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Biospeleology of the Piemonte |
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Biospeleology of the Piemonte |
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An outline of Biospeleology
Hypogean evolution
What's the evolution?
The thematic on the evolution is much
wide; in the last century has engaged the naturalists giving to
biological sciences a strong transformation of thought and a extraordinary
development.
It's necessary to dwell first on the meaning of evolution, defined as historical changes, that is in the time, of the whole of the instructions necessary for the formation of an organism; this change is due to the genetic variability and to the environmental selection, for which the descendants of a population are different from their ancestors.
The adaptation is the point of departure of all the evoluzionistic theories and is the main aspect that every theory must explain. In the adaptation is comprised the improvement of the suitability of an organism to its environment and therefore the evolution must be the answer to an environmental change.
As an example, if we consider two species of Coleopters derived from a same ancestor, but today living in different habitats, can be thought that they have had changes and adaptations: differences in the morphology of the exoskeleton, of the length of the legs and antennas, differences in the pigmentation, etc.
The evoluzionistic theories.
Two main points of view exist through which it was
attempted to explain the mechanisms of adaptation to the environment.
1. The theory of Lamarck presupposes the existence of a natural tendency in the living beings to their improvement, for which the "need" of one determined function would be sufficient to induce the appearance of the organs and the apparatus necessary to execute it: "the necessity creates the organ, the use of an organ leans to perfect it, the disuse leans to regress it". But this theory turned out indefensible. It would be approximately like supposing that an athlete, after to have perfected, by means of suited training, some of his muscular apparatus, suitable to the execution of some kinds of exercises, he have the expectation to generate sons already supplied of those same advantages acquired by him with his activity.
2. According to the theory of Darwin the evolution must be conceived as one consequence of the adaptation of the organisms to the environment. Such evolution is expressed by means of the fight for the existence with the destruction of the individuals less adapted to the environmental conditions and with the survival of the individuals that casually possessed some characteristics that makes them fittest to that determining environment of life.
This concept of evolution that grants a preference to the survival of the individuals more adapted, constitutes one of the more original mechanisms that rewards, through the natural selection in the course of the reproduction, from a part the individuals of a population that possess combinations and characteristics adapted to develop and to multiply in an certain environment in the time, while from the other part, it eliminates the less adaptable ones to the survival in those environmental conditions. Therefore, the exemplary less adapted will be destined to succumb before to reach the reproductive age.
In short the theory of Darwin is founded on these three main
factors: individual variability completely fortuitous, natural
selection of the fittest characteristics, hereditariness of the advantages achieved
through the natural selection.
Currently are in course studies that would revalue partially the theory of
Lamarck, to molecular level, for which the
environment would influence the formation of determined molecules that would
favour the organism in the fight for the survival and the genes -
that allow the synthesis of such compound - would be genetically
transmissible (olistic theory).
We will spoke of favorable mutations,
if to the inside of a species it will increase the
number of individuals and these will spread in new environments, and of
disadvantageous mutations, if
their number will be reduced also until the disappearance.
- temporary environmental modifications (minimal temperatures in the winter or summery maximum temperatures,
floodings, drought, chemical composition of waters, glaciations, etc.);
- action of enemies , predators, parasites and pathogenic agents;
- competition in the feeding, in the choice of the territory of life
(competition);
- competition in the choice of the partner for the copulation
(sexual selection);
All these factors can act in measure much various and several
combinations; the "pressure of mutation" or the "pressure of selection" can
moreover prevail depending if
the genic accidental mutations that make adapted the
individuals to the environment are preponderant, or the environmental factors have preponderant
selective action on genetically stable individuals.
In conclusion, in the evolutionary history of a species, series of adaptations
and selections have been, until reaching the current evolutionary phase, with passing of the
millenia. Therefore, the hypogean fauna hasn't entered in cave suddenly, but through a
gradual adaptation during million of years. Moreover in much species not
yet specialized to the hypogean life, periods of transition or of regression can
be observe. The main characteristics of a hypogean environment are:
1. absence or reduction of the light ;
2. elevated and constant relative
humidity ;
3. constant temperature.
These are extreme conditions that cannot not have consequences
on the population of a cavity. In fact the extreme conditions of such environment
determine different and unusual adaptations. Absence of light. A consequence is the anophthalmy
(the absence of eyes or blindness). According to the
more or less marked adaptation to the light absence, various morphologies can be
observed in some Coleopters: in the Trechus the functional eyes
have a semispherical configuration, in the Duvalius the reduction
of the visual organ become much more evident and in the Agostinia genus
the eyes are totally disappeared.
