In biology , a SPECIES is the basic unit of classification and a
taxonomic rank , as well as a unit of biodiversity , but it has proven
difficult to find a satisfactory definition. Scientists and
conservationists need a species definition which allows them to work,
regardless of the theoretical difficulties. If as
species were fixed, there would be no problem, but evolutionary
processes cause species to change continually, and to grade into one
another. A species is often defined as the largest group of organisms
in which two individuals can produce fertile offspring , typically by
sexual reproduction . While this definition is often adequate, when
looked at more closely it is problematic . For example, with
hybridisation , in a species complex of hundreds of similar
microspecies , or in a ring species , the boundaries between closely
related species become unclear. Among organisms that reproduce only
asexually , the concept of a reproductive species breaks down, and
each clone is potentially a microspecies. Problems also arise when
dealing with fossils , since reproduction cannot be examined; the
concept of the chronospecies is therefore used in palaeontology .
Other ways of defining species include their karyotype ,
morphology , behaviour or ecological niche .
All species are given a two-part name , a "binomial". The first part
of a binomial is the genus to which the species belongs. The second
part is called the specific name or the specific epithet (in botanical
nomenclature , also sometimes in zoological nomenclature ). For
Boa constrictor is one of four species of the Boa genus.
Species were seen from the time of
Aristotle until the 18th century
as fixed kinds that could be arranged in a hierarchy, the great chain
of being . In the 19th century, biologists grasped that species could
evolve given sufficient time.
Charles Darwin 's 1859 book The Origin
Species explained how species could arise by natural selection .
That understanding was greatly extended in the 20th century through
genetics and population ecology . Genetic variability arises from
mutations and recombination , while organisms themselves are mobile,
leading to geographical isolation and genetic drift with varying
selection pressures. Genes can sometimes be exchanged between species
by horizontal gene transfer ; new species can arise rapidly through
hybridisation and polyploidy ; and species may become extinct for a
variety of reasons.
Viruses are a special case, driven by a balance of
mutation and selection , and can be treated as quasispecies .
As a practical matter, species concepts may be used to define species
that are then used to measure biodiversity , though whether this is a
good measure is disputed, as other measures are possible.
* 1 History
* 1.1 Classical forms
* 1.2 Fixed species
* 1.3 The possibility of change
* 2 Taxonomy and naming
* 2.1 Common and scientific names
* 2.3 Abbreviations
* 2.4 Identification codes
* 2.5 Lumping and splitting
* 2.6 Broad and narrow senses
* 3 Mayr\'s biological species concept
* 4 The species problem
* 4.1 When Mayr\'s concept breaks down
* 4.2 Aggregates of microspecies
* 4.3 Hybridisation
* 5 Attempts at definition
* 5.1 Typological or morphological species
* 5.2 Recognition and cohesion species
* 5.3 Genetic similarity and barcode species
* 5.4 Phylogenetic, cladistic, or evolutionary species
* 5.5 Ecological species
* 5.6 Genetic species
Evolutionarily significant unit
* 6 Change
* 6.2 Exchange of genes between species
* 7 Practical implications
* 8 See also
* 9 Notes
* 10 Cited sources
* 11 External links
Main article: Aristotle\'s biology
In his biology ,
Aristotle used the term γένος (génos) to mean
a kind, such as a bird or fish , and εἶδος (eidos) to mean a
specific form within a kind, such as (within the birds) the crane ,
eagle , crow , or sparrow . These terms were translated into Latin as
"genus" and "species", though they do not correspond to the Linnean
terms thus named; today the birds are a class , the cranes are a
family , and the crows a genus . A kind was distinguished by its
attributes ; for instance, a bird has feathers, a beak, wings, a
hard-shelled egg, and warm blood. A form was distinguished by being
shared by all its members, the young inheriting any variations they
might have from their parents.
