Theories on Evolution.. Lamarckian theory..

This theory named, "Use and Disuse", was proposed by a scientist Jean Baptiste de Lamarck, real name:Jean-Baptiste Pierre Antoine de Monet Chevalier de Lamarck. It says that based on heritability of acquired characteristics, the once widely accepted idea that an organism can acquire characteristics during its lifetime and pass them on to its offspring.

It proposed that individual efforts during the lifetime of the organisms were the main mechanism driving species to adaptation, as they supposedly would acquire adaptative changes and pass them on to offspring. After publication of Charles Darwin's theory of natural selection, the importance of individual efforts in the generation of adaptation was considerably diminished. Later, Mendelian genetics supplanted the notion of inheritance of acquired traits, eventually leading to the development of the modern evolutionary synthesis, and the general abandonment of the Lamarckian theory of evolution in biology. In a wider context, Lamarckism is of use when examining the evolution of cultures and ideas, and is related to the theory of Memetics.

There are two basis of his theory:

1. Use and Disuse-wherein:

Use- certain structures could be developed when they are used
Disuse-certain structures are lost or becomes vestigial when they are not used

2. Inheritance of acquired traits- individuals acquire the traits of thir predecessors or ancestors

Because of these 2 ideas, Lamarck has developed two laws:
1. In every animal which has not passed its limit of development, a more frequent and continuous use of any organ gradually strenghtens, develops and enlarges that organ, and gives it a power proportional to the length of time it has been so used; while the permanent disuse organ imperceptibly weakens and deteriorates it, and progressively diminishes its functional capacity, until it finally disappears.
2. All the acquisitions or losses wrought by nature on individuals, through the influence of the environment in which their race has long been placed, and hence through the influence of the predominantly use and permanently disuse of any organ; all these are preserved by reproduction to the new individuals which arise, provided that the acquired modifications are common to both sexes, or at least to the individuals which produce the young.

Examples:

• It has been said that long ago giraffes have long necks but due to the scarcity of food (grass) , the giraffes stretched their necks, to reach the trees, thus lengthening it and developing it.. The offsprings will have slightly longer necks.
  
• The blacksmith is also a good example for this theory. Because of his work, the blacksmith's developing his muscles. His offspring will have the same muscular development when they mature.

But his theory was discredited by other experts for reasons:


In essence, a change in the environment brings about change in "needs" (besoins), resulting in change in behavior, bringing change in organ usage and development, bringing change in form over time — and thus the gradual transmutation of the species. While such a theory might explain the observed diversity of species and the first law is generally true, the main argument against Lamarckism is that experiments simply do not support the second law — purely "acquired traits" do not appear in any meaningful sense to be inherited. For example, a human child must learn how to catch a ball even though his or her parents learned the same feat when they were children.

(Though it is important to note that just because the skill of catching a ball can not be passed on from parent to child does not mean that we can rule out all other traits from being passed on in a Lamarckian fashion).

The argument that instinct in animals is evidence for hereditary knowledge is generally regarded within science as false. Such behaviours are more probably passed on through a mechanism called the Baldwin effect. Lamarck’s theories gained initial acceptance because the mechanisms of inheritance were not elucidated until later in the 19th Century, after Lamarck's death.

Several historians have argued that Lamarck's name is linked somewhat unfairly to the theory that has come to bear his name, and that Lamarck deserves credit for being an influential early proponent of the concept of biological evolution, far more than for the mechanism of evolution, in which he simply followed the accepted wisdom of his time. Lamarck died 30 years before the first publication of Charles Darwin's Origin of Species. As science historian Stephen Jay Gould has noted, if Lamarck had been aware of Darwin's proposed mechanism of natural selection, there is no reason to assume he would not have accepted it as a more likely alternative to his "own" mechanism. Note also that Darwin, like Lamarck, lacked a plausible alternative mechanism of inheritance - the particulate nature of inheritance was only to be observed by Gregor Mendel somewhat later, published in 1866. Its importance, although Darwin cited Mendel's paper, was not recognised until the Modern evolutionary synthesis in the early 1900s. An important point in its favour at the time was that Lamarck's theory contained a mechanism describing how variation is maintained, which Darwin’s own theory lacked.

Many experts have been thinking of considering Lamarck's theory as a basis of cultural evolution.
(source:
http://en.wikipedia.org/wiki/Lamarckian)