Similar examples in arthropodology are a lot, but they are much
less in the superior animals: as an example in some bony fish the same cavernicole
family comprises both cavernicole species and abysmal ones.
The visual organ occupies in the chromosomes a good
part of genetic potential; the elimination of this organ, that don't serves in the caves,
represents therefore an earn not indifferent of space on the chromosomes.
The reduction of the visual organs, until to the total anophthalmy, generate the development of other sensory organs
in the true cavernicoles. This animals in fact perfect usually the
tactile sensibility, as it is demonstrated by the
particular development of the specific receptors, like the
trichobotres (bristle) of the Pseudoscorpiones or the sensory hairs of the Coleopters
and other Insects. The lengthening of the antennas and of special bristles can be
observed in this organisms and renders these animals also most sensitive to the minimum
airflows.
The legs sometimes are lengthened, while the membraneous wings
of the troglobite insects are very reduced or lack at all. The bats, in the absolute dark, orient themselves with the ultrasounds generated by their larinx that
rebound on the obstacles and are tapped by their specialized auditory apparatus
(a system similar to the sonar called echolocation). In some
specialized cavernicole insects appear new organs of sense like the organ of Hamann
that is situated on the antennas of some Coleopters Cholevidae and that
seems deputy to the perception of the atmospheric humidity variations
(hygroreceptor).
Another effect reulting from the total absence of light is the depigmentation (loss of color).
Many animals appear white or with light colors (to have the
exoskeleton of red or blue color in the hypogean environment it serves very
little). For some aquatic Crustaceans as the Niphargus the light can be
even lethal. Also with the depigmentation an energetic earn is realized.
Humidity (or dampness). The exoskeleton of many Arthropods is constituted of chitine, an
ammino-polysaccaridic substance, that renders it leathery and to whose
formation concurs the solar light. For the Arthropods living in the epigean environment
an exoskeleton containing a lot of chitine is often an advantage because it defends them
from the parasites, the predators and many external physical agents that
are absent inside the caves. In the cavernicole Arthropods the exoskeleton often appears
reduced; this is an ulterior example of energetic saving, but it involves
advantages and disadvantages; the reduction of chitine in the exoskeleton causes a weakening of the spiracles (the openings of the tracheas), which
cannot more keep the inner humidity of the organism. The insects, in fact, do
not have lungs and their respiration is made by means of the tracheas, small aerial
channels that lead oxygen to all the organs through fine ramifications. An epigean
insect, placed in an environment with reduced relative humidity, would be exposed to a
lethal dehydration without its particular adaptations of the spiracles
of the tracheas that are fortified by curious tufts of hairs and without the
microscopical structures able to keep the inner humidity. Nevertheless in several
cavernicole insects these structures are not more functional and for this in the fossil
branches of a hypogean system (with water lack and therefore with scarce humidity) very rarely we can meet living organisms: the fossil cave environment can be azoic (lacking in life).
The humidity, in fact, represents an environmental factor of
critical importance for most of the terrestrial cavernicoles, that are
in uneasiness conditions as soon as they are removed from an environment
almost saturate of aqueous vapor; this is due to the marked stenohygry (minimal endurance of
small humidity variations) of these organisms. Just in connection with this fact it
would be evolued the pseudophysogastry of some Coleopters, mainly
Cholevidae. As an example, in Leptodirus hohenwarti, (the first
described blind Cholevidae, discovered in 1831 by Franz von Hohenwart, an Austrian naturalist), the elytras, very developed and knitted between themselves don't serve to cover an
big abdomen, but in order to keep an bubble of very humid air useful for
short periods in which it is forced to cross zones with low relative humidity.
In the hypogean environment all is turned to an energetic
saving. An epigean grasshopper has
antennas and legs very different from a hypogean grasshopper. In fact the hypogean fauna has
indirectly renounces to several "facilitations" typical of the epigean one (eyes,
wings, colors, etc). This genetic saving has been used for the mutations
with "facilitations" suitable to lead hypogean life: the lengthening of the
antennas and legs is a classic example.
When our home fall in the dark because of a black-out we spontaneously
stretch out one's arms in order to sound the ambient around us;
the length of the legs and the appendages is without doubt an
advantage, and often the antennas and palps of many hypogen arthropods
are provided with sensitive apparatus, that is specific structures, as an example,
act to locate biochemically the proximity of food or of other individuals of the same or different species. Others
sensitive structures can be the silked hairs posizioned in very precise points on the
exoskeleton of many Coleopters like in Doderotrechus (troglobite
Coleopter Carabidae).