Aristotle believed all kinds and forms
to be distinct and unchanging. His approach remained influential until
John Ray believed that species breed true and do not change,
even though variations exist. Main article:
Great chain of being
When observers in the
Early Modern period began to develop systems of
organization for living things, they placed each kind of animal or
plant into a context. Many of these early delineation schemes would
now be considered whimsical: schemes included consanguinity based on
colour (all plants with yellow flowers) or behaviour (snakes,
scorpions and certain biting ants).
John Ray , an English naturalist,
was the first to attempt a biological definition of species in 1686,
No surer criterion for determining species has occurred to me than
the distinguishing features that perpetuate themselves in propagation
from seed. Thus, no matter what variations occur in the individuals or
the species, if they spring from the seed of one and the same plant,
they are accidental variations and not such as to distinguish a
species ... Animals likewise that differ specifically preserve their
distinct species permanently; one species never springs from the seed
of another nor vice versa. Carl
Linnaeus created the binomial
system for naming species.
In the 18th century, the Swedish scientist Carl
organisms according to shared physical characteristics, and not simply
based upon differences. He established the idea of a taxonomic
hierarchy of classification based upon observable characteristics and
intended to reflect natural relationships. At the time, however, it
was still widely believed that there was no organic connection between
species, no matter how similar they appeared. This view was influenced
by European scholarly and religious education, which held that the
categories of life are dictated by God, forming an Aristotelian
hierarchy, the scala naturae or great chain of being . However,
whether or not it was supposed to be fixed, the scala (a ladder)
inherently implied the possibility of climbing.
THE POSSIBILITY OF CHANGE
Faced with evidence of hybridisation,
Linnaeus came to accept that
species could change, and the struggle for survival, but not that new
species could freely evolve. By the 19th century, naturalists
understood that species could change form over time, and that the
history of the planet provided enough time for major changes.
Jean-Baptiste Lamarck , in his 1809 Zoological Philosophy, described
the transmutation of species , proposing that a species could change
over time, in a radical departure from Aristotelian thinking.
Charles Darwin and
Alfred Russel Wallace
Alfred Russel Wallace provided a
compelling account of evolution and the formation of new species.
Darwin argued that it was populations that evolved, not individuals,
by natural selection from naturally occurring variation among
individuals. This required a new definition of species. Darwin
concluded that species are what they appear to be: ideas,
provisionally useful for naming groups of interacting individuals,
I look at the term species as one arbitrarily given for the sake of
convenience to a set of individuals closely resembling each other ...
It does not essentially differ from the word variety, which is given
to less distinct and more fluctuating forms. The term variety, again,
in comparison with mere individual differences, is also applied
arbitrarily, and for convenience sake.
TAXONOMY AND NAMING
A cougar, mountain lion, panther, or puma, among other common
names: its scientific name is
Puma concolor .
COMMON AND SCIENTIFIC NAMES
The commonly used names for kinds of organisms are often ambiguous:
"cat" could mean the domestic cat,
Felis catus , or the cat family,
Felidae . Another problem with common names is that they often vary
from place to place, so that puma, cougar, catamount, panther, painter
and mountain lion all mean
Puma concolor in various parts of America,
while "panther" may also mean the jaguar (Panthera onca) of Latin
America or the leopard (Panthera pardus) of Africa and Asia. In
contrast, the scientific names of species are chosen to be unique and
universal; they are in two parts used together: the genus as in Puma,
and the specific epithet as in concolor.
The type specimen (holotype ) of Lacerta plica , described by
Linnaeus in 1758 Main article:
A species is given a taxonomic name when a type specimen is described
formally, in a publication that assigns it a unique scientific name.
The description typically provides means for identifying the new
species, differentiating it from other previously described and
related or confusable species and provides a validly published name
(in botany) or an available name (in zoology) when the paper is
accepted for publication. The type material is usually held in a
permanent repository, often the research collection of a major museum
or university, that allows independent verification and the means to
compare specimens. Describers of new species are asked to choose
names that, in the words of the International Code of Zoological
Nomenclature , are "appropriate, compact, euphonious, memorable, and
do not cause offence".
