The world is believed to have been occupied by millions of different plants and animals. These plant

are   animal are adapted to different kind of habitats.  There are two theories that support the history of life.

There are ; divine creation and organic evolution

The theory of divine creation states with regard to ORIGIN of life (not species) that every living thing was designed and created by God and did not just occur by chance as evolution alleges.

I    The theory of organic evolution believes that the first living organism is formed from certain molecules such as methane, ammonia, hydrogen and water vapour which combined under particular ultraviolet radiation from the sun and electrical lighting of the hot atmosphere to form the living thing about two billion years ago.

Organic evolution is therefore defined as a series of gradual changes which living organism had undergone in response to changes in environment since the beginning life.

Lamarck’s theory of organic Evolution

Jean Baptist Lamarck, a French biologist was the first to put forward the theory of organic evolution in 1801. His idea of evolution was based on theories:

1. The theory of need

This theory states that the development of a new part or organ by an organism (plant or animal) result from the need of part or organ to the organism .

For instance, the early ancestors of snake had short bodies and legs. As the land was charging during  its formation, many narrow places and dense vegetation, were formed, for         snake to walk through narrow places and dense vegetation , they started stretching their bodies to become elongated so that they could easily crawl through the vegetation   instead  of walking would be gather preferred.

The theory states that organ become well developed and achieve, become functionless or disappeared with disuse.

For instance , each generation of snakes continue to                                                                                                 stretch  their bodies  resulting in the strengthening of their bodies . The  become useless and strengthless and finally disappeared since they would hinder crawling through narrow places and dense vegetation.

iii        Theory of acquired characterizes of  inheritance .(structures ) or variation acquired by organism during their life time are transmitted to the next generation by inheritance.

The modern giraffe believed to have evolved from a giraffe with short neck and short  legs. When competition for low grasses among herbivorous animal become been , the short neck and short legged giraffe started to starch their neck legged giraffe started to stretch their  neck and four legs so as to reach the leave of the  trees.

The load and duck originally had  no webs between their toes, but as they continue frequently visiting water  food therefore the need to develop webs for swimming become a necessities.

DARWIN’S THEORY OF ORGANIC EVOLUTION BY NATURAL SELECTION

Charlie’s Darwin , a British biologist in 1859  propounded a theory of organic devolution by  natural selection leading to the origin of species. His theory of natural selection is as follows:

i-Overpopulation

All living organisms should be allowed to produce more offspring than can survive.

ii-Struggle for existence

Due to overpopulation of offspring, there is a constant competition for existence among offspring

iii-Offspring shows variation

No two individual are exactly the same .

Iv Adaptive characteristics

Some of the offspring’s are well adapted  or fitted to survive the competition than  others.

v-inheritance of adaptive characteristics.

The well adapted ones or the fittest will transmit such variation to their offspring’s.

Those with poor adaptation will die off (extinct).

vi-Development of new species.

An accumulation of favourable variation will in a long time head a divergence (spread) from the original stock resulting in the formation of   new species.

TYPES OF EVOLUTION

1-Divergent Evolution is that for which related species or their parts become  different  because of the availability of niches in  the environment. For instance, the month part of insects, birds and limbs of vertebrate.

Ii- Convergent Evolution – is were  unrelated parts if organisms  are modified to look   similar which arises because of a  common need in the environment. Example, the need to fly caused   the birds and bat  to develop wings .Birds   are aves while bat are mammals.

Modern theories of organic   Evolution.

Theory of organic evolution  as stated by Darwin has been modified. The modern

The modern theories of evolution are now based on genetics ecology cytology, physiology , anatomy and embryology.

EVIDENCES OF EVOLUTION

Evolution any process are continuous but slow the evidences have been derived from several  sources including.

i-         CYTOLOGY

Biochemical similarities- The studies on the structure and function of cells have revealed that biochemical and fine  structure of the  cell of different organisms are very alike with respect to .

–          Nucleic  acids

–          ATP and cytochrome

–          Organelle like mitochondria

–          Serological  test  – is the study of the test tube reactions between antigens and antibodies. Possible evolutionary     relationship can be detected by comparing the blood  proteins.

ii-        Serological test has shown that human blood is much closer to that of the great  apes like chimpanzees.

iii-       Physiology (molecular record) The evolutionary relationship has been established from  molecular records, based on the fact that hereditary information is stored by all organisms in  nucleic  acids mainly DNA.

iv-       Paleontology (fossil Record).

Plants and animals that lived in the past have been studied from fossil  record, fossils are generally preserved in sedimentary rocks which were formed by the deposition of silt ,sand or  calcium carbonate over thousands or even million of years.

v-. Comparative  anatomy

Many groups of organisms especially vertebrates show similarities in  anatomical  features.

• Pent dactyl limb
• Brain
• Heart
• V Embryos of many organism reassemble the embryo of their ancestor especially during the early stage of development the early  human embryo for example has wee developed fail and a series of pouches (fold of skin) and slit making it look very much like an early embryo of rabbit.
• Vi Evidence from vestigial organs .

Vestigial organs are rudimentary or organs still found in animals and plant examples are

• Appendix in man which functionless in man but useful in herbivore ;
• Coccyx (caudal vertebrae)of man are together and reduced to small size has no specific function in man.
• Pyramidal nasi muscle of forehead and  posterior auricular muscles of the ears
• Posterior auricular muscle is functionless in man but used by other mammals in moving the pinnae.

MUTATION THEORY OF EVOLUTION

Mutation is defined as a sudden changes  in genetic make up (DNA) either in gene or  chromosomes resulting in a new characteristic or trait that can be inherited . Mutation occurs during meiosis when the sperm and egg are being formed. When mutation occurs it will lead to production of offspring with marked difference in appearance from other member of the population of the species called a mutant.

Mutation provides raw materials for organic evolution through natural selection of new species.

TYPES OF MUTATION

i    Gene point mutation are sudden changes in the structure of DNA (gene code) at a  particular point or area.

Ii   Chromosome   mutation are caused  either by breaking off or fragmentation of chromosomes during  mutation.

CAUSES OF MUTATION

I             X- rays

Ii           Ultra – violet light

Iii          Cosmic radiation

Iv          Re-assortment of  gene

Forces responsible for evolution

(i)Mutation

(ii)Re-assortment of gene

(iii)Recombination of gene

This may interest you: