LEVELS OF ORGANIZATION – SPECIES AND POPULATION

Levels of Organization

The living organism is a system of interwoven and overlapping hierarchies of organization. Starting from atom, each level makes the base for the next higher level and thus forms different levels of organization. The properties of those levels of organization are unique and are different from any levels of organization, e.g. water is a molecule of hydrogen and oxygen. It has its own properties which are different from hydrogen and oxygen, both. Each level of organization is more complex and has fewer units than the previous one i.e. there are lesser tissues than cells. The molecules of life are packed into highly organized, self enclosed units called cells. Each cell has a complex structure that gives it the capability of self-reproduction a unique characteristic of living things. Cell is the lowest level among the levels of organization that is considered to be alive. The structural hierarchy represents how matter has become more and more organized with each levels of organization. At every shift of organizational level, energy is required. This new level of organization has its own properties. Energy is necessary to maintain that level of organization. There is an intimate relationship between organism and environment in every levels of organization. Levels of organization are the basis of evolution. Levels of organization has determined the kind of life that existed in past and is existing today however, the activities of an organism also produce changes in environment so the relationship is in both ways.

A living organism – the most observable unit can be studied at various levels as given:

Genes → Cells → Tissues → Organs → Organisms → Populations → Biotic Community → Ecosystem

In this part headed "Levels of Organization" we will study the living organisms at the species and population level.

The Individual Organism

A living organism is able to carry out its life processes within its body independently. The individuals can be counted, measured and studied as they are the most concrete objects. The individual organism is always derived from the pre – existing ones through vegetative, asexual or sexual reproduction. An individual always has a life span with a definite beginning and a definite end. An individual is able to transmit its characters to the offspring. The aggregation of individuals of one kind forms the population.

Population
Population is a geographically isolated group of the same kind (so called species). It is a group of organisms of same species that occupy a specific area. The population may be closer or widely dispersed geographically. The number of population distributed over a large geographical area may form one species.

Species
Species is supposed to be the basis unit of taxonomy. A species may be defined in number of ways in various biological aspects.

Genetically – A species having group of organisms that show a distinct similarity in the genetic karyotype.
Morphologically – A species is a distinct and recognizable group of organisms.
Ecological – A species having group of organisms that share the same ecological niche. (No two species can have the same niche)
Interbreeding – A species having group of organisms that can interbreed and produce a fertile offspring in nature. The criteria is too simple to understand a species. There are number of species where the organisms may look different and yet they belong to the same species.

Amazing Biology

Biotic Community

Characteristics of a Biotic Community

In any natural environment of only one species cannot exist by themselves. It is always a group of population that live together in the same area. An association of a number of different inter-related populations of different species in a common environment in a nature forms a Biotic Community. The members of a biotic community can be producers, consumers or decomposers. The members of a biotic community are all interdependent. A biotic community may be small or large e.g. pond community occupies a limited area while the desert or grass land communities could spread to miles. A biotic community may primarily be of plants or animals and generally includes both. As different species share a common environment, number of direct or indirect interactions exists between them inside a biotic community. A biotic community interacts with the abiotic factors and forms an ecosystem.

Cells – Tissues – Organs – Organisms – Population = Biotic Community

Every biotic community needs abiotic factors for survival. At times the dominance of one factor determines the predominance of particular kind of population in a biotic community. For example: In a biotic community of fruit bearing trees the frugivorous bats and insects would be in plenty. A grass land biotic community has seed eating birds, mice and predatory birds living on small insects. A damp marsh has frogs, toads, fish, water insects and water birds that feed on small creatures. Aquatic plants of different kinds adapted for different intensities of light at the surface or at different depths would be present in a biotic community.

Interspecific Interact

ions in a Biotic Community

When various species live together in a biotic community, numbers of interactions take place according to specific needs of food, shelter and habits. We will discuss some of these in this chapter.

1. Predation 2. Scavenging 3. Parasitism 4. Commensalism 5. Symbiosis 6. Competition

1. Predation

Predation is a direct and often complex interaction of two species in a biotic community. The stronger animal called predator captures and feeds on the weak animal called prey. The decline in number of predators leads to a increase in number of the prey. An increase in the predator population leads to decrease in the prey population in a biotic community. These fluctuations play an important part in regulating natural population. Examples: Tiger feeds on deer, owls on rats etc. At times an animal could be a predator as well as a prey.

2. Scavenging

Scavenging is a direct food relationship where animals called scavengers feed on other dead animals which have either died naturally or have been killed by another animal in a biotic community. They play an important role in a biotic community as food is not wasted and also is disposed off. Examples: - Vulture feeds on dead bodies. Hyenas and jackals feed on left over killed animals by lion. They may feed on big animals like zebra and giraffe killed by other animals.

3. Parasitism

Parasitism is a negative interaction in a biotic community where one not only derives nourishment but also lives a part or the whole of life on another organism. The parasite is an organism that lives on another organism, the host from which it obtains food as well as shelter in a biotic community. It could be between animals between plants or between animals and plants in a biotic community. In the host – parasite relationship the weak attacks the stronger. It is beneficial to parasite and harmful to host. Generally a parasite may cause an illness but not kill the host except in severe cases. There may be number of parasites on one host.

4. Commensalism

In Commensalism one organism or a population is benefited while the other is neither benefited nor harmed. In some cases the host may be able to derive some minor benefit. There is no physiological exchange of any kind. The association may be temporary or permanent in a biotic community. Example

: Remora – a small fish attaches itself to the lower side of a shark. Remora feeds on scraps of shark's food and is not harmed.

5. Symbiosis

Symbiosis is an association of two populations in a biotic community where both the populations are benefited. There is often a close or permanent association. In some cases both are so interdependent that neither can live alone in that biotic community.

6. Competition

The two species in a biotic community interact in such a way that it affects their growth and survival. Both the species share the same resources like water, nutrients, space, sunlight, food, etc of a biotic community. Members of both species compete to survive in that respective biotic community. There is a direct inhibition of one by another. Many a times there is no set pattern as any one can succeed. Examples:- Carnivorous animals like tiger and lion compete for the prey. Trees, herbs and shrubs compete for sunlight and nutrients in a biotic community.

7. Some Passive Interactions

There are many interactions that exist in nature of a biotic community in between the animals, or between animals and the surrounding, which also help an organism to survive in that biotic community. Camouflaging – The blending of an animal with its surrounding is called camouflaging. It is also known as protective coloration. They camouflage their body shape and colour to suit the environment. Examples:- Stick insect Carausius morosus resembles a thin dry branch. Dead Leaf butterfly Kalima parolecta resembles a dry leaf. Praying mantis Mantis religiosa resembles the green foliage. Mimicry – Mimicry is also a type of protective resemblance in a biotic community. In this an animal mimics another animal so as to avoid predation. The viceroy butterfly mimics the common Monarch butterfly which is avoided by birds for its unpleasant taste.

Other Interactions

There may be many other interactions which may not be apparent. The association of birds and animals bringing about seed dispersal and pollination. The birds feeding on ticks – the parasites on the body of cattle. The red billed and yellow billed ox peckers perch on the black rhinoceros of Africa and feed on ticks and thus make the animal rid of the parasites to survive in a biotic community. They also warn the animal of the approaching danger. The rufus woodpecker makes a nest in a hole in the ball shaped nest of the ants. The ferocious ants do not harm the eggs or young ones but the birds feed on these ants and keep a check on their population to continue their generation in a biotic community.

Biotic Stability

A biotic community is a naturally occurring assemblage of plants and animals living in the same environment. They all interact to make the

community stable. It has been seen that more the number of species, more stable is the community. For example a biotic community containing a large population of Eucalyptus or any other plant may be totally wiped out by a fungal disease or insect infestation. But if a biotic community contains many species and kinds of plants, only one would wiped out at a time and the rest would survive. Example – In Serengeti plains of Africa, 20 species of antelope graze in the same area. Each species eats on a different kind of grass or shrubs. Some that feed on the same species, feed at different stages of grass. This makes the biotic community rich and stable.

Changes in a Biotic Community – Ecological Succession

A community is built up over a period of time. As the time passes, communities change. In a biotic community there are interactions among the organisms (biotic factors) and between the biotic and abiotic factors (climate, light, soil, etc) All these bring about changes in a biotic community. A biotic community is a dynamic unit where tropic levels exist, there is

a flow of energy and cycling of nutrients. It is a living part of an ecosystem. The wind, fire, volcanic activity or any other event in nature or man may destroy the organisms living in a biotic community. Now if this area is left alone, a succession would start and ultimately a permanent biotic community would take shape. This process from the beginning to the climax may take many years. During this time there will be an orderly and progressive replacement of one biotic community by another till a relatively stable biotic community is established. This is called as ecological succession. A complete succession is called as Sere. A Sere is made up of a number of seral stages. A climax community is the final or the seral stage.

Kinds of Ecological Succession

(1) Primary Succession (2) Secondary Succession.

1. Primary Succession

Primary Succession occurs where no biotic community has previously existed like sand dunes, volcanic islands, lava flows, etc. The area is devoid of any organisms. It may take upto 1000 years before climax community gets established.

Various stages in a Primary Succession – Let us take an example of a bare rock or volcanic islands. Trees, herbs or shrubs can not grow because of absence of good soil. The first organisms often called pioneer organisms are lichens that invade the rocky area. The fungal component holds and the algae provides food. The usually rode the rock surface and tiny crevices or fissures open in the rock in a biotic community. The erosion of rock, sand, dead and decayed lichens provide sufficient soil for larger plant species to invade. Next mosses, ferns and grasses grow on the rocky surface. Certain insects and small animals are also seen. Then the herbs, shrubs and trees replace the earlier vegetation and new animals continue to invade. Ultimately these plants will be succeeded by large seed bearing plants, tall grasses. This would also support number of different kinds of animals populations in a biotic community. This final stable self growing community is called as climax community. The animals also show a succession but these are governed by the plant succession.

Climax Community

It may take hundreds of years for a climax forest community to be established on a sand dune. A climax community is in equilibrium with the environment. A climax community often has one or more dominant species. It is generally referred to those species which collectively form a greater biomass and are generally larger in size. Climax community supports large number of species. Climax community has a high bio mass and rich organic soil. Climax community provides a wide range of food material. Climax community has a diverse variety of niches for animals.

2. Secondary Succession

It occurs when a biotic community has been disrupted and the surface is completely or largely devoid of vegetation. It may be due to earthquake, fire or even clearing of forest by man. As the spores, seeds, rhizomes the organs of vegetative reproduction are present beneath the soil, often grasses, shrubs, weeds are first to appear. The same principle of primary succession applies but it occurs at a much faster pace. It has been observed that a destroyed grass

land may take 50 – 100 years and a destroyed forest about 200 years. Overgrazing of grasslands is like reversion of the community succession. The introduction of exotic weeds in a cleared forest seriously affects the succession and it may become impossible to regenerate the same old type of forest in a biotic community.

Dominance of Species

Each sere has one or more dominant species which are a part of climax community. These could be Pine, Oak, Sal or Teak depending on the area and climatic conditions in that biotic community. The dominant species are the most numerous plants. They are generally large in size and have the greatest biomass. They influence the local environment. The species may be fewer in number in temperate forests – 90% of trees may be pines and oaks of a biotic community. The species may be more in tropical rain forests like Andaman group of islands where there are about 12 dominant species.

Evidences from Palaeontology and Comparative Cytology

Evidences from Paleontology

Paleontology is the study of past life based on fossil and fossilization record. The fossils are the petrified remains or impressions or imprints of the hard parts of the ancient organisms. They are preserved in the sedimentary rocks (gradual deposition of soil particles in layer after layer) or other media like volcanic ash, ice, sand, mud, etc. to study about evidences from Paleontology. Examples: Archaeopteryx left bones and feathers in the form of impression.

Note:

Dinosaurs left foot print on the hard rocks called Imprints.

Leonardo da Vinci (1452-1519) is called father of Paleontology.

Georges Cuvier (1800) is called father of modern Paleontology.

Paleozoology is the branch of Paleontology which deals about the study of animal fossils.

Four general types of Fossils

They are Unaltered, Petrified, Moulds and Casts and Prints

Unaltered

Whole bodies of extinct organisms have been found frozen in ice at the poles (fossilized resin of confers). Example: Elephant like Wooly mammoths were found buried in ice in Siberia.

Petrified

According to evidences from Paleontology the fossil includes the hard parts of extinct organisms such as bones, shell, teeth, etc. Sometimes these hard parts are found unchanged but mostly they are found with their organic parts or completely replaced by deposition by minerals. Replacement of organic parts by minerals deposition is called petrification.

Moulds and Casts

Evidences from Paleontology suggests that moulds of hardened and fossilized mud that surrounds an extinct organisms have retained true copies of their shapes. During moulding buried organisms are completely replaced by minerals.

Prints

It includes print of print of foot, wings, bone, skin, etc. in soft mud.

Evidences from Comparative Cytology

According to evidences from comparative cytology all forms of life come from cellular level. The cell is structural and functional unit of life. This indicates basic relationship among living forms. All cells contain cell membrane made up of double layered lipo proteins. They have DNA - RNA protein information and communication system. All the cells utilize the glycolytic pathway and have a Krebs cycle and an electron transport system as pointed by the evidences from comparative cytology.

Evidences from Embryology

Embryology is the study of development of an egg into the adult. Evidences from Embryology based on comparative developmental studies of embryos of various organisms. Some examples are: All multicellular start their life as a Zygote. Zygote is a single celled and comparable to Protozoa. Zygote undergoes a series of events like morula, blastula and gastrula. Cell of blastula organize to differentiate into two germ layers i.e. Ectoderm and Endoderm (in diploblastic animals). Blastula changes into gastrula. During gastrula stage (in triploblastic) the third germ layers mesoderm originates. According to Evidences from Embryology the developmental stages upto gastrula stage in all metazoans are similar i.e. Monophyltic origin. The development of heart in embryos of fish is two chambered and same condition of the heart retained in adult stage. The development of heart starts as two chambered in amphibia, reptiles, birds and mammals but become three chambered in adult amphibia, three and half in reptile (in crocodile 4 chambered), in aves and mammals four chambered. The presence of gills slits is one of salient characters of chordates; which remains as such in primitive characters (protochordata). The gill pouch and gill slits develops into gills in fish for aquatic respiration. But in adult amphibia, reptiles, aves and mammals replaced by lungs for Aerial respiration.

Von Baer (1828)
Von Baer suggests basic principles of Evidences from Embryology development, which are as follows: During development general characters appear before special characters. From more general, the less general and finally the special characters appear. During development, an animal develops progressively from the form of other animals. Young stages are like young stage of ancestors or embryonic stages of lower animal, but not like adults of those animals.

Biogenetic Law
Biogenetic Law was proposed by Ernest Haeckel (1866), also called Recapitulation Theory. He studied a lot about Evidences from Embryology. It states that ontogeny repeats phylogeny. It means all organisms during their development receives ancestral characters (ontogeny). This historical evolution is called phylogeny. Ontogeny: It means development of organs, Phylogeny: It means ancestral characters. Example: Gills, gill slits, tail, tail fin, lateral line, sense organs, etc. in the tadpole larva of frog.

Evidences of Organic Evolution

Direct and indirect proofs of existence of evolution are:

Evidences of Organic Evolution from Comparative Anatomy
Study of functional anatomy is called Tectology.

Evidences of Organic Evolution from Homology and Homologous Organs
The structures which are different in appearance and perform different functions but have similar basic structure and origin are called homologous organs. The relationship between the structures having common basic plan and similar development origin is called homology or divergent evolution. Some important examples are: Fore limbs in vertebrates like flippers (paddles) of whale or seal, wings of bat, cat's paw, front foot of horse, human hand and wings of birds. They are built on same pentadactyle plan (5- digits) but performs different functions. They show similar arrangement of structure like bones, blood vessels, muscles, nerves, etc. Function: In seal – swimming, bird and bat – flying, in cat – running and walking, in horse – running, in man – grasping.
Evidences of Organic Evolution from Analogy and Analogous Organs
The organs which are different in the basic structures and developmental origin but appear similar and perform similar function. The relationship between structures of different groups of animals due to their similar function is called analogy or convergent evolution. Example: Wings of insects and birds. They are different in basic structure and origin. Insects wing are formed from integument while the bird wing is modified fore limb. They are analogous organ, as both are flat and are adapted for flight. Fins of fishes and the flippers of whale are of similar function but structurally different. The sting of honey bees and scorpions perform similar function and look alike. They are analogous structures because the sting of honey bee is a modification of its ovipositor while that of scorpion is modified by last abdominal segment.

Evidences of Organic Evolution from Vestigial Organs
Those organs of the body which were functional in ancestors but non-functional in the descendants. Example: There are about 90-100 vestigial organs in man. Important vestigial organs in man are – Coccyx (tail bone), nictitating membrane (third eyelid), muscles of ear pinna, caecum, vermiform appendix, canine, body hair, mammary gland in male, segmental muscles on abdomen, etc. Hind limbs in Python, Pelvic girdle and pinna of whale, wings of flightless birds, etc are example of vestigial organs. Snake do not possess legs because legs are degenerated during evolution.

Evidences of Organic Evolution from Connecting Link
The living organisms having intermediate characters between two group are called connection link. Example: Virus, Euglena, Proterospongia, Peripatus, Neoplina, Balanoglossus, Dipnoi, Archaeopteryx, Prototheria. Protopterus (Lung Fish): It is a connecting link between bony fish and Amphibia. In fish paired fins, dermal scales, lateral line system and gills. In amphibians internal nares, lungs and 3 chambered heart. Ornithorhynchus (Duck billed Platypus) and Tachyglossus (spiny and eater) are egg laying mammals. They act as connecting links between mammals. Mammalian characters are hair, diaphragm, mammary glands etc. while reptilian characters are large coracoid, being oviparous, laying eggs and having cloaca.

Evidences of Organic Evolution from Living Fossils
Those animals which underwent little change during long geological periods. Some important living fossils are:

Limulus and Peripatus – Arthopoda
Nautilus and Neopilina – Mollusca
Lingula – Brachiopoda
Latimeria – Coelacanth fish
Sphenodon – Reptilia
Didelphis – Opossum

Evidences of Organic Evolution from Missing Links
Those extinct organisms which had the characters of two different groups of animals and confirm the path of evolution as lie in between these groups. Arcaeopteryx, also called lizard bird. It is a missing link between reptiles and birds.
Evidences of Organic Evolution from Reptilian Characters:
Presence of similar teeth in Jaws.
Each finger ending into a claw.
A long tail with free caudal vertebrae.
Presence of keelless sternum.

Evidences of Organic Evolution from Avian Characters:
Presence of feathers of the body. Rounded cranium. Forelimbs modified into wings, have three fingers. Presence of furcula or wish bone. Above mention characters show that the birds have evolved from reptilian ancestors. So birds are glorified reptiles (Huxley).

Evidences of Organic Evolution from Atavism or Reversion
The sudden appearance of some ancestral features in some individuals is called atavism. It means non functional organs of the ancestors become functional in descendants. Examples: Large canine in man, thick hair on body, functional nipples in male, elongation of tail bone, etc.