Books and articles sometimes intentionally do not identify species
fully and use the abbreviation "sp." in the singular or "spp."
(standing for species pluralis, the Latin for multiple species) in the
plural in place of the specific name or epithet (e.g. Canis sp.) This
commonly occurs when authors are confident that some individuals
belong to a particular genus but are not sure to which exact species
they belong, as is common in paleontology . Authors may also use
"spp." as a short way of saying that something applies to many species
within a genus, but not to all. If scientists mean that something
applies to all species within a genus, they use the genus name without
the specific name or epithet. The names of genera and species are
usually printed in italics . Abbreviations such as "sp." should not be
italicised. When a species identity is not clear a specialist may use
"cf." before the epithet to indicate that confirmation is required.
The abbreviations "nr." (near) or "aff." (affine) may be used when the
identity is unclear but when the species appears to be similar to the
species mentioned after.
With the rise of online databases, codes have been devised to provide
identifiers for species that are already defined, including:
National Center for Biotechnology Information
National Center for Biotechnology Information (NCBI) employs a
numeric 'taxid' or Taxonomy identifier, a "stable unique identifier",
e.g., the taxid of
Homo sapiens is 9606.
Kyoto Encyclopedia of Genes and Genomes (KEGG) employs a three- or
four-letter code for a limited number of organisms; in this code, for
example, H. sapiens is simply hsa.
UniProt employs an "organism mnemonic" of not more than five
alphanumeric characters, e.g., HUMAN for H. sapiens.
Integrated Taxonomic Information System
Integrated Taxonomic Information System (ITIS) provides a unique
number for each species. The LSID for
Homo sapiens is
LUMPING AND SPLITTING
Lumpers and splitters
The naming of a particular species, including which genus (and higher
taxa) it is placed in, is 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. Sometimes, especially in the past when communication was more
difficult, taxonomists working in isolation have given two distinct
names to individual organisms later identified as the same species.
When two named species are discovered to be of the same species, the
older species name is given priority and usually retained, and the
newer name considered as a junior synonym, a process called
synonymisation. Dividing a taxon into multiple, often new, taxa is
called splitting. Taxonomists are often referred to as "lumpers" or
"splitters" by their colleagues, depending on their personal approach
to recognising differences or commonalities between organisms.
BROAD AND NARROW SENSES
The nomenclatural codes that guide the naming of species, including
the ICZN for animals and the ICN for plants, do not make rules for
defining the boundaries of the species. Research can change the
boundaries, also known as circumscription, based on new evidence.
Species may then need to be distinguished by the boundary definitions
used, and in such cases the names may be qualified with sensu stricto
("in the narrow sense") to denote usage in the exact meaning given by
an author such as the person who named the species, while the antonym
sensu lato ("in the broad sense") denotes a wider usage, for instance
including other subspecies. Other abbreviations such as "auct."
("author") and "non." ("not") may be used to further clarify the sense
in which the specified authors delineated or described the species.
MAYR\'S BIOLOGICAL SPECIES CONCEPT
Ernst Mayr proposed the widely used Biological
of reproductive isolation in 1942. Main article: Biological species
Most modern textbooks make use of
Ernst Mayr 's 1942 definition,
known as the Biological
Species Concept as a basis for further
discussion on the definition of species. It is also called a
reproductive or isolation concept. This defines a species as
groups of actually or potentially interbreeding natural populations,
which are reproductively isolated from other such groups.
It has been argued that this definition is a natural consequence of
the effect of sexual reproduction on the dynamics of natural
selection. Mayr's use of the adjective "potentially" has been a
point of debate; some interpretations exclude unusual or artificial
matings that occur only in captivity, or that involve animals capable
of mating but that do not normally do so in the wild.
THE SPECIES PROBLEM
It is difficult to define a species in a way that applies to all
organisms. The debate about species delimitation is called the
species problem . The problem was recognized even in 1859, when
Darwin wrote in On the Origin of
No one definition has satisfied all naturalists; yet every naturalist
knows vaguely what he means when he speaks of a species. Generally the
term includes the unknown element of a distinct act of creation.
