The biological species concept has wide application to the

majority of organisms. It focusses attention on the really

important criterion of reproductive isolation, which is the

mechanism by which species integrity is maintained. Be-

cause reproductive isolation is often brought about as a

result of differences in behaviour, ecology, and genetics,

much attention has been given to these characteristics;

this has resulted in enrichment of the field of systematic

biology and a better understanding of the nature of spe-

cies.

It is useful to present the exceptions-examples of situa-

tions in which the concept is difficult to apply-for these

have been the objects of intense study and have resulted

in much new biological information.

Allopatric species. The biological species concept is

difficult to apply to allopatric species-i.e., populations

that appear to be closely related but differ in enough

characteristics to be called different species and are geo-

graphically isolated from one another. The barrier of

geography prevents natural interbreeding, but sometimes

the two problematic groups can be artificially bred in the

laboratory by taking eggs from the female of one "spe-

cies" and fertilizing them with sperm from the male of

another "species." Normal development of the hybrid

offspring is justification for stating that the two "species"

are closely related but does not provide enough evidence

for considering them a single species. Even if they came

into contact naturally, they might still behave as two

species and not interbreed because of reproductive isola-

tion, as a result of differences that preclude the produc-

tion of normal offspring. Similar experiments with plants,

in which one species is pollinated with pollen from the

flower of another species, suggest little about the natural

forces that might prevent cross-pollination or discourage

germination of the hybrid seed.

The decision as to whether to consider two allopatric

populations as species often can be made on a fairly

reasonable basis. If two such populations are as different

from one another as related species that are known not to

interbreed, then they may be considered separate species.

The failure of artificial crossing would be further confir-

mation of the presumption that the species are distinct;

the two populations have indeed reached a point at which

interbreeding would not be possible even if their ranges

were rejoined. Similarly, if the two allopatric populations

differ in characteristics that would make mating difficult

should their ranges meet, they may be considered sepa-

rate biological species. Differences in time of flowering,

in mating calls, or in soil requirement might prohibit or

greatly restrict interbreeding. Generally then, the biologi-

cal species concept has been extended to allopatric popu-

lations by emphasizing that a species is a group of popu-

lations that actually-or potentially-exchange genes

and that are reproductively isolated from other groups of

populations.

Hybridization. Hybridization between populations that

are considered on all other grounds to be a species is

a common occurrence in some frogs and toads, some

freshwater fishes, and many plants. In some instances the

hybrids that result are abnormal and soon die. In some

cases the hybrid matures but is sterile, or incapable of

reproduction. In still other instances the hybrid may ma-

ture and reproduce, but subsequent breeding with either

parental species results in "hybrid breakdown," in which

later generations of hybrids rather than the first suffer

from gene incompatibility. In these instances of hybrid

inviability, sterility, or breakdown there is effective repro-

ductive isolation: little or no transfer of genes occurs

between the parental types, and they are therefore distinct

species even though interbreeding occurs.

There are instances, however, of interbreeding between

organisms that are generally considered to be separate

species and in which there appears to be fairly free gene

exchange in the area of species overlap. In these cases the

gene flow is limited to a narrow geographic band or is

associated with unnatural disturbances that dissolve the

barriers to interbreeding or provide habitats in which

hybrids can survive; nevertheless, the entities still main-

tain their distinctiveness as two species over the vast ma-

jority of their ranges.

Speciation is a dynamic process, and there exist popula-

tions in all stages of becoming species. Some populations

are completely isolated reproductively in zones of over-

lap, and there is no question about the status of these as

species. Others hybridize but produce unsuccessful

offspring, again raising no doubts about their status as

species. Others interbreed rather freely in some areas but

not in others, perhaps because the process of Speciation

has not been completed.

Variants within a single species. Variations within a

species are common in both animals and plants. In some

instances there may be distinct colour or pattern differ-

ences within a single population with no intermediates

between the types. Such instances are called polymorph-

isms, Males and females of some species are so different

in appearance that they were originally described as sepa-

rate species. Other polymorphisms are not so evident and

may involve such characteristics as chromosome struc-

ture and number. If such differences pose no barrier to

successful interbreeding, then they offer no real difficulty

to the application of the biological species concept.

In many species, striking differences occur between pop-

ulations in different parts of the geographic range. At the

boundaries between these geographic groups-called

races or subspecies iu animals and varieties in plants-

are zones of intergradation in which the characteristics

of one group are replaced by those of another. In some

instances it may be difficult to decide whether there is

sufficient restriction of interbreeding in these intergrada-

tion zones to call the populations involved species.

Asexual species. If a species is defined as a group of

organisms reproductively isolated from other groups but

with freely interbreeding individual members, then every

individual of an asexual species is potentially a new spe-

cies. Asexual species of animals usually produce only

females; in such species as bees the males are produced

asexually, the females sexually. In other species there

may be alternation between sexual and asexual stages of

reproduction in the life cycle. Such an alternation of gen-

erations is the rule among plants.

Among plants, many types of asexual reproduction are

grouped under the term apomixis. This category includes

vegetative reproduction by the rooting of broken stems or

specialized buds, self-pollination of a flower by pollen

from the same flower, or development of the egg without

fertilization. All such mechanisms tend to restrict recom-

bination of genes to a greater or lesser extent, and the

resulting offspring resemble the parent more closely than

is true in unrestricted sexual reproduction.

The biological species concept cannot really be applied

to organisms that reproduce asexually. In such cases com-

parative morphology of such species is relied upon to

reach a decision concerning its status. Fortunately, most

such species have characteristics that are shared by the

majority of individuals within the species and are differ-

ent from those of other asexual species.

Fossil species. It is difficult to apply the biological con-

cept to species that have a time dimension- Living species

exemplify various points in the evolutionary process. It is

relatively easy, for example, to discern three separate liv-

ing species; however, the distinctions between these spe-

cies become fewer and fewer as the fossil record ap-

proaches the common ancestor from which they arose.

They begin to lose their separate identities.

It is difficult to apply the biological species concept to

fossil species in general. Often the species is represented

only by fragments, but even given complete fossil materi-

al the species can be defined only on a morphological

basis. If fossil species show striking structural differences,

however, it is generally a safe presumption that they were

incapable of interbreeding. Were this not the case it is

unlikely that the differences would have been maintained

through the course of evolution.

THE PROPERTIES OF SPECIES

Distinctiveness. The ability of sexes to recognize mates

within their own species and to distinguish them from

sexes of another species is exceedingly important; an er-

ror may produce sterile or defective offspring,

Isolating mechanisms are employed to decrease the like-

lihood of such untoward events. Fireflies use the flash as a

means of species identification; different species differ in

the timing of the flash signal produced. In frogs and

toads, the distinctive call of the males attracts the females

to the proper mate. In some animals, such as moths, mate

attraction depends upon volatile scents produced by an-

other individual of the same species. In some fishes, the

pattern of movements made by the male during courtship

allows the female to discriminate between a male of her

own species and one of another.

Isolating mechanisms in plants are less obvious. Differ-

ences in flower structure between related species, how-

ever, may serve the same function as do differences in

mating behaviour between animal species. Insect pollina-

tors transfer pollen grains from the male reproductive

organs of one plant to the female organs of another plant

of the same species. Differences between related plant

species in flower colour, shape, and fragrance may result