Some insects are sensitive also to more the minimal airflows and are
equipped with bristles encircled from cells that inform them in case of
dehydration danger. The sense of smell too is very developed. The
velocity with which the Leptodirinae, and some aquatic organisms, reach the
smelling decoys, also from considerable distances, is a clear indication of their sense of smell.
In a cave we can distinguish predators, detritivores, saprophages and limivores. The feeding is obviously the main problem of a living being and every
species, beacuse of the ecological niche that occupies, has an suitable mouth apparatus:
we can think to the jaws of the Carabidae or to the mouth apparatus suitable to suck
of the Diptera (flies and mosquitos) and of the Lepidoptera (butterflies).
Another morphologic modification in several insects, is the
apterism (absence of wings); in the case of the Coleopters it isn't
possible to observe it directly because the membraneous wings are covered from elytras
of chitine. In the Diptera, instead, with external membraneous
wings, the apterism of the cavernicoles is evident, like in Chionea alpina.
Also the metabolism of the cavernicole fauna has
suffered some modifications. As an example the oxygen consumption decrease
respect to the surface fauna: the life in the caves is much less animated, there are
less predators, less environmental risks, etc.
Lacking the light also the nictemeral and seasonal
rhythms vanish; all the biological rhythm suffer a slowing down, in the opinion of some authors,
due also to an insufficient and sporadic food regimen.
The more studied physiological adaptations
concern the reproduction:
- decrease of the fecundity;
- correlated increase of the volume of
eggs and the vitelline reserves;
- decrase of the egg number;
- lengthening of the duration of
embryonic and postembryonic development and of the
adult life and the entire biological cycle with modifications of the larval
cycle (like in the contracted cycle of the Leptodirinae);
- fall of the reproductive regularity
or, more frequently, a change of the seasonality
determined by the characteristics of the cave.
In the hypogean environment every action is based on the energetic
saving; to have too many sons would mean to give life of too many
individuals destined to die of hunger: there aren't so many predators like in the
epigean environment, isn't necessary the selection in an environment
already selective and moreover not would be sufficient maintenance for numerically abundant descendants.
The less prolific animals, are generally those
that reserve to the descendants the greater parental cares, and this
happens also for some among the more specialized cavernicole insects.
As an example the Cholevidae with globular elytras matures
a single egg to the time, two or three in the course of their entire existence,
but in compensation they avoid every existential problem to their larvas:
the greater part of the juvenile increase is effected inside to the maternal
body and the larva, contrarily to what happens in the overwhelming
majority of the insect, does not lead active life, but very soon it is transformed
in pupa and then in adult.
The cavernicole organisms adopt like survival strategy the K-selection that foresee a delay
of the maturation, small litters, abundant parental cares and
birth in advanced stages of developement.
A curious case is the reproduction of the
bats: the copulation happens at the end of the summer or more frequently in autumn, then
follows the dormancy; it would be improper for a female that don't eat to carry forward a gestation;
in fact the spermatozoa are kept in the
uterus during all the winter and are fed by a nourishing secretion of the
uterine epithelium and the fecundation happens only in spring after the
beginning of the normal activity.
In the epigean environment the choice of the partner is "made in the
sun light": there are courtships, colorful wedding liveries,
very remarkable sexual dimorphism.
In the hypogean environment "all is made in the dark", therefore there are
biochemical sexual callbacks, that is hormones secreted by specialized glands. As an example, some females of Diplopoda leave as
callback a thinnest thread, a slime that the male perceives and follows in the dark
for to catch up the female, hoping to find her not yet occupied.
All these mechanisms have made possible the evolution in the
underground environments in much different way than in the solar light
and we must remember that, in a certain sense, the mammals are the
product of a selection happened in the dark for hundred of million years, by night
or in natural holes and cavities, in the time when the Earth surface was
kept of by the voracious dinosaurs; this fact has allowed them to develop senses and
characteristics, as the intelligence, that has made the mammals
the organisms with greater evolutionary success during the Tertiary, after
the decline of the great reptiles. An outline of Biospeleology
Biospeleo SUMMARY Systematic Index Some embellishments, Javascript, etc. have been modified
from them situated on the site
Mechanisms of hereditariness.
Factors of selection. The selection factors are multiple, the main ones are:
Characteristics of the hypogean living
beings
We must remember that it's possible to meet epigean Diptera in caves at depths of
800-900 m; this is an demonstration that these bugs have the particular
organs that allow them to find one's way in the dark for great distances.
BIBLIOGRAPHY
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