WHEN MAYR\'S CONCEPT BREAKS DOWN
Palaeontologists are limited to morphological evidence when
deciding whether fossil life-forms like these
formed a separate species.
A simple textbook definition, following Mayr's concept, works well
for most multi-celled organisms , but breaks down in several
* When organisms reproduce asexually , as in single-celled organisms
such as bacteria and other prokaryotes , and parthenogenetic or
apomictic multi-celled organisms. The term quasispecies is
sometimes used for rapidly mutating entities like viruses.
* When scientists do not know whether two morphologically similar
groups of organisms are capable of interbreeding; this is the case
with all extinct life-forms in palaeontology , as breeding experiments
are not possible.
* When hybridisation permits substantial gene flow between species.
* In ring species , when members of adjacent populations in a widely
continuous distribution range interbreed successfully but members of
more distant populations do not.
The willow warbler and chiffchaff are almost identical in
appearance but do not interbreed.
Species identification is made difficult by discordance between
molecular and morphological investigations; these can be categorized
as two types: (i) one morphology, multiple lineages (e.g.
morphological convergence , cryptic species ) and (ii) one lineage,
multiple morphologies (e.g. phenotypic plasticity , multiple
life-cycle stages). In addition, horizontal gene transfer (HGT) makes
it difficult to define a species. All species definitions assume that
an organism acquires its genes from one or two parents very like the
"daughter" organism, but that is not what happens in HGT. There is
strong evidence of HGT between very dissimilar groups of prokaryotes ,
and at least occasionally between dissimilar groups of eukaryotes ,
including some crustaceans and echinoderms .
The evolutionary biologist
James Mallet concludes that
there is no easy way to tell whether related geographic or temporal
forms belong to the same or different species.
Species gaps can be
verified only locally and at a point of time. One is forced to admit
that Darwin's insight is correct: any local reality or integrity of
species is greatly reduced over large geographic ranges and time
AGGREGATES OF MICROSPECIES
Species complex and
The species concept is further weakened by the existence of
microspecies , groups of organisms, including many plants, with very
little genetic variability, usually forming species aggregates . For
example, the dandelion
Taraxacum officinale and the blackberry Rubus
fruticosus are aggregates with many microspecies—perhaps 400 in the
case of the blackberry and over 200 in the dandelion, complicated by
hybridisation , apomixis and polyploidy , making gene flow between
populations difficult to determine, and their taxonomy debatable.
Species complexes occur in insects such as
vertebrates such as
Hypsiboas treefrogs, and fungi such as the fly
Blackberries belong to any of hundreds of microspecies of the Rubus
fruticosus species aggregate .
The butterfly genus
Heliconius contains many similar species.
Hypsiboas calcaratus –fasciatus species complex contains at
least six species of treefrog.
Natural hybridisation presents a challenge to the concept of a
reproductively isolated species, as fertile hybrids permit gene flow
between two populations. For example, the carrion crow Corvus corone
and the hooded crow Corvus cornix appear and are classified as
separate species, yet they hybridise freely where their geographical
* Hybridisation of carrion and hooded crows permits gene flow
Hybrid with dark belly, dark gray nape
Hybrid with dark belly
A ring species is a connected series of neighbouring populations,
each of which can sexually interbreed with adjacent 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 flow between each "linked" population. Such
non-breeding, though genetically connected, "end" populations may
co-exist in the same region thus closing the ring.
Ring species thus
present a difficulty for any species concept that relies on
reproductive isolation. However, ring species are at best rare.
Proposed examples include the herring gull -lesser black-backed gull
complex around the North pole, the
Ensatina eschscholtzii group of 19
populations of salamanders in America, and the greenish warbler in
Asia, but many so-called ring species have turned out to be the
result of misclassification leading to questions on whether there
really are any ring species.
