The Species Enigma New Scientist 13 Jun 98

Martin Brookes

THE latest estimates suggest that perhaps as many as 30 million species live
on planet Earth, though only 1.5 million have so far been described. Many
more millions have already been and gone, a few leaving fossil remains. But
what exactly is a species? The term is used to classify both living and
extinct organisms. For instance, the modern horse, known as Equus caballus,
is distinguished from the mountain zebra, Equus zebra, and both are
distinguished from the extinct Equus giganteus. The first name is the genus
name, and the second, the species name (Box 1). Most biologists define a
living species as a distinct reproductive group-a population of organisms
which naturally mate in the wild to produce viable offspring. This is called
the biological species concept (BSC) (Box 2). Though widely used, the BSC is
a relatively recent idea, and only one of many interpretations of the word
"species". Biologists have been arguing among themselves over the exact
meaning of the term for the past two hundred years, and disagreements
continue to this day All species definitions have been found to have their
limitations. The BSC, for example, relies on a knowledge of reproductive
behaviour, so it is impossible to apply it among fossil species.

Disappearing life Going extinct

BIOLOGISTS will agree, however, that biological diversity is now
disappearing at rates comparable only with the handful of major extinctions
that have occurred in the 3.5 billion years of life on the planet human
population growth, pollution and habitat destruction is largely to blame.
Because species are perceived as a prime currency of biological diversity,
how they are defined plays a crucial part in attempts to conserve life on
Earth. For more than 2000 years, the ideas of the Greek philosopher Plato
dominated scientists' views on species. Plato believed that the natural
world was underpinned by "ideas" or "essences", of which individual animals
and plants were imperfect copies. The species "type" was immutable. By the
early 1800s, it was widely understood that fossils were the remains of
ancient creatures. But while some fossils had no living relatives, others
seemed to be primitive versions of living species. If species were
immutable, what did this mean? Some writers explained the demise of these
ancient species by sudden catastrophes, such as the Biblical flood, perhaps
followed by the creation by God of new species. But in 1859, Charles Darwin
offered, in his famous book On the Origin of Species, an explanation of
fossils so compelling that it ended forever the notion of species as
Godgiven and unchanging. After many years of study around the world, Darwin
suggested that all living organisms arose from pre-existing forms through a
slow and gradual process of change and modification-evolution by natural
selection.

At any point in time, the characteristics of animals and plants might seem
fixed and unchanging. But over thousands or millions of years, these
characteristics were likely to be modified as organisms adapted to changes
in their environment. Before Darwin, one of the problems confronting natural
historians was how to interpret variation within species. Many species vary
in size, shape or colour over their geographic range. House sparrows in
North America, for example, are larger in the northern states than in the
south. The terms variety, race or subspecies are used to describe this
variation within a species. Before Darwin, local variants were thought of as
imperfections in the underlying "essence", as deviations from the ideal. But
to Darwin, the existence of variation was not a mistake, but the key to his
evolutionary theory. Darwin observed that much of the variation seen among
individuals is passed on to offspring. Those variants which were better
adapted to their environment would leave more offspring. Varieties could be
the basis for new species, especially if they became geographically
isolated, or carried adaptations which allowed them to exploit new
resources. Despite the title of his book, Darwin offered no concrete
definition of species. Nor did he see a need to do so. For Darwin, varieties
and species were relative terms abstract boundaries we impose on a world
always in transition. As he wrote in On the Origin of Species: "In short we
shall have to treat species in the same manner as those naturalists treat
genera, who admit that genera are merely artificial combinations made for
convenience. This may not be a cheering prospect, but we shall at least be
freed from the vain search for the undiscovered and undiscoverable essence
of the term species." Darwin's evolutionary ideas shifted species from an
idealised concept to an arbitrary definition. They also forced those who
classified the living world, known as taxonomists, to consider descent as an
underlying principle. Taxonomists were no longer attempting to decipher
God's essences, but identifying individuals with a shared evolutionary
history. Despite this philosophical shift, in practice the business of
defining species changed little. Defining species boiled down to finding
those common features in a group of individual organisms which distinguished
them from other groups of organisms. A hundred years ago, taxonomists would
use morphological features-the shape, size and colour of organisms and body
parts -to make their species distinctions. This view of species, as clusters
of individuals physically distinct from other clusters, became known as the
morphological species definition. The main problem with this approach is the
question of how different two groups have to be before they can be called
separate species? Since there is no absolute criterion governing the
demarcation of the boundaries between species, taxonomists regularly
indulged in minor skirmishes over what was, or what wasn't, a species.

Reproductive glue In the breeding

IN THE 1930s two American evolutionary biologists, Ernst Mayr and Theodosius
Dobzhansky, both highly critical of the morphological species definition,
introduced the BSC. By placing all living things into distinct reproductive
units, they believed that the BSC would remove much of the ambiguity
inherent in the morphological definition. The criterion was now simple. If a
group of individuals could potentially breed together in the wild to produce
fertile offspring, then they belonged to the same species. Reproductive
compatibility was seen as the glue which maintained species integrity. An
emphasis on reproduction and the life cycle would also help resolve a range
of incidental problems that had dogged morphological species definitions,
such as the obvious morphological difficulty of classifying a caterpillar
and a butterfly in the same species, or males and females which were
markedly different. Even Carolus Linnaeus, the founding father of modern
taxonomy (see Box 1), was confused by the mallard duck, classifying males
and females as distinct species. Morphological definitions had also missed
the existence of "sibling species", natural populations which look
morphologically identical but do not interbreed with one another. Sibling
species occur in all groups of animals, but seem to be much more common in
insects and other invertebrates. One celebrated case of confusion caused by
morphological definitions and sibling species arose out of attempts to
control malaria in Europe earlier this century.

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MODERN taxonomy owes a great debt to the Swedish botanist Carolus Linnaeus
(17071778), who took on the onerous task of categorising and classifying all
living things. Linnaeus's hierarchical system of classification, which
persists to this day, set out to group similar-looking species into genera,
genera into families, families into orders, and so on. Linnaeus also
established the binomial system of nomenclature. Each species is given a
scientific name consisting of two Latinised names, which are italicised or
underlined, followed by the authority or organisation responsible for the
name. Corvus frugilegus L., for example, is the scientific name for the
raven. Corvus is the name of the genus, while frugilegus denotes the
particular species. Other members of the genus include the crow, Corvus
corone L. The capital "L' is an abbreviation for Linnaeus, who was
responsible for the name. A third Latinised name following the species
epithet denotes a subspecies. The crow, for example, is recognised as two
distinct subspecies, the carrion crow Corvus corone corone L. and the hooded
crow Corvus corone cornix L. Although common names like rook and raven are
easier to remember, they differ between regions or countries. A formal
system of naming ensures that information about animals and plants can be
communicated worldwide without confusion.

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At one time, it was thought that there was just one type of mosquito that
carried malaria, Anopheles maculipennis. Various attempts made to control
the disease by spraying insecticides on the mosquito populations had only
limited success until it was realised that this single "species" was
actually made up of six sibling species, only three of which transmitted the
disease.

Hybrid hiatus Mules and tigons

THE BSC had immense conceptual appeal. But it introduced new problems of its
own. If a species could only be defined if its members were shown to be
reproductively compatible, the only way to finally prove the point would be
to conduct countless breeding experiments and discover which hybrid
offspring were sterile or stillborn. Mating a horse with a donkey produces a
sterile mule, while crossing a tiger with a lion yields a sterile tigon.
There are thousands of plants in which well defined varieties produce
sterile offspring when crossed. But even if fertile offspring were produced
in an experiment, did this have any relevance to what actually happened in
the wild, where organisms might have little opportunity to interbreed
because of differences in their behaviour or habitat? As a rule of thumb,
biologists argued that if there were no naturally occurring hybrids in those
areas where two similar organisms shared the same environment, then this was
a good indication that they were "reproductively isolated". But when two
populations are geographically separated, it becomes impossible to test
their species status this way. This was not the only difficulty A definition
which depended on the notion of interbreeding reproductive communities
necessarily excluded fossils. The BSC also complicated the picture for
species which normally reproduce asexually, that is without the exchange of
genetic material between the sexes. This method of reproduction occurs
widely in nature, and is very common among bacteria.

DNA to the rescue? Genetic distance

THE discovery of the DNA code revolutionised taxonomy Each individual
organism inherits a sequence of nucleotide bases which is highly preserved
from generation to generation. Chance changes in this code happen fairly
infrequently, and if these genetic mutations are not damaging to the
individual, they can be passed on to the next generation. A tree of
mutations should give biologists the tree of evolutionary descent. Though
this approach is complicated by the differences in rates of mutation between
genes and species, and has failed to provide an ultimate arbiter on what
constitutes a species, it is now adding immensely to our understanding of
the relatedness of different populations. One recurring problem is that
variability in DNA is often not correlated to variability in morphology or
reproductive compatibility. Take the case of the chimpanzee Pan troglodytes
and the pygmy chimpanzee or bonobo, Pan paniscus. To most biologists, these
are distinct species. They certainly look and act differently. Bonobos are
smaller, have different habits and their unique and promiscuous sex lives
are legendar-y. Yet DNA tests reveal virtually no differences between the
two "species". The case of the dusky seaside sparrow provides another
interesting example. In 1987, the last dusky seaside sparrow, previously
found all over Florida, died in captivity. its dark colour led taxonomists
to define it as a subspecies, distinct from other seaside sparrows found
along the Atlantic and Gulf coasts of North America. Despite the efforts of
conservationists, the bird was wiped out along with its salt marsh habitat.
Using some fancy genetic techniques, it was possible to extract DNA from
preserved specimens and analyse it. Remarkably, this retrospective genetic
study revealed that dusky seaside sparrows were genetically
indistinguishable from Atlantic forms. Furthermore, the morphologically
identical Gulf and Atlantic forms were genetically quite different from one
another. The practical difficulties of using a single system for defining
species make taxonomy a source of endless controversy. In general,
taxonomists keep the BSC as an ideal concept, while taking into account
morphological, ecological, behavioural and DNA differences as a measure of
likely reproductive isolation. There are cases where all these measures tell
the same story. Humans and chimpanzees, for example, do not breed, differ
from one another in their morphology, and are on average about 1-3 per cent
different in their DNA. In other cases, the stories are not the same. This
tells us something about the complex processes that lead to speciation, the
formation of new species, and the variety of means by which populations
remain distinct in nature beyond simple reproductive incompatibility.

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Species Defined

THE WORD "species" is actually Latin for "kind" or "form". Many alternatives
have been suggested, and the list below outlines just a few of the more
popular modern definitions.

    * Biological species concept-a group of actually or potentially
      interbreeding populations which are reproductively isolated from other
      such groups. The BSC encapsulates the idea that species are the real
      and fundamental units of evolution, while higher taxonomic cate gories
      such as genera, families and orders are more artificial collections
      made for convenience, though loosely reflecting evolutionary
      relationships.
    * Morphological species definition a species is defined by a given set of
      common features not shared by other groups. Some of these features will
      dis tinguish morphological discontinuities or "gaps" between
      populations.
    * Evolutionary species concept-a species is defined by its shared evolu
      tionary history and descent from a common ancestor. Unlike the BSC, the
      evolutionary concept also allows fossils and asexual organisms to be
      classified as species. Although conceptually appealing, discovering the
      precise evo lutionary history of organisms is practi cally impossible.
    * Genotypic cluster definition-a recently introduced definition which is
      essentially a genetic version of the mor phological definition. Genetic
      rather than morphological "gaps" identify the distinctions between
      species.

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INTERESTINGLY, Darwin foresaw the problem of supposing that speciation was
due solely to reproductive incompatibility. He rejected reproductive
incompatibility as an absolute guide to whether two forms were distinct
species because of inconsistencies between the degree of external difference
between related forms and their interbreeding potential. All breeds of
domestic dog, for example, can mate with one another to produce fertile
offspring, despite the fact that they look remarkably different.

And as recent studies are increasingly showing, many apparently distinct
species not only freely hybridise in nature but also remain distinct from
one another. Darwin's finches were once cited as a classic example of how
new species evolved, or "speciated", when geographically separated on
islands. The 14 distinct "species" are all endemic to the Galapagos Islands,
a volcanic archipelago lying a few hundred miles off the coast of Ecuador in
the Pacific Ocean. Each species has a differently shaped beak, adapted for
feeding on one type of seed. But here is the twist. Most people would think
of Darwin's finches as "good" biological species. But recent studies show
that some of these "species" freely interbreed in the wild to produce
healthy hybrids. it is not clear why the groups remain distinct, but
presumably hybrids with intermediate beak shapes are less well adapted for
feeding, and lose out to the "pure" forms in the competition for food. When
El Nifio triggered significant but temporary changes in the vegetation of
the islands in the 1980s, the number of hybrids increased, and there are
signs that differences between the forms are being further eroded. Darwin's
finches are not an isolated example. Current estimates suggest that at least
10 per cent of animal species hybridise in nature. Birds, butterflies and
coral reef fish all include huge numbers of hybridising species. These
hybrids are not necessarily weak, sterile or deformed, but are frequently
found to be healthy individuals capable of reproducing. Even the blue whale,
the largest animal that has ever lived, has been found to breed in the wild
with the fin whale, producing fertile offspring. In plants, hybridisation is
so common that botanists have found the BSC practically unworkable.

Conserving Species Think biodiversity

THE LESSON steadily emerging from such studies is that planning to preserve
simply the maximum number of species is probably not the best approach to
good conservation policy. Instead we must focus our efforts on preserving
biodiversity-and conserve habitats with the broadest range of species or
varieties. At present, it is usually the number of species in a particular
habitat that is taken as a measure of the value of the environment in
conservation terms. Governments or conservation groups confronted with the
choice of preserving either habitat A or habitat B will usually count up the
number of species in each habitat as the first step in their decision
making. Using the BSC as the sole guide to con servation thinking has thrown
up several anomalies. In 1973, the US Congress passed the Endangered Species
Act (ESA), based on the BSC. But the weakness of this approach soon became
apparent in 1977, when the Hybrid Policy dictated that hybrids between
endangered species should not receive protection, because hybrids, it was
argued, belonged to neither one parental species nor the other. The Hybrid
Policy had further ramifica tions. There were suggestions that endan gered
species which hybridised in nature should also have their protected status
removed, because hybridisation meant that their genetic integrity - and
hence their species status, had already been violated. There have been calls
for the protected status of the threatened red wolf to be removed now that
DNA tests have revealed that the wolves hybridised with coyotes at some
stage in their recent evolutionary history. So far, these requests have not
been implemented. As several biologists have pointed out, applying the BSC
to the letter will leave few species on the endangered list, as it is
probable that most species of animals and plants have hybridised at some
stage in the past. So instead of thinking only about species, biologists are
now looking more at the raw material of species and sub species - the range
of variation in their genes, morphology, behaviours and habitats.
Unfortunately, not even this approach gets us entirely out of the woods. The
two populations of orangutans in Borneo and Sumatra are geographically
separated, but virtually identical in physical characteristics. However,
they are distinguished by  slight differences in the sequence of their
mitochondrial DNA. Some biologists 5 believe that these differences are
sufficient for both populations to be afforded protected status. if the
ranges of the two orangutan pop ulations ever merged, there is little doubt
that they would form one reproductive group. But even avoiding the murky
question of whether or not the two populations are distinct species, we are
left with the problem of whether each population deserves independently
protected status based on a tiny difference in their DNA. If you look hard
enough at two populations, you will almost inevitably find differences
between them. Organisations such as the Natural History Museum, London, are
now using the World Wide Web to advance new models for conservation
decision making. These are based on the idea of preserving bio diversity-the
diversity in genes, species and habitats. The new models suggest, for
example, that conservationists should measure not only the number of
species, but also how different they are. A habitat containing two closely
related rare daisies would possibly rank lower than one containing a rare
daisy and a rare orchid. The meaning of the term "species" has gone through
many changes, driven on by new methods, the differing priorities of each
scientific age and the varied fields of biological research. But all species
definitions have been shown to have their limitations, and perhaps it is
unrealistic to assume that we can impose and apply a single definition on a
natural world made restless by evolutionary change. The natural world is an
everchanging mosaic of form and function, with a staggering variety of
morphological, behavioural, ecological and genetic differences. Perhaps
biological and conservation thinking will, in the future, focus more on
these differences, rather than the meaning and definition of one enigmatic
word.

   1. "Hybridisation rules OK!", Martin Brookes (New Scientist, 20 April
      1996);
   2. Species, Microsoft Encarta, 1998;
   3. On the Origin of Species by Charles Darwin (John Murray, 1859);
   4. "A species definition for the modern synthesis", James Mallet, (Trends
      in Ecology and Evolution, vol 10, p 294).


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