Darwin's Theory or Theory of Natural Selection -Notes4subject.blogspot.com

Darwin's Theory or Theory Of Natural Selection: 

    Charles Darwin (1809-1882) proposed this theory in his book "Orig ion of Species by Natural Selection". His theory based on his observation which he passed five Lear's on ship Beagle. He visited many islands and coats, and recorded every thing in his book which he observed. The following are main factors/topics of his theory.

1. Over Production
2. Struggle for existence
3. Variation & Heredity 
4. Natural Selection or Survival of fittest
5. Formation of species.

Over Production: Every species in the absence of enviormental checks, tends to increase their number (children) in a gecinatrical mammer. A population of given species double/triple or quadruple in a year, if there are no checks, For example: Oyster gives about 80 millions eggs in one season.
- Salmon fish gives about 28 millions eggs in a season.
- Cat fish lays about 5-7 millions eggs in a single season.
- A house fly lays about 120 eggs six times in every summer.
- Star fish gives about one million eggs in one season.
- A rabbit gives Six children every four times in a year, six months old rabbit start to give children.
       It indicates that over production is the characters of all living organisms, if such reproduction of individuals will continue, only that species occupy the particular habitat and no space, food etc will be available for others species.

Struggle For Existence: If the all numbers of living organism reproduce in over production. so result in a competition for food, space, light and air to alive, this is called "struggle for existence". Under different competition for basic needs many individuals are killed. This struggle may be:
Intrapecific struggle: This struggle is between the same kind of species.
Inter specific struggle: This struggle is between the different kinds of species.
Environmental struggle: This struggle against the climate condition like cold, rain, temp: etc.

Variation And Heredity: Variation means changes, it is a fact that two individuals are not same, while they belongs to same parents. All organisms show suitable variation in form, size colour, Physislogical and habit. Those individuals who show suitable variation continue to exists and reproduce, while those individuals who show unfavourable or no variation are eliminated. Variations are two types: favourable variations are those help to adjust the living organism in the suitable environment, while Unfavourable variations are those don't help in the adjustment of living organism in the environment, so it sh art to disappear.

Natural Selection & Survival Of Fittest: Darwin said natural selection is the driving force behind the evolution, the idea came in his through practises of Artificial selection to produce strains of crops and domestic animals.
     According to Darwin the environment plays the role of the breeder in Natural selection which generate population whose member are better adopted (mast fit) in the environment and the species that tack the ability to copy with environment factors or change will shrink in number or even become extinct.

Formation Of Species: According to Darwin the individuals species most survival and show continue and gradual variation that after several generation / species may produce discontents which are quite different from their anertors, different enough to be separate species. Further more, certain members of a population with one group of variations may become adapted to be environment in one way, or become adapted to be different set of variations, become adapted in a different way, or become adapted to a different environment. In this way or more species may rise from a single ancestral stock.