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Species is a taxonomic concept used in biology to refer to a population of
organisms that are in some important ways similar. The idea of species has a
long history. After thousands of years of use, the concept remains central
to biology and a host of related fields, and yet also remains at times
ill-defined and controversial. There are several main lines of thought in
the definition of species:
* A morphological species is a group of organisms that have a distinctive
form: for example, we can distinguish between a chicken and a duck
because they have different shaped bills and the duck has webbed feet.
Species have been defined in this way since well before the beginning
of recorded history. Although much criticised, the concept of
morphological species remains the single most widely used species
concept in everyday life, and still retains an important place within
the biological sciences, particularly in the case of plants.
* The biological species or isolation species concept identifies a
species as a set of actually or potentially interbreeding organisms.
This is generally the most useful formulation for scientists working
with living examples of the higher taxa like mammals, fish, and birds,
but meaningless for organisms that do not reproduce sexually. It
distinguishes between the theoretical possibility of interbreeding and
the actual likelihood of gene flow between populations. For example, it
is possible to cross a horse with a donkey and produce offspring,
however they remain separate species—in this case for two
different reasons: first because horses and donkeys do not normally
interbreed in the wild, and second because the fruit of the union is
rarely fertile. The key to defining a biological species is that there
is no significant cross-flow of genetic material between the two
* A mate-recognition species is defined as a group of organisms that are
known to recognise one another as potential mates. Like the isolation
species concept above, it is not applicable to organisms that do not
* A phylogenetic or evolutionary or Darwinian species is a group of
organisms that shares a common ancestor; a lineage that maintains its
integrity with respect to other lineages through both time and space.
At some point in the progress of such a group, members may diverge from
one another: when such a divergence becomes sufficiently clear, the two
populations are regarded as separate species.
In practice, these definitions often coincide, and the differences between
them are more a matter of emphasis than of outright contradiction.
Nevertheless, no species concept yet proposed is entirely objective, or can
be applied in all cases without resorting to judgement.
The naming of a particular species should be regarded as a hypothesis about
the evolutionary relationships and distinguishability of that group of
organisms. As further information comes to hand, the hypothesis may be
confirmed or refuted. As a result of the revolutionary (and still ongoing)
advance in microbiological research techniques in the later years of the
20th century, a great deal of extra knowledge about the differences and
similarities between species has become available. Many populations which
were formerly regarded as separate species are now considered to be a single
biological unit, and many formerly grouped populations have been split. At
higher taxonomic levels, these changes have been still more profound.
The isolation species concept in more detail
In general, for large, complex, organisms that reproduce sexually (such as
mammals and birds) one of several variations on the isolation or biological
species concept is employed. Often, the distinction between different
species, even quite closely related ones, is simple. Horses (Equus caballus)
and donkeys (Equus asinus) are easily told apart even without study or
training, and yet are so closely related that they can interbreed after a
fashion. Because the result, a mule or hinny, is not usually fertile, they
are clearly separate species.
But many cases are more difficult to decide. This is where the isolation
species concept diverges from the evolutionary species concept. Both agree
that a species is a lineage that maintains its integrity over time, that is
diagnosably different to other lineages (else we could not recognise it), is
reproductively isolated (else the lineage would merge into others, given the
chance to do so), and has a working intra-species recognition system
(without which it could not continue). In practice, both also agree that a
species must have its own independent evolutionary history—otherwise
the characteristics just mentioned would not apply. The species concepts
differ in that the evolutionary species concept does not make predictions
about the future of the population: it simply records that which is already
known. In contrast, the isolation species concept refuses to assign the rank
of species to populations that, in the best judgement of the researcher,
would recombine with other populations if given the chance to do so.
The isolation question
There are, essentially, two questions to resolve. First, is the proposed
species consistently and reliably distinguishable from other species?
Secondly, is it likely to remain so in the future? To take the second
question first, there are several broad geographic possibilities.
* The proposed species are sympatric—they occupy the same habitat.
Observation of many species over the years has failed to establish even
a single instance of two diagnostically different populations that
exist in sympatry and have then merged to form one united population.
Without reproductive isolation, population differences cannot develop,
and given reproductive isolation, gene flow between the populations
cannot merge the differences. This is not to say that cross breeding
does not take place at all, simply that it has become negligible.
Generally, the hybrid individuals are less capable of successful
breeding than pure-bred individuals of either species.
* The proposed species are allopatric—they occupy different
geographical areas. Obviously, it is not possible to observe
reproductive isolation in allopatric groups directly. Often it is not
possible to achieve certainty by experimental means either: even if the
two proposed species interbreed in captivity, this does not demonstrate
that they would freely interbreed in the wild, nor does it always
provide much information about the evolutionary fitness of hybrid
individuals. A certain amount can be inferred from other experimental
methods: for example, do the members of population A respond
appropriately to playback of the recorded mating calls of population B?
Sometimes, experiments can provide firm answers. For example, there are
seven pairs of apparently almost identical marine snapping shrimp
(Altheus) populations on either side of the Isthmus of Panama (which
did not exist until about 3 million years ago). Until then, it is
assumed, they were members of the same 7 species. But when males and
females from opposite sides of the isthmus are placed together, they
fight instead of mating. Even if the isthmus were to sink under the
waves again, the populations would remain genetically isolated:
therefore they are now different species. In many cases, however,
neither observation nor experiment can produce certain answers, and the
determination of species rank must be made on a 'best guess' basis from
a general knowledge of other related organisms.
* The proposed species are parapatric—they have breeding ranges
that abut but do not overlap. This is fairly rare, particularly in
temperate regions. The dividing line is often a sudden change in
habitat (an ecotone) like the edge of a forest or the snow line on a
mountain, but can sometimes be remarkably trivial. The parapatry itself
indicates that the two populations occupy such similar ecological roles
that they cannot coexist in the same area. Because they do not
crossbreed, it is safe to assume that there is a mechanism, often
behavioral, that is preventing gene flow between the populations, and
therefore that they should be classified as separate species.
* There is a hybrid zone where the two populations mix. Typically, the
hybrid zone will include representatives of one or both of the 'pure'
populations, plus first-generation and back-crossing hybrids. The
strength of the barrier to genetic transmission between the two pure
groups can be assessed by the width of the hybrid zone relative to the
typical dispersal distance of the organisms in question. (The dispersal
distance of oaks, for example, is the distance that a bird or squirrel
can be expected to carry an acorn; the dispersal distance of Numbats is
about 15 kilometres, as this is as far as young Numbats will normally
travel in search of vacant territory to occupy after leaving the nest.)
The narrower the hybrid zone relative to the dispersal distance, the
less gene flow there is between the population groups, and the more
likely it is that they will continue on separate evolutionary paths.
Nevertheless, it can be very difficult to predict the future course of
a hybrid zone; the decision to define the two hybridizing populations
as either the same species or as separate species is difficult and
* The variation in the population is clinal—at either extreme of
the population's geographic distribution, typical individuals are
clearly different, but the transition between them is seamless and
gradual. For example, the Koalas of northern Australia are clearly
smaller and lighter in colour than those of the south, but there is no
particular dividing line: the further south an individual Koala is
found, the larger and darker it is likely to be; Koalas in intermediate
regions are intermediate in weight and colour. In contrast, over the
same geographic range, black-backed (northern) and white-backed
(southern) Australian Magpies do not blend from one type to another:
northern populations have black backs, southern populations white
backs, and there is an extensive hybrid zone where both 'pure' types
are common, as are crossbreeds. The variation in Koalas is clinal (a
smooth transition from north to south, with populations in any given
small area having a uniform appearance), but the variation in magpies
is not clinal. In both cases, there is some uncertainty regarding
correct classification, but the consensus view is that species rank is
not justified in either. The gene flow between northern and southern
magpie populations is judged to be sufficiently restricted to justify
terming them subspecies (not full species); but the seamless way that
local Koala populations blend one into another shows that there is
substantial gene flow between north and south. As a result, experts
tend to reject even subspecies rank in this case.
The difference question
Obviously, when defining a species, the geographic circumstances become
meaningful only if the populations groups in question are clearly different:
if they are not consistently and reliably distinguishable from one another,
then we have no grounds for believing that they might be different species.
The key question in this context, is "how different is different?" and the
answer is usually "it all depends".
In theory, it would be possible to recognise even the tiniest of differences
as sufficient to delineate a separate species, provided only that the
difference is clear and consistent (and that other criteria are met). There
is no universal rule to state the smallest allowable difference between two
species, but in general, very trivial differences are ignored on the twin
grounds of simple practicality, and genetic similarity: if two population
groups are so close that the distinction between them rests on an obscure
and microscopic difference in morphology, or a single base substitution in a
DNA sequence, then a demonstration of restricted gene flow between the
populations will probably be difficult in any case.
More typically, one or other of the following requirements must be met:
* It is possible to reliably measure a quantitative difference between
the two groups that does not overlap. A population has, for example,
thicker fur, rougher bark, longer ears, or larger seeds than another
population, and although this characteristic may vary within each
population, the two do not grade into one another, and given a
reasonably large sample size, there is a definite discontinuity between
them. Note that this applies to populations, not individual organisms,
and that a small number of exceptional individuals within a population
may 'break the rule' without invalidating it. The less a quantitative
difference varies within a population and the more it varies between
populations, the better the case for making a distinction.
Nevertheless, borderline situations can only be resolved by making a
* It is possible to distinguish a qualitative difference between the
populations; a feature that does not vary continuously but is either
entirely present or entirely absent. This might be a distinctively
shaped seed pod, an extra primary feather, a particular courting
behaviour, or a clearly different DNA sequence.
Sometimes it is not possible to isolate a single difference between species,
and several factors must be taken in combination. This is often the case
with plants in particular. In eucalypts, for example, Corymbia ficifolia
cannot be reliably distinguished from its close relative Corymbia calophylla
by any single measure (and sometimes individual trees cannot be definitely
assigned to either species), but populations of Corymbia can be clearly told
apart by comparing the colour of flowers, bark, and buds, number of flowers
for a given size of tree, and the shape of the leaves and fruit.
When using a combination of characteristics to distinguish between
populations, it is necessary to use a reasonably small number of factors (if
more than a handful are needed, the genetic difference between the
populations is likely to be insignificant and is unlikely to endure into the
future), and to choose factors that are functionally independent (height and
weight, for example, should usually be considered as one factor, not two).
Historical development of the species concept
In the earliest works of science, a species was simply an individual
organism that represented a group of similar or nearly identical organisms.
No other relationships beyond that group were implied. When early observers
began to develop systems of organization for living things, they began to
place formerly isolated species into a context. To the modern mind, many of
the schemes delineated are whimsical at best, such as those that determined
consanguinity based on color (all plants with yellow flowers) or behavior
(snakes, scorpions and certain biting ants).
In the 18th century Carolus Linnaeus classified organisms according to
differences in the form of reproductive apparatus. Although his system of
classification sorts organisms according to degrees of similarity, it made
no claims about the relationship between similar species. At the time, it
was common to believe that there is no organic connection between species,
no matter how similar they appear; every species was individually created by
God, a view today called creationism. This approach also suggested a type of
idealism: the notion that each species exists as an "ideal form". Although
there are always differences (although sometimes minute) between individual
organisms, Linnaeus considered such variation problematic. He strove to
identify individual organisms that were exemplary of the species, and
considered other non-exemplary organisms to be deviant and imperfect.
By the 19th century most naturalists understood that species could change
form over time, and that the history of the planet provided enough time for
major changes. As such, the new emphasis was on determining how a species
could change over time. Lamarck suggested that an organism could pass on an
acquired trait to its offspring. As an example, imagine an animal that
repeatedly stretches its neck in order to reach the treetops: the longer
neck that it has acquired would then, according to this theory, be passed on
to its offspring. This well-known and simplistic example, however, does not
do justice to the breadth and subtly of Lamarck's ideas.
Lamarck's most important insight may have been that species can be
extraordinarily fluid; his 1809 Zoological Philosophy contained one of the
first logical refutations of creationism. With the advent of Darwin,
Lamarck's reputation suffered gravely. It was not until the late 20th
century that his work began to be reexamined, and took its place as a
fundamental stepping stone to the modern theory of adaptive mutation.
Lamarck's long-discarded ideas of the goal-oriented evolution of species,
also known the teleological process, have also received renewed attention,
particularly by proponents of artificial selection.
Charles Darwin and Alfred Wallace provided what scientists now consider the
most powerful and compelling theory of evolution. Basically, Darwin argued
that it is populations that evolve, not individuals. His argument relies on
a radical shift in perspective from Linnaeus: rather than defining species
in ideal terms (and searching for an ideal representative and rejecting
deviations), Darwin considered variation among individuals to be natural. He
further argued that such variation, far from being problematic, is actually
a good thing.
Following Thomas Malthus, he suggested that population would often exceed
the amount of food available, and that some organisms would die. Darwin
suggested that those organisms that would die would be those less adapted to
their environment, and that those that survived -- and reproduced -- would
be those best adapted to their environment. Variation among members of a
species is important because different and changing environments favor
different traits (i.e. there is no ideal trait; whether a trait is
beneficial or not depends on the environment).
These survivors would not pass acquired traits on to their offspring; they
would pass their inherited traits on to their offspring. But since the
environment effectively selected which organisms would live to reproduce,
the environment would select which traits would be passed on. This is the
theory of evolution by "natural selection." For example, among a group of
animals some have longer necks, others have shorter necks. If all the leaves
are high up, those with shorter necks will die; those with longer necks will
thrive. This process is evident today as resistant strains of bacteria
The development of the field of genetics (many years after Darwin) has
revealed the mechanisms that generate variation as well as those through
which traits are passed on from generation to generation.
The theory of the evolution of species through natural selection has two
important implications for discussions of species -- consequences that
fundamentally challenge the assumptions behind Linnaeus' taxonomy. First, it
suggests that species are not just similar, they may actually be related.
Some students of Darwin argue that all species are descended from a common
ancestor. Second, it supposes that "species" are not homogeneous, fixed,
permanent things; members of a species are all different, and over time
species change. This suggests that species do not have any clear boundaries
but are rather momentary statistical effects of constantly changing
gene-frequencies. One may still use Linnaeus' taxonomy to identify
individual plants and animals, but one can no longer think of species as
independent and immutable.
The rise of a new species from a parental line is called speciation. There
is no clear line demarcating the ancestral species from the descendant species.
Although the current scientific understanding of species suggests there is
no principled, black and white way to distinguish between different species
in all cases, biologists continue to seek concrete ways to operationalize
the idea. One of the most popular biological definitions of species is in
terms of reproductive isolation; if two creatures cannot reproduce to
produce fertile offspring, then they are in different species. This
definition captures a number of intuitive species boundaries, but
nonetheless has some problems, however. It has nothing to say about species
that reproduce asexually, for example, and is it very difficult to apply to
extinct species. Moreover, boundaries between species are often fuzzy: there
are examples where members of one population can produce fertile offspring
with a second population, and members of the second population can produce
fertile offspring with members of a third population, but members of the
first and third population cannot produces fertile offspring. Consequently,
some people reject this notion of species.
Richard Dawkins defines two organisms as conspecific if and only if they
have the same number of chromosomes and, for each chromosome, both organisms
have the same number of nucleotides. (The Blind Watchmaker, p. 118)
The classification of species has been profoundly affected by technological
advances that have allowed researchers to determine relatedness based on
genetic markers. The results have been nothing short of revolutionary,
resulting in the reordering of vast expanses of the phylogenetic tree (see
also molecular phylogeny).
A species name can be:
* A noun in apposition with the genus: Panthera leo. The words agree in
case but not necessarily in gender.
* An adjective, agreeing in case and gender with the genus: Allium
* A noun or adjective in the genitive. This is common in parasites: Xenos
vesparum, Anaticola phoenicopteri. Also, names of people and places are
used in the genitive: Latimeria chalumnae.
There are several common species names. Most of these are adjectives.
Linnaean taxonomy discusses how the category "species" meshes with other
classification categories, such as "kingdom" and "genus".
In chemistry, a species indicates that two particles are the same atomic
nucleus, atom, molecule, or ion.