric speciation there is no specific extrinsic
barrier to gene flow. The population is continuous, but
nonetheless, it does not mate randomly. Individuals are
more likely to mate with their geographic neighbors than
with individuals in a different part of the population’s
range. In this mode, divergence may happen because of reduced
gene flow within the population as a whole and varying selection
pressures across the population’s range.
An example of this is the grass Anthoxanthum, which has
been known to undergo parapatric speciation in such cases
as mine contamination of an area. This creates a selection
pressure for tolerance to those metals. Flowering time generally
changes (tending toward character displacement—strong selection
against interbreeding—as the hybrids are generally ill-suited
to the environment) and often plants will become self-pollinating.
Similarly, a recent study provided evidence for parapatric
speciation in Tennessee cave salamanders, involving divergence
with gene flow between cave and surface populations.
Observed instances - ring species
- ·The Larus gulls form a ring species around the
- ·The Ensatina salamanders, which form a ring round
the Central Valley in California
- ·The Greenish Warbler (Phylloscopus trochiloides),
around the Himalayas
Sympatric speciation is the process through which new
species evolve from a single ancestral species while inhabiting
the same geographic region. In evolutionary biology and
biogeography, sympatric and sympatry are terms referring
to organisms whose ranges overlap or are even identical,
so that they occur together at least in some places. If
these organisms are closely related (e.g. sister species),
such a distribution may be the result of sympatric speciation.
Etymologically, sympatry is derived from the Greek roots
συν ("together", "with") and πατρίς ("homeland" or "fatherland").
The term was invented by Poulton in 1904, who explains the
Sympatric speciation is one of three traditional geographic
categories for the phenomenon of speciation. Allopatric
speciation is the evolution of geographically isolated populations
into distinct species. In this case, divergence is facilitated
by the absence of gene flow, which tends to keep populations
genetically similar. Parapatric speciation is the evolution
of geographically adjacent populations into distinct species.
In this case, divergence occurs despite limited interbreeding
where the two diverging groups come into contact. In sympatric
speciation, there is no geographic constraint to interbreeding.
These categories are special cases of a continuum from zero
(sympatric) to complete (allopatric) spatial segregation
of diverging groups.
In multicellular eukaryotic organisms, sympatric speciation
is thought to be an uncommon but plausible process by which
genetic divergence (through reproductive isolation) of various
populations from a single parent species and inhabiting
the same geographic region leads to the creation of new
species. In bacteria, however, the analogous process
(defined as "the origin of new bacterial species that occupy
definable ecological niches") might be more common because
bacteria are less constrained by the homogenizing effects
of sexual reproduction and prone to comparatively dramatic
and rapid genetic change through horizontal gene transfer.
Example of three-spined sticklebacks
The three-spined sticklebacks, freshwater fishes, that
have been studied by Dolph Schluter (who received his Ph.D.
for his work on Darwin's finches with Peter J. Grant) and
his current colleagues in British Columbia, were once thought
to provide an intriguing example best explained by sympatric
speciation. Schluter and colleagues found:
·Two different species of three-spined sticklebacks in
each of five different lakes
·a large benthic species with a large mouth that feeds
on large prey in the littoral zone
·a smaller limnetic species — with a smaller mouth —
that feeds on the small plankton in open water
·DNA analysis indicates that each lake was colonized
independently, presumably by a marine ancestor, after the
last ice age
·DNA analysis also shows that the two species in each
lake are more closely related to each other than they are
to any of the species in the other lakes
·The two species in each lake are reproductively isolated;
neither mates with the other.
However, aquarium tests showed:
·the benthic species from one lake will spawn with the benthic
species from the other lakes and
·likewise the limnetic species from the different lakes will
spawn with each other.
·These benthic and limnetic species even display their mating
preferences when presented with sticklebacks from Japanese lakes;
that is, a Canadian benthic prefers a Japanese benthic over
its close limnetic cousin from its own lake.
Their conclusion: in each lake, what began as a single population
faced such competition for limited resources that:
·disruptive selection — competition favoring fishes at either
extreme of body size and mouth size over those nearer the mean
— coupled with:
·assortative mating — each size preferred mates like it —
favored a divergence into two subpopulations exploiting different
food in different parts of the lake.
The fact that this pattern of speciation occurred the same
way on three separate occasions suggests strongly that ecological
factors in a sympatric population can cause speciation.
However, the DNA evidence cited above is from mitochondrial
DNA (mtDNA), which can often move easily between closely related
species ("introgression") when they hybridize. A more recent
study, using genetic markers from the nuclear genome, shows
that limnetic forms in different lakes are more closely related
to each other (and to marine lineages) than to benthic forms
in the same lake. The three-spine stickleback is now usually
considered an example of "double invasion" (a form of allopatric
speciation) in which repeated invasions of marine forms have
subsequently differentiated into benthic and limnetic forms.
The three-spine stickleback provides an example of how molecular
biogeographic studies that rely solely on mtDNA can be misleading,
and that consideration of the genealogical history of alleles
from multiple unlinked markers (i.e. nuclear genes) is necessary
to infer speciation histories.
Reinforcement, also called the Wallace effect, is the process
by which natural selection increases reproductive isolation.
It may occur after two populations of the same species are separated
and then come back into contact. If their reproductive isolation
was complete, then they will have already developed into two
separate incompatible species. If their reproductive isolation
is incomplete, then further mating between the populations will
produce hybrids, which may or may not be fertile. If the hybrids
are infertile, or fertile but less fit than their ancestors,
then there will be further reproductive isolation and speciation
has essentially occurred (e.g., as in horses and donkeys.)
The reasoning behind this is that if the parents of the hybrid
offspring each have naturally selected traits for their own
certain environments, the hybrid offspring will bear traits
from both, therefore would not fit either ecological niche as
well as either parent. The low fitness of the hybrids would
cause selection to favor assortative mating, which would control
hybridization. This is sometimes called the Wallace effect after
the evolutionary biologist Alfred Russel Wallace who suggested
in the late 19th century that it might be an important factor
Conversely, if the hybrid offspring are more fit than their
ancestors, then the populations will merge back into the same
species within the area they are in contact.
Reinforcement favoring reproductive isolation is required
for both parapatric and sympatric speciation. Without reinforcement,
the geographic area of contact between different forms of the
same species, called their "hybrid zone," will not develop into
a boundary between the different species. Hybrid zones are regions
where diverged populations meet and interbreed. Hybrid offspring
are very common in these regions, which are usually created
by diverged species coming into secondary contact. Without reinforcement,
the two species would have uncontrollable inbreeding. Reinforcement
may be induced in artificial selection experiments as described
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