after collection a vast amount of information, Darwin began to realised that under the intention competition of members in a population, any variation, which favour survival in a particular environment would increase the individual ability to reproduce and leave fertile offspring. a less favourable variation would be at disadvantage and organism possessing them would, therefore, have decreased chance of their successful reproduction.
this data provide Darwin with the framework to formulate, a theory of evolution by natural selection in 1839. he did not publish his finding at that time. indeed, Darwin greatest contribution to science was not so much to show that evolution occurs but how it might occur .
in the meantime, another naturalist Alford Russel Wallace, came to the same conclusion as Darwin regarding natural selection . in 1858, Wallace wrote a 20 pages essay outlining his theory and send it Darwin. Darwin and Wallace presented paper on their ideas at a meeting of the Linnea Society in London
in November 1859 Darwin publish his thought on evolution in 'Origin of Species ' by the name of natural Selection.
POSTULATE OF DARWINSM :
the main postulate, which formed the basics of Darwin theory of natural selection are as follows
overproduction (rapid multiplication)
All organism possess enormous fertility. they multiply in a geometric proportion with some organism producing very large number of species. Despite of this high rate of reproduction of a species, its number remain constant under fairly stable environment. the production of more offspring by some organism and fewer by other is termed as differential reproduction.
limited resource:
the resource like food, space, etc. remain limited inspite of rapid multiplication of the individual of all the species.
struggle for existence :
the limited resource present in an environment are the main cause of struggle for existence, which may occy at following three levels.
intraspecific struggle :
it is the struggle among the individual of same species for their common requirement like food, shelter, mate, breeding, places, etc. intraspecific struggle is very severe, e.g. young tree in a forest, cannibalism, (eating individual of its own species), human wars, etc.
interspecific struggle : it is the struggle between the individual of different species for their similar requirement like food and space. for example, a frog feed on insect and it prayed upon by snake while kite feed on both as well as snakes.
environmental struggle : it is the struggle of living from against the environment condition like extreme heat, cold, drought, earthquakes, storms, disease, volcanic eruption, etc.
4 appearanceof variation :
all individuals are dissimilar in some of their characters except the identical twins. this dissimilarity is mainly due to the variation .
those are the small or large difference among the individual . variation allow some individual to better adjust with their environment.
variation can be categorised into following type
somatic variation : these variation affect the somatic cell of an organism. they are also called modifications or acquired characters because they are caused by various environment factors, use and disuse of organs and conscious efforts, etc.
germinal variation : these are inheritable variation recognised by Darwin . they are formed mostly in germinal cell . they are further two type
(a) continuous variation : these are fluctuation variation, which oscillate due to race, variety and species .
(b)discontinuous variation : these appear suddenly and show no spots gradation . this variation were termed as' spots ' by Darwin and Mutation by Hugo de Vries . Darwin regarded continuous variation to be more important because the discontinuous variation being mostly harmful would not be selected again.
the organism which are provide with favourable variation would survive, which the unfit are destroyed. originally it was an idea of Herbert spencer ( 1820-1903) who used the phrase ' survival of the fittest ' first time Darwin named it natural selection.
survival of the fittest refers to the idea that the natural selection tents to favore those organism that are most fit to survive with the reproductive age in a particular environment and to produce offspring.
for example, according to the theory of Darwin , the long necked giraffe evolved due to natural selection.
he started that initially both long neck and legs and short neck and leg containing giraffes were present , but due to the scarcity of grass on land ,they had to feed on the leaves of tall trees.
Now giraffe with long neck and longer legs could get food more easily and better change of survival . as a result, long neck and legs containing giraffes become abundant and giraffes with short neck legs become extinct .
the giraffes which short necked and limb soon died due to starvation when climate change due to their natural selection .
evidences of natural selection :
it has the following evidence
(1) rate of reproductive , limitation of resources , competition and struggle for existence for existence can be seen in natural normally.
(2) abundance of variation is evidenced by the absence of two similar individual except monozygotic twins , which also show some environmental variation
(3) preproduction of new varieties of plant and animals through artificial selection . these show evidence that the natural having vast resources can easily that the natural having vast resources can easily produced new species through the natural selection .
(4) pedigrees of some animals (horse, camel, elephant), mimicry and protective colouration also supports natural selection.
formation of new species (speciation)
Darwin considered that as a result of struggle for existence, variability and inheritance, species become better adapted to their environment . this beneficial adaptation are preserved and accumulation in the individual of species generation after generation . this result into the origin of species or speciation.
the first attempt to explain the origin of species and their adaptation to the environment was done by French naturalist Jean Baptiste de Lamarck (1744-1829). he gave the first theory of evolution known as Lamarckism or the theory of inheritance to acquire characters. it states that all changes, which organisms acquire during their lifetime, are transmitted to their offspring by the process of inheritance. Lamarck believed that a nervous fluid within each species helps to progress up the change of being. the more strongly exercised organ attract more of the nervous fluid which in turn enlarge this organ and such alternation are inherited.
Lamarck publishes this theory in 1809 (the same year of Darwin's birth )in his famous book "philosophy zoologique". he also introduces certain terms such as invertebrates, vertebrates, and Annelid.
POSTULATE OF LAMARCKISM
the theory of inheritance of acquired characters was explained by Lamarck using four main postulates.
these postulate along with the classical example of the evolution of long necks is in giraffes are as follows
1 internal vital force
organism and their organ have a natural tendency to continuously increase in size, generation after generation. for example, the ancestors of parent day giraffes were bearing small neck and forelimbs. however, these had an internal vital force to increase their size and become relatively larger in due course of time.
effect of environment and new needs
continuous change in the environment condition directly influences the nature, habit, way of living, of an organism, and their structural organization. for example, early giraffes used to feed on the grasses and other surface plants of an area. with time the climate of these areas becomes arid. it means that the grasses and other surface vegetation dried up living only a few taller trees. thus, the leaves of these tall trees were the only food available to ancestors. it is called the doctrine of appetency | desire.
uses and disuses of organ
the grown of less used part decline, while that of better-used part progresses. for example, to full fill their new need the giraffes continuously stretched their neck to reach the taller trees. as the necks were comprehensively used to reach to the tree, its permanent elongation took place and the character was acquired.
inheritance of acquired characters
the growth of organ ( either better or poorer) acquired during the lifetime of an organism in hereditary. as in the case of giraffes where, the acquired characters, i.e. along the neck was transmitted in the next generation.
other examples in Favour of Lamarckism
snakes: the present day limbless snake with a long slender body were developed from the limbed ancestors. it is due to continued disuse of limb and stretching of their body to suit their creeping mode of locomotion and fossorial mode of living.
aquatic bird: aquatic bird-like ducks, geese, etc. developed, from their terrestrial ancestors by the inheritance of acquired characters like a reduction of wings due to their continued discus, development of webs between their toes for wading purposes, etc. these changes were mainly induced due to the deficient of food on land and severe competition.
flightless birds: the development of flightless birds like an ostrich from flying ancestors is considered due to continued disuse of wing as they were found in well-protected areas with plenty of food.
flat fishes : these are deep sea fishes present at the bottom of sea where there is no sunlight . they lead an inactive life.
whales : they lost there hind limb as the consequence of there disuse.
wading bird : they developed long legs through the generation of sustained stretching to keep the body above the water level, example : jacana bird
cave dwellers : eyes are reduce in cave dwellers ( mole) since , they live underground.
Emergent hydrophytes : in emergent hydrophytes like a ranunculus aquatilis the submerged leaves are dissected while the emerged ones are simply lobed. when the plant is grown out of water , all the leaves are undissected. in the submerged environment, all leaves are dissected.
there are other effect also, such as vestigial organ in some living animals, strong claws and canines in carnivores, sensitive skin and tactile point on the ventral side of the body and callosities of palm in hard workers, exemplifying the Lamarckian theory.
this is all about Lamarckism above example are common example to gives the Lamarckism theory .
to under stand more about above topic we are to gives some question are related with Lamarckism theory.
(1) the term 'evolution' was first used by?
(a) spencer
(b) Darwin
(c) Lamarck
(d) Mendel
solution: the term evolution was first used by spencer
Q(2) ' Phiosophie zoologique' was written by?
(a) De varies
(b) Lamarck
(c) Mendel
(d) Spencer
solution: phiolosophie of zoologoque was written by Lamarck
(3) according to the Lamarck , acquired characters are?
(a) inherited
(b) artificial
(c) superficial
(d) all of the above
solution : inherited
(4) whose theory of evolution believes that every organism has an internal vital force?
