Species
Species is a taxonomic concept used in biology to refer to a population of organisms that are in some important ways similar.
Importance in biological classification
The idea of species has a long history. In formal scientific classification, the species level lies below the genus and above the subspecies. It is one of the most important levels of classification, for several reasons:- It often corresponds to what lay people treat as the different basic kinds of organism - dogs are one species, cats another.
- It appears in the standard binomial nomenclature (or trinomial nomenclature) by which scientists typically refer to organisms/
- It is the only taxonomic level which has empirical content,
in the sense that asserting that two animals are of different
species is saying something more than classificatory about
them.
Definitions of Species
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.
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- 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 populations.
- 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 reproduce sexually.
- 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.
Implications of assignment of species status
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 taxon, and many formerly grouped populations have been split. At higher taxonomic levels, these changes have been still more profound.From a taxonomical point of view, groups within a species can be defined as being of a taxon hierachically lower than a species. In zoology only the subspecies is used while in botany the variety, subvariety and form are used as well. The term for persons lumping taxons together into one is lumpers and the term for splitting a taxon into multiple, often new, taxons is splitters.
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 potentially
controversial.
- 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
'best-guess' judgement.
- 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.
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 evolve.
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 it is 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 sativum.
- 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.
Linnaean taxonomy discusses how the taxon "species" meshes with other classification categories, such as "kingdom" and "genus".