Species
are
the only real entities in biological classification. All other categories,
from genus up to kingdom, are philosophical constructs. But the species
represents an aggregate of individuals that share a common ancestry and a
common evolutionary history. Charles Darwin was one of many
biologists concerned with the origin of species, and species are the focal
point of most biological studies. It is thus particularly ironic that
"species"
is still a term lacking a universally accepted definition.
In the 1940s, Ernst
Mayr coined the term "Biological
Species Concept" which was
subsequently widely embraced. The definition stated that species were
members of a potentially interbreeding population that could produce
viable offspring. With the discovery that some species hybridized in
captivity--such as lions and tigers--the definition was reworded to state
that species were members of a potentially interbreeding population that
could produce viable offspring in nature. Additional exception continued
to be discovered. Many geographically distinct population of a species
(termed "subspecies") may breed and produce offspring called intergrades.
A more seriousness weakness to the biological species concept occurs when
members of different species, sometimes from different genera, produce
young called hybrids. If nothing else, trying to define
"species" has demonstrated that biology is the science of
exceptions.
By the late 1970s,
several new methods of approaching taxonomy came into prominence, and with
them many new species concepts were devised. Among these are the Recognition
Species Concept which
is also based on reproductive isolation mechanisms, assuming that
potential mates will have the ability to recognize each other and ignore
similar but different species. There are also the Paleontological ( or Temporal) Species Concept
which
relates a species to its age and fossil history, the Typological
(Morphological) Species Concept which identifies
species by resemblances, and the Phylogenetic
Species Concept, stating that a
species is a member of a population that shares a recent common
ancestor.
There are strengths
and weaknesses to all these concepts, and many biologists apply a
particular definition based on the group with which they are working. In
part, this reflects the problem in biology that different organisms, even
some closely related types, display a remarkable and bewildering number of
ways of reproducing. For
example, animals that reproduce unisexually (parthenogenetically) do not
interbreed at all, yet they produce viable offspring. Nevertheless, all
the individuals in such a population of parthenogenetic organisms share a
common line of descent or ancestry. In this case a phylogenetic definition
is particularly useful. On the other hand, many animals and most of the
orchids (a huge family of flowers with some 26,000 species--more than all
the fishes) can hybridize and produce new species in a single generation.
Among the American whiptail lizards are several parthenogenetic species
that we now know are the hybrid result of matings between sexual species
sometime in the past (the formation of a new species via hybridization is
termed reticulate evolution).
Add to this variety species, such as many snails and flatworms, that are
hermaphroditic (containing the reproductive systems of both sexes) and occasionally
self-fertilizing, and you can see how the problem of agreeing on a single,
all-encompassing species definition becomes quite unlikely.
The traditional
typological species concept appears straight-forward enough--you group
individuals that look very similar into the same species. This method,
though often useful, has many pitfalls. The most conspicuous difficulty
occurs in species where males and females look radically different. The
angler fish female is a bulb-shaped creature that is recognizable as a
fish, while the males is a fraction (often 1% or less) of her mass and
resembles either a growth or a stunted leech. Other species, particularly
insects and amphibians, are extremely variable at different life stages.
Who could unite a tadpole and a frog based only on external appearance, or
a caterpillar and a butterfly?
The business of
recognizing species and determining if individuals are this species or
that, or merely variations or aberrant forms (after all, a mutant 5-legged
frog is still a frog) is the job of a taxonomist.
A responsible taxonomist is a scientist who undergoes a very detailed and
broad training in biology, and may examine hundreds or thousands of
individuals before coming to a well-founded conclusion about species
identity. The business of taxonomy is the first step in most biological
research, but there are few people training in taxonomy as more
"exciting" and lucrative fields of biology emerge. This is sad
because we are still discovering new species at a rapid rate (see our New
Species listings in the Museum Library for a partial list of animals named
just in the past year), yet we have fewer and fewer trained people to
properly describe and name them. Without good taxonomy, evaluation of the
number and kinds of species in any given area, termed biodiversity,
is in jeopardy.
Genera with many
species, or regions of high biodiversity, are properly considered to be species
rich, not "speciose" (see below). Defining, recognizing and
naming species is still essential to biology. Such taxonomic activity is
needed to ascertain regions of high or significant biodiversity, to plan
preserves, enact laws and regulations, and conduct proper (sometimes
life-critical) evolutionary studies.
To be
continued...
A
note about the
commonly
used term
"speciose,"
taken
from Douglas E. Gill:
"I take pleasure in using the adjectives
"speciose" and species-rich in the same sentence to call
attention to the fact that they are not synonymous and that
"speciose" is not even a proper word: it is not found in any
standard dictionary. If it were, it would derive from the Latin speciosus,
and would mean "beautiful," as it does in its common use
as speciosa in many scientific names, and the etymologically
identical specious (= deceptively beautiful). I urge that the
misuse of "speciose" in the evolutionary biological literature
cease."
("Fruiting
failure, pollinator inefficiency, and speciation in orchids," fn. p.
458,
In Otte & Endler, eds., Speciation and its Consequences,
1989, Sinauer Associates, ISBN 0-87893-657-2.)