Seven "species" of
Larus gulls interbreed in a ring around the
Opposite ends of the ring: a herring gull (
Larus argentatus ) (front)
and a lesser black-backed gull (
Larus fuscus )
A greenish warbler , Phylloscopus trochiloides
Presumed evolution of five "species" of greenish warblers around
ATTEMPTS AT DEFINITION
Biologists and taxonomists have made many attempts to define species,
beginning from morphology and moving towards genetics . Early
taxonomists such as
Linnaeus had no option but to describe what they
saw: this was later formalised as the typological or morphological
species concept. Mayr emphasised reproductive isolation, but this,
like other species concepts, is hard or even impossible to test.
Later biologists have tried to refine Mayr's definition with the
recognition and cohesion concepts, among others. Many of the concepts
are quite similar or overlap, so they are not easy to count: the
biologist R. L. Mayden recorded about 24 concepts, and the
philosopher of science John Wilkins counted 26. Wilkins further
grouped the species concepts into seven basic kinds of concepts: (1)
agamospecies for asexual organisms (2) biospecies for reproductively
isolated sexual organisms (3) ecospecies based on ecological niches
(4) evolutionary species based on lineage (5) genetic species based on
gene pool (6) morphospecies based on form or phenotype and (7)
taxonomic species, a species as determined by a taxonomist.
TYPOLOGICAL OR MORPHOLOGICAL SPECIES
All adult Eurasian blue tits share the same coloration,
unmistakably identifying the morphospecies.
A typological species is a group of organisms in which individuals
conform to certain fixed properties (a type), so that even
pre-literate people often recognise the same taxon as do modern
taxonomists. The clusters of variations or phenotypes within
specimens (such as longer or shorter tails) would differentiate the
species. This method was used as a "classical" method of determining
species, such as with
Linnaeus early in evolutionary theory. However,
different phenotypes are not necessarily different species (e.g. a
Drosophila born to a two-winged mother is not a different
Species named in this manner are called morphospecies.
In the 1970s,
Robert R. Sokal , Theodore J. Crovello and Peter Sneath
proposed a variation on this, a phenetic species, defined as a set of
organisms with a similar phenotype to each other, but a different
phenotype from other sets of organisms. It differs from the
morphological species concept in including a numerical measure of
distance or similarity to cluster entities based on multivariate
comparisons of a reasonably large number of phenotypic traits.
RECOGNITION AND COHESION SPECIES
A mate-recognition species is a group of sexually reproducing
organisms that recognize one another as potential mates. Expanding
on this to allow for post-mating isolation, a cohesion species is the
most inclusive population of individuals having the potential for
phenotypic cohesion through intrinsic cohesion mechanisms; no matter
whether populations can hybridize successfully, they are still
distinct cohesion species if the amount of hybridization is
insufficient to completely mix their respective gene pools . A
further development of the recognition concept is provided by the
biosemiotic concept of species.
GENETIC SIMILARITY AND BARCODE SPECIES
A region of the gene for the cytochrome c oxidase enzyme is used
to distinguish species in the
Life Data Systems database.
In microbiology , genes can move freely even between distantly
related bacteria, possibly extending to the whole bacterial domain. As
a rule of thumb, microbiologists have assumed that kinds of Bacteria
16S ribosomal RNA gene sequences more similar than 97%
to each other need to be checked by DNA-
DNA hybridisation to decide if
they belong to the same species or not. This concept was narrowed in
2006 to a similarity of 98.7%.
DNA hybridisation is outdated, and results have sometimes led to
misleading conclusions about species, as with the pomarine and great
skua . Modern approaches compare sequence similarity using
DNA barcoding has been proposed as a way to distinguish species
suitable even for non-specialists to use. The so-called barcode is a
region of mitochondrial
DNA within the gene for cytochrome c oxidase .
Life Data Systems (BOLD) contains
sequences from over 190,000 species. However, scientists such as Rob
DeSalle have expressed concern that classical taxonomy and DNA
barcoding, which they consider a misnomer, need to be reconciled, as
they delimit species differently. Genetic introgression mediated by
endosymbionts and other vectors can further make barcodes ineffective
in the identification of species.
PHYLOGENETIC, CLADISTIC, OR EVOLUTIONARY SPECIES
The cladistic or phylogenetic species concept is that a species
is the smallest lineage which is distinguished by a unique set of
either genetic or morphological traits. No claim is made about
reproductive isolation, making the concept useful also in
palaeontology where only fossil evidence is available.
A phylogenetic or cladistic species is an evolutionarily divergent
lineage, one that has maintained its hereditary integrity through time
and space. A cladistic species is the smallest group of populations
that can be distinguished by a unique set of morphological or genetic
traits. Molecular markers may be used to determine genetic
similarities in the nuclear or mitochondrial
DNA of various species.
For example, in a study done on fungi , studying the nucleotide
characters using cladistic species produced the most accurate results
in recognising the numerous fungi species of all the concepts studied.
Versions of the Phylogenetic
Species Concept may emphasize monophyly
Unlike the Biological
Species Concept, a cladistic species does not
rely on reproductive isolation, so it is independent of processes that
are integral in other concepts. It works for asexual lineages, and
can detect recent divergences, which the Morphological
cannot. However, it does not work in every situation, and may
require more than one polymorphic locus to give an accurate result.
The concept may lead to splitting of existing species, for example of
Bovidae , into many new ones.
An evolutionary species, suggested by
George Gaylord Simpson in 1951,
is "an entity composed of organisms which maintains its identity from
other such entities through time and over space, and which has its own
independent evolutionary fate and historical tendencies". This
differs from the biological species concept in embodying persistence
over time. Wiley and Mayden state that they see the evolutionary
species concept as "identical" to
Willi Hennig 's species-as-lineages
concept, and assert that the biological species concept, "the several
versions" of the phylogenetic species concept, and the idea that
species are of the same kind as higher taxa are not suitable for
biodiversity studies (with the intention of estimating the number of
species accurately). They further suggest that the concept works for
both asexual and sexually-reproducing species.
An ecological species is a set of organisms adapted to a particular
set of resources, called a niche, in the environment. According to
this concept, populations form the discrete phenetic clusters that we
recognise as species because the ecological and evolutionary processes
controlling how resources are divided up tend to produce those
A genetic species as defined by Robert Baker and Robert Bradley is a
set of genetically isolated interbreeding populations. This is similar
to Mayr's Biological
Species Concept, but stresses genetic rather than
reproductive isolation. In the 21st century, a genetic species can be
established by comparing
DNA sequences, but other methods were
available earlier, such as comparing karyotypes (sets of chromosomes )
and allozymes (enzyme variants).
EVOLUTIONARILY SIGNIFICANT UNIT
An evolutionarily significant unit (ESU) or "wildlife species" is a
population of organisms considered distinct for purposes of
A chronospecies is defined in a single lineage (solid line)
whose morphology changes with time. At some point, palaeontologists
judge that enough change has occurred that two species (A and B),
separated in time and anatomy, once existed. Main article:
In palaeontology , with only comparative anatomy (morphology) from
fossils as evidence, the concept of a chronospecies can be applied.
During anagenesis (evolution, not necessarily involving branching),
palaeontologists seek to identify a sequence of species, each one
derived from the phyletically extinct one before through continuous,
slow and more or less uniform change. In such a time sequence,
palaeontologists assess how much change is required for a
morphologically distinct form to be considered a different species
from its ancestors.
Viruses have enormous populations, are doubtfully living since they
consist of little more than a string of
DNA or RNA in a protein coat,
and mutate rapidly. All of these factors make conventional species
concepts largely inapplicable. A viral quasispecies is a group of
genotypes related by similar mutations, competing within a highly
mutagenic environment, and hence governed by a mutation–selection
balance . It is predicted that a viral quasispecies at a low but
evolutionarily neutral and highly connected (that is, flat) region in
the fitness landscape will outcompete a quasispecies located at a
higher but narrower fitness peak in which the surrounding mutants are
unfit, "the quasispecies effect" or the "survival of the flattest".
There is no suggestion that a viral quasispecies resembles a
traditional biological species.
Species are subject to change, whether by evolving into new species,
exchanging genes with other species, merging with other species or by
The evolutionary process by which biological populations evolve to
become distinct or reproductively isolated as species is called
Charles Darwin was the first to describe the role of
natural selection in speciation in his 1859 book The Origin of Species
Speciation depends on a measure of reproductive isolation , a
reduced gene flow. This occurs most easily in allopatric speciation,
where populations are separated geographically and can diverge
gradually as mutations accumulate.
Reproductive isolation is
threatened by hybridisation, but this can be selected against once a
pair of populations have incompatible alleles of the same gene, as
described in the
Bateson–Dobzhansky–Muller model . A different
mechanism, phyletic speciation, involves one lineage gradually
changing over time into a new and distinct form, without increasing
the number of resultant species.
EXCHANGE OF GENES BETWEEN SPECIES
Horizontal gene transfers between widely separated species
complicate the phylogeny of bacteria . Main article: Horizontal gene
Horizontal gene transfer between organisms of different species,
either through hybridisation , antigenic shift , or reassortment , is
sometimes an important source of genetic variation.
transfer genes between species.
Bacteria can exchange plasmids with
bacteria of other species, including some apparently distantly related
ones in different phylogenetic domains , making analysis of their
relationships difficult, and weakening the concept of a bacterial
Louis-Marie Bobay and Howard Ochman suggest, based on analysis of the
genomes of many types of bacteria, that they can often be grouped
"into communities that regularly swap genes", in much the same way
that plants and animals can be grouped into reproductively isolated
Bacteria may thus form species, analogous to
Mayr's biological species concept, consisting of asexually reproducing
populations that exchange genes by homologous recombination.
A species is extinct when the last individual of that species dies,
but it may be functionally extinct well before that moment. It is
estimated that over 99 percent of all species that ever lived on
Earth, some five billion species, are now extinct. Some of these were
in mass extinctions such as those at the ends of the
Cretaceous periods. Mass extinctions had a variety of
causes including volcanic activity , climate change , and changes in
oceanic and atmospheric chemistry, and they in turn had major effects
on Earth's ecology, atmosphere, land surface, and waters. Another
form of extinction is through the assimilation of one species by
another through hybridization. The resulting single species has been
termed as a "compilospecies".
Biologists and conservationists need to categorise and identify
organisms in the course of their work. Difficulty assigning organisms
reliably to a species constitutes a threat to the validity of research
results, for example making measurements of how abundant a species is
in an ecosystem moot. Paul Michael-Agapow and colleagues found that
surveys using a phylogenetic species concept reported 48% more species
and accordingly smaller populations and ranges than those using
nonphylogenetic concepts; they note that this "taxonomic inflation"
could cause a false appearance of change to the number of endangered
species and consequent political and practical difficulties. The
evolutionary biologist Jody Hey observes that there is an inherent
conflict between the desire to understand the processes of speciation
and the need to identify and to categorise.
Conservation laws in many
countries make special provisions to prevent species from going
extinct. Hybridization zones between two species, one that is
protected and one that is not, have sometimes led to conflicts between
law-makers, land owners and conservationists. One of the classic cases
in North America is that of the protected northern spotted owl which
hybridizes with the unprotected
California spotted owl and the barred
owl ; this has led to legal debates.
The botanist Brent D. Mishler states that the species problem is
created by the many ways that people want to use the species category,
but argues that the solution is to abandon the traditional ranks and
just use monophyletic groups of different inclusivess. A species is
then "simply the least inclusive taxon" of whatever type. He argues
that ecology, evolution, and conservation work are all better served
by this approach. Since in his view species are not comparable,
counting them is not a valid measure of biodiversity , and he calls
for new ways to measure it, noting that other authors such as R. I.
Vane-Wright and colleagues have proposed quantitative measures for
phylogenetic biodiversity that make use of "the number of